Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Obsoletes: 5077, 5246 (if approved) July 06, 2017
Updates: 4492, 5705, 6066, 6961 (if
approved)
Intended status: Standards Track
Expires: January 7, 2018

The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-latest

Abstract

This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 7, 2018.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for this draft is maintained in GitHub. Suggested changes should be submitted as pull requests at https://github.com/tlswg/tls13-spec. Instructions are on that page as well. Editorial changes can be managed in GitHub, but any substantive change should be discussed on the TLS mailing list.

The primary goal of TLS is to provide a secure channel between two communicating peers. Specifically, the channel should provide the following properties:

These properties should be true even in the face of an attacker who has complete control of the network, as described in [RFC3552]. See Appendix E for a more complete statement of the relevant security properties.

TLS consists of two primary components:

TLS is application protocol independent; higher-level protocols can layer on top of TLS transparently. The TLS standard, however, does not specify how protocols add security with TLS; how to initiate TLS handshaking and how to interpret the authentication certificates exchanged are left to the judgment of the designers and implementors of protocols that run on top of TLS.

This document defines TLS version 1.3. While TLS 1.3 is not directly compatible with previous versions, all versions of TLS incorporate a versioning mechanism which allows clients and servers to interoperably negotiate a common version if one is supported by both peers.

This document supersedes and obsoletes previous versions of TLS including version 1.2 [RFC5246]. It also obsoletes the TLS ticket mechanism defined in [RFC5077] and replaces it with the mechanism defined in Section 2.2. Section 4.2.6 updates [RFC4492] by modifying the protocol attributes used to negotiate Elliptic Curves. Because TLS 1.3 changes the way keys are derived it updates [RFC5705] as described in Section 7.5 it also changes how OCSP messages are carried and therefore updates [RFC6066] and obsoletes [RFC6961] as described in section Section 4.4.2.1.

1.1. Conventions and Terminology

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 [RFC2119].

The following terms are used:

client: The endpoint initiating the TLS connection.

connection: A transport-layer connection between two endpoints.

endpoint: Either the client or server of the connection.

handshake: An initial negotiation between client and server that establishes the parameters of their subsequent interactions.

peer: An endpoint. When discussing a particular endpoint, “peer” refers to the endpoint that is not the primary subject of discussion.

receiver: An endpoint that is receiving records.

sender: An endpoint that is transmitting records.

server: The endpoint which did not initiate the TLS connection.

1.2. Change Log

RFC EDITOR PLEASE DELETE THIS SECTION.

(*) indicates changes to the wire protocol which may require implementations to update.

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1.3. Major Differences from TLS 1.2

The following is a list of the major functional differences between TLS 1.2 and TLS 1.3. It is not intended to be exhaustive and there are many minor differences.

1.4. Updates Affecting TLS 1.2

This document defines several changes that optionally affect implementations of TLS 1.2:

An implementation of TLS 1.3 that also supports TLS 1.2 might need to include changes to support these changes even when TLS 1.3 is not in use. See the referenced sections for more details.

2. Protocol Overview

The cryptographic parameters used by the secure channel are produced by the TLS handshake protocol. This sub-protocol of TLS is used by the client and server when first communicating with each other. The handshake protocol allows peers to negotiate a protocol version, select cryptographic algorithms, optionally authenticate each other, and establish shared secret keying material. Once the handshake is complete, the peers use the established keys to protect the application layer traffic.

A failure of the handshake or other protocol error triggers the termination of the connection, optionally preceded by an alert message (Section 6).

TLS supports three basic key exchange modes:

Figure 1 below shows the basic full TLS handshake:

       Client                                               Server

Key  ^ ClientHello
Exch | + key_share*
     | + signature_algorithms*
     | + psk_key_exchange_modes*
     v + pre_shared_key*         -------->
                                                       ServerHello  ^ Key
                                                      + key_share*  | Exch
                                                 + pre_shared_key*  v
                                             {EncryptedExtensions}  ^  Server
                                             {CertificateRequest*}  v  Params
                                                    {Certificate*}  ^
                                              {CertificateVerify*}  | Auth
                                                        {Finished}  v
                                 <--------     [Application Data*]
     ^ {Certificate*}
Auth | {CertificateVerify*}
     v {Finished}                -------->
       [Application Data]        <------->      [Application Data]

              +  Indicates noteworthy extensions sent in the
                 previously noted message.

              *  Indicates optional or situation-dependent
                 messages/extensions that are not always sent.

              {} Indicates messages protected using keys
                 derived from a [sender]_handshake_traffic_secret.

              [] Indicates messages protected using keys
                 derived from [sender]_application_traffic_secret_N

Figure 1: Message flow for full TLS Handshake

The handshake can be thought of as having three phases (indicated in the diagram above):

In the Key Exchange phase, the client sends the ClientHello (Section 4.1.2) message, which contains a random nonce (ClientHello.random); its offered protocol versions; a list of symmetric cipher/HKDF hash pairs; either a set of Diffie-Hellman key shares (in the “key_share” extension Section 4.2.7), a set of pre-shared key labels (in the “pre_shared_key” extension Section 4.2.10) or both; and potentially additional extensions.

The server processes the ClientHello and determines the appropriate cryptographic parameters for the connection. It then responds with its own ServerHello (Section 4.1.3), which indicates the negotiated connection parameters. The combination of the ClientHello and the ServerHello determines the shared keys. If (EC)DHE key establishment is in use, then the ServerHello contains a “key_share” extension with the server’s ephemeral Diffie-Hellman share which MUST be in the same group as one of the client’s shares. If PSK key establishment is in use, then the ServerHello contains a “pre_shared_key” extension indicating which of the client’s offered PSKs was selected. Note that implementations can use (EC)DHE and PSK together, in which case both extensions will be supplied.

The server then sends two messages to establish the Server Parameters:

EncryptedExtensions:
responses to ClientHello extensions that are not required to determine the cryptographic parameters, other than those that are specific to individual certificates. [Section 4.3.1]
CertificateRequest:
if certificate-based client authentication is desired, the desired parameters for that certificate. This message is omitted if client authentication is not desired. [Section 4.3.2]

Finally, the client and server exchange Authentication messages. TLS uses the same set of messages every time that authentication is needed. Specifically:

Certificate:
the certificate of the endpoint and any per-certificate extensions. This message is omitted by the server if not authenticating with a certificate and by the client if the server did not send CertificateRequest (thus indicating that the client should not authenticate with a certificate). Note that if raw public keys [RFC7250] or the cached information extension [RFC7924] are in use, then this message will not contain a certificate but rather some other value corresponding to the server’s long-term key. [Section 4.4.2]
CertificateVerify:
a signature over the entire handshake using the private key corresponding to the public key in the Certificate message. This message is omitted if the endpoint is not authenticating via a certificate. [Section 4.4.3]
Finished:
a MAC (Message Authentication Code) over the entire handshake. This message provides key confirmation, binds the endpoint’s identity to the exchanged keys, and in PSK mode also authenticates the handshake. [Section 4.4.4]

Upon receiving the server’s messages, the client responds with its Authentication messages, namely Certificate and CertificateVerify (if requested), and Finished.

At this point, the handshake is complete, and the client and server must derive the keying material required by the record layer to exchange application-layer data protected through authenticated encryption. Application data MUST NOT be sent prior to sending the Finished message and until the record layer starts using encryption keys. Note that while the server may send application data prior to receiving the client’s Authentication messages, any data sent at that point is, of course, being sent to an unauthenticated peer.

2.1. Incorrect DHE Share

If the client has not provided a sufficient “key_share” extension (e.g., it includes only DHE or ECDHE groups unacceptable to or unsupported by the server), the server corrects the mismatch with a HelloRetryRequest and the client needs to restart the handshake with an appropriate “key_share” extension, as shown in Figure 2. If no common cryptographic parameters can be negotiated, the server MUST abort the handshake with an appropriate alert.

         Client                                               Server

         ClientHello
         + key_share             -------->
                                 <--------         HelloRetryRequest
                                                         + key_share

         ClientHello
         + key_share             -------->
                                                         ServerHello
                                                         + key_share
                                               {EncryptedExtensions}
                                               {CertificateRequest*}
                                                      {Certificate*}
                                                {CertificateVerify*}
                                                          {Finished}
                                 <--------       [Application Data*]
         {Certificate*}
         {CertificateVerify*}
         {Finished}              -------->
         [Application Data]      <------->        [Application Data]

Figure 2: Message flow for a full handshake with mismatched parameters

Note: The handshake transcript includes the initial ClientHello/HelloRetryRequest exchange; it is not reset with the new ClientHello.

TLS also allows several optimized variants of the basic handshake, as described in the following sections.

2.2. Resumption and Pre-Shared Key (PSK)

Although TLS PSKs can be established out of band, PSKs can also be established in a previous connection and then reused (“session resumption”). Once a handshake has completed, the server can send to the client a PSK identity that corresponds to a unique key derived from the initial handshake (see Section 4.6.1). The client can then use that PSK identity in future handshakes to negotiate the use of the associated PSK. If the server accepts it, then the security context of the new connection is cryptographically tied to the original connection and the key derived from the initial handshake is used to bootstrap the cryptographic state instead of a full handshake. In TLS 1.2 and below, this functionality was provided by “session IDs” and “session tickets” [RFC5077]. Both mechanisms are obsoleted in TLS 1.3.

PSKs can be used with (EC)DHE key exchange in order to provide forward secrecy in combination with shared keys, or can be used alone, at the cost of losing forward secrecy for the application data.

Figure 3 shows a pair of handshakes in which the first establishes a PSK and the second uses it:

       Client                                               Server

Initial Handshake:
       ClientHello
       + key_share               -------->
                                                       ServerHello
                                                       + key_share
                                             {EncryptedExtensions}
                                             {CertificateRequest*}
                                                    {Certificate*}
                                              {CertificateVerify*}
                                                        {Finished}
                                 <--------     [Application Data*]
       {Certificate*}
       {CertificateVerify*}
       {Finished}                -------->
                                 <--------      [NewSessionTicket]
       [Application Data]        <------->      [Application Data]


Subsequent Handshake:
       ClientHello
       + key_share*
       + psk_key_exchange_modes
       + pre_shared_key          -------->
                                                       ServerHello
                                                  + pre_shared_key
                                                      + key_share*
                                             {EncryptedExtensions}
                                                        {Finished}
                                 <--------     [Application Data*]
       {Finished}                -------->
       [Application Data]        <------->      [Application Data]

Figure 3: Message flow for resumption and PSK

As the server is authenticating via a PSK, it does not send a Certificate or a CertificateVerify message. When a client offers resumption via PSK, it SHOULD also supply a “key_share” extension to the server to allow the server to decline resumption and fall back to a full handshake, if needed. The server responds with a “pre_shared_key” extension to negotiate use of PSK key establishment and can (as shown here) respond with a “key_share” extension to do (EC)DHE key establishment, thus providing forward secrecy.

When PSKs are provisioned out of band, the PSK identity and the KDF hash algorithm to be used with the PSK MUST also be provisioned. Note: When using an out-of-band provisioned pre-shared secret, a critical consideration is using sufficient entropy during the key generation, as discussed in [RFC4086]. Deriving a shared secret from a password or other low-entropy sources is not secure. A low-entropy secret, or password, is subject to dictionary attacks based on the PSK binder. The specified PSK authentication is not a strong password-based authenticated key exchange even when used with Diffie-Hellman key establishment.

2.3. Zero-RTT Data

When clients and servers share a PSK (either obtained externally or via a previous handshake), TLS 1.3 allows clients to send data on the first flight (“early data”). The client uses the PSK to authenticate the server and to encrypt the early data.

When clients use a PSK obtained externally to send early data, then the following additional information MUST be provisioned to both parties:

As shown in Figure 4, the 0-RTT data is just added to the 1-RTT handshake in the first flight. The rest of the handshake uses the same messages as with a 1-RTT handshake with PSK resumption.

         Client                                               Server

         ClientHello
         + early_data
         + key_share*
         + psk_key_exchange_modes
         + pre_shared_key
         (Application Data*)     -------->
                                                         ServerHello
                                                    + pre_shared_key
                                                        + key_share*
                                               {EncryptedExtensions}
                                                       + early_data*
                                                          {Finished}
                                 <--------       [Application Data*]
         (EndOfEarlyData)
         {Finished}              -------->

         [Application Data]      <------->        [Application Data]

               +  Indicates noteworthy extensions sent in the
                  previously noted message.

               *  Indicates optional or situation-dependent
                  messages/extensions that are not always sent.

               () Indicates messages protected using keys
                  derived from client_early_traffic_secret.

               {} Indicates messages protected using keys
                  derived from a [sender]_handshake_traffic_secret.

               [] Indicates messages protected using keys
                  derived from [sender]_application_traffic_secret_N

Figure 4: Message flow for a zero round trip handshake

IMPORTANT NOTE: The security properties for 0-RTT data are weaker than those for other kinds of TLS data. Specifically:

  1. This data is not forward secret, as it is encrypted solely under keys derived using the offered PSK.
  2. There are no guarantees of non-replay between connections. Protection against replay for ordinary TLS 1.3 1-RTT data is provided via the server’s Random value, but 0-RTT data does not depend on the ServerHello and therefore has weaker guarantees. This is especially relevant if the data is authenticated either with TLS client authentication or inside the application protocol. The same warnings apply to any use of the early_exporter_master_secret.

0-RTT data cannot be duplicated within a connection (i.e., the server will not process the same data twice for the same connection) and an attacker will not be able to make 0-RTT data appear to be 1-RTT data (because it is protected with different keys.) Appendix E.5 contains a description of potential attacks and Section 8 describes mechanisms which the server can use to limit the impact of replay.

3. Presentation Language

This document deals with the formatting of data in an external representation. The following very basic and somewhat casually defined presentation syntax will be used.

3.1. Basic Block Size

The representation of all data items is explicitly specified. The basic data block size is one byte (i.e., 8 bits). Multiple byte data items are concatenations of bytes, from left to right, from top to bottom. From the byte stream, a multi-byte item (a numeric in the example) is formed (using C notation) by:

   value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
           ... | byte[n-1];

This byte ordering for multi-byte values is the commonplace network byte order or big-endian format.

3.2. Miscellaneous

Comments begin with “/*” and end with “*/”.

Optional components are denoted by enclosing them in “[[ ]]” double brackets.

Single-byte entities containing uninterpreted data are of type opaque.

3.3. Vectors

A vector (single-dimensioned array) is a stream of homogeneous data elements. The size of the vector may be specified at documentation time or left unspecified until runtime. In either case, the length declares the number of bytes, not the number of elements, in the vector. The syntax for specifying a new type, T’, that is a fixed-length vector of type T is

   T T'[n];

Here, T’ occupies n bytes in the data stream, where n is a multiple of the size of T. The length of the vector is not included in the encoded stream.

In the following example, Datum is defined to be three consecutive bytes that the protocol does not interpret, while Data is three consecutive Datum, consuming a total of nine bytes.

   opaque Datum[3];      /* three uninterpreted bytes */
   Datum Data[9];        /* 3 consecutive 3-byte vectors */

Variable-length vectors are defined by specifying a subrange of legal lengths, inclusively, using the notation <floor..ceiling>. When these are encoded, the actual length precedes the vector’s contents in the byte stream. The length will be in the form of a number consuming as many bytes as required to hold the vector’s specified maximum (ceiling) length. A variable-length vector with an actual length field of zero is referred to as an empty vector.

   T T'<floor..ceiling>;

In the following example, mandatory is a vector that must contain between 300 and 400 bytes of type opaque. It can never be empty. The actual length field consumes two bytes, a uint16, which is sufficient to represent the value 400 (see Section 3.4). Similarly, longer can represent up to 800 bytes of data, or 400 uint16 elements, and it may be empty. Its encoding will include a two-byte actual length field prepended to the vector. The length of an encoded vector must be an exact multiple of the length of a single element (e.g., a 17-byte vector of uint16 would be illegal).

   opaque mandatory<300..400>;
         /* length field is 2 bytes, cannot be empty */
   uint16 longer<0..800>;
         /* zero to 400 16-bit unsigned integers */

3.4. Numbers

The basic numeric data type is an unsigned byte (uint8). All larger numeric data types are formed from fixed-length series of bytes concatenated as described in Section 3.1 and are also unsigned. The following numeric types are predefined.

   uint8 uint16[2];
   uint8 uint24[3];
   uint8 uint32[4];
   uint8 uint64[8];

All values, here and elsewhere in the specification, are stored in network byte (big-endian) order; the uint32 represented by the hex bytes 01 02 03 04 is equivalent to the decimal value 16909060.

3.5. Enumerateds

An additional sparse data type is available called enum. Each definition is a different type. Only enumerateds of the same type may be assigned or compared. Every element of an enumerated must be assigned a value, as demonstrated in the following example. Since the elements of the enumerated are not ordered, they can be assigned any unique value, in any order.

   enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;

Future extensions or additions to the protocol may define new values. Implementations need to be able to parse and ignore unknown values unless the definition of the field states otherwise.

An enumerated occupies as much space in the byte stream as would its maximal defined ordinal value. The following definition would cause one byte to be used to carry fields of type Color.

   enum { red(3), blue(5), white(7) } Color;

One may optionally specify a value without its associated tag to force the width definition without defining a superfluous element.

In the following example, Taste will consume two bytes in the data stream but can only assume the values 1, 2, or 4 in the current version of the protocol.

   enum { sweet(1), sour(2), bitter(4), (32000) } Taste;

The names of the elements of an enumeration are scoped within the defined type. In the first example, a fully qualified reference to the second element of the enumeration would be Color.blue. Such qualification is not required if the target of the assignment is well specified.

   Color color = Color.blue;     /* overspecified, legal */
   Color color = blue;           /* correct, type implicit */

The names assigned to enumerateds do not need to be unique. The numerical value can describe a range over which the same name applies. The value includes the minimum and maximum inclusive values in that range, separated by two period characters. This is principally useful for reserving regions of the space.

   enum { sad(0), meh(1..254), happy(255) } Mood;

3.6. Constructed Types

Structure types may be constructed from primitive types for convenience. Each specification declares a new, unique type. The syntax for definition is much like that of C.

   struct {
       T1 f1;
       T2 f2;
       ...
       Tn fn;
   } [[T]];

The fields within a structure may be qualified using the type’s name, with a syntax much like that available for enumerateds. For example, T.f2 refers to the second field of the previous declaration. Structure definitions may be embedded. Anonymous structs may also be defined inside other structures.

3.7. Constants

Fields and variables may be assigned a fixed value using “=”, as in:

   struct {
       T1 f1 = 8;  /* T.f1 must always be 8 */
       T2 f2;
   } T;

3.8. Variants

Defined structures may have variants based on some knowledge that is available within the environment. The selector must be an enumerated type that defines the possible variants the structure defines. The body of the variant structure may be given a label for reference. The mechanism by which the variant is selected at runtime is not prescribed by the presentation language.

   struct {
       T1 f1;
       T2 f2;
       ....
       Tn fn;
       select (E) {
           case e1: Te1;
           case e2: Te2;
           ....
           case en: Ten;
       } [[fv]];
   } [[Tv]];

For example:

   enum { apple(0), orange(1) } VariantTag;

   struct {
       uint16 number;
       opaque string<0..10>; /* variable length */
   } V1;

   struct {
       uint32 number;
       opaque string[10];    /* fixed length */
   } V2;

   struct {
       VariantTag type;
       select (VariantRecord.type) {
           case apple:  V1;
           case orange: V2;
       };
   } VariantRecord;

4. Handshake Protocol

The handshake protocol is used to negotiate the security parameters of a connection. Handshake messages are supplied to the TLS record layer, where they are encapsulated within one or more TLSPlaintext or TLSCiphertext structures, which are processed and transmitted as specified by the current active connection state.

   enum {
       client_hello(1),
       server_hello(2),
       new_session_ticket(4),
       end_of_early_data(5),
       hello_retry_request(6),
       encrypted_extensions(8),
       certificate(11),
       certificate_request(13),
       certificate_verify(15),
       finished(20),
       key_update(24),
       message_hash(254),
       (255)
   } HandshakeType;

   struct {
       HandshakeType msg_type;    /* handshake type */
       uint24 length;             /* bytes in message */
       select (Handshake.msg_type) {
           case client_hello:          ClientHello;
           case server_hello:          ServerHello;
           case end_of_early_data:     EndOfEarlyData;
           case hello_retry_request:   HelloRetryRequest;
           case encrypted_extensions:  EncryptedExtensions;
           case certificate_request:   CertificateRequest;
           case certificate:           Certificate;
           case certificate_verify:    CertificateVerify;
           case finished:              Finished;
           case new_session_ticket:    NewSessionTicket;
           case key_update:            KeyUpdate;
       } body;
   } Handshake;

Protocol messages MUST be sent in the order defined in Section 4.4.1 and shown in the diagrams in Section 2. A peer which receives a handshake message in an unexpected order MUST abort the handshake with an “unexpected_message” alert.

New handshake message types are assigned by IANA as described in Section 11.

4.1. Key Exchange Messages

The key exchange messages are used to determine the security capabilities of the client and the server and to establish shared secrets including the traffic keys used to protect the rest of the handshake and the data.

4.1.1. Cryptographic Negotiation

In TLS, the cryptographic negotiation proceeds by the client offering the following four sets of options in its ClientHello:

If the server does not select a PSK, then the first three of these options are entirely orthogonal: the server independently selects a cipher suite, an (EC)DHE group and key share for key establishment, and a signature algorithm/certificate pair to authenticate itself to the client. If there is no overlap between the received “supported_groups” and the groups supported by the server then the server MUST abort the handshake with a “handshake_failure” or an “insufficient_security” alert.

If the server selects a PSK, then it MUST also select a key establishment mode from the set indicated by client’s “psk_key_exchange_modes” extension (at present, PSK alone or with (EC)DHE). Note that if the PSK can be used without (EC)DHE then non-overlap in the “supported_groups” parameters need not be fatal, as it is in the non-PSK case discussed in the previous paragraph.

If the server selects an (EC)DHE group and the client did not offer a compatible “key_share” extension in the initial ClientHello, the server MUST respond with a HelloRetryRequest (Section 4.1.4) message.

If the server successfully selects parameters and does not require a HelloRetryRequest, it indicates the selected parameters in the ServerHello as follows:

If the server is unable to negotiate a supported set of parameters (i.e., there is no overlap between the client and server parameters), it MUST abort the handshake with either a “handshake_failure” or “insufficient_security” fatal alert (see Section 6).

4.1.2. Client Hello

When a client first connects to a server, it is REQUIRED to send the ClientHello as its first message. The client will also send a ClientHello when the server has responded to its ClientHello with a HelloRetryRequest. In that case, the client MUST send the same ClientHello (without modification) except:

Because TLS 1.3 forbids renegotiation, if a server has negotiated TLS 1.3 and receives a ClientHello at any other time, it MUST terminate the connection with an “unexpected_message” alert.

If a server established a TLS connection with a previous version of TLS and receives a TLS 1.3 ClientHello in a renegotiation, it MUST retain the previous protocol version. In particular, it MUST NOT negotiate TLS 1.3.

Structure of this message:

   uint16 ProtocolVersion;
   opaque Random[32];

   uint8 CipherSuite[2];    /* Cryptographic suite selector */

   struct {
       ProtocolVersion legacy_version = 0x0303;    /* TLS v1.2 */
       Random random;
       opaque legacy_session_id<0..32>;
       CipherSuite cipher_suites<2..2^16-2>;
       opaque legacy_compression_methods<1..2^8-1>;
       Extension extensions<8..2^16-1>;
   } ClientHello;

legacy_version
In previous versions of TLS, this field was used for version negotiation and represented the highest version number supported by the client. Experience has shown that many servers do not properly implement version negotiation, leading to “version intolerance” in which the server rejects an otherwise acceptable ClientHello with a version number higher than it supports. In TLS 1.3, the client indicates its version preferences in the “supported_versions” extension (Section 4.2.1) and the legacy_version field MUST be set to 0x0303, which is the version number for TLS 1.2. (See Appendix D for details about backward compatibility.)
random
32 bytes generated by a secure random number generator. See Appendix C for additional information.
legacy_session_id
Versions of TLS before TLS 1.3 supported a “session resumption” feature which has been merged with Pre-Shared Keys in this version (see Section 2.2). This field MUST be ignored by a server negotiating TLS 1.3 and MUST be set as a zero length vector (i.e., a single zero byte length field) by clients that do not have a cached session ID set by a pre-TLS 1.3 server.
cipher_suites
This is a list of the symmetric cipher options supported by the client, specifically the record protection algorithm (including secret key length) and a hash to be used with HKDF, in descending order of client preference. If the list contains cipher suites that the server does not recognize, support or wish to use, the server MUST ignore those cipher suites and process the remaining ones as usual. Values are defined in Appendix B.4. If the client is attempting a PSK key establishment, it SHOULD advertise at least one cipher suite indicating a Hash associated with the PSK.
legacy_compression_methods
Versions of TLS before 1.3 supported compression with the list of supported compression methods being sent in this field. For every TLS 1.3 ClientHello, this vector MUST contain exactly one byte set to zero, which corresponds to the “null” compression method in prior versions of TLS. If a TLS 1.3 ClientHello is received with any other value in this field, the server MUST abort the handshake with an “illegal_parameter” alert. Note that TLS 1.3 servers might receive TLS 1.2 or prior ClientHellos which contain other compression methods and MUST follow the procedures for the appropriate prior version of TLS. TLS 1.3 ClientHellos are identified as having a legacy_version of 0x0303 and a supported_versions extension present with 0x0304 as the highest version indicated therein.
extensions
Clients request extended functionality from servers by sending data in the extensions field. The actual “Extension” format is defined in Section 4.2. In TLS 1.3, use of certain extensions is mandatory, as functionality is moved into extensions to preserve ClientHello compatibility with previous versions of TLS. Servers MUST ignore unrecognized extensions.

All versions of TLS allow an extensions field to optionally follow the compression_methods field. TLS 1.3 ClientHello messages always contain extensions (minimally “supported_versions”, otherwise they will be interpreted as TLS 1.2 ClientHello messages). However, TLS 1.3 servers might receive ClientHello messages without an extensions field from prior versions of TLS. The presence of extensions can be detected by determining whether there are bytes following the compression_methods field at the end of the ClientHello. Note that this method of detecting optional data differs from the normal TLS method of having a variable-length field, but it is used for compatibility with TLS before extensions were defined. TLS 1.3 servers will need to perform this check first and only attempt to negotiate TLS 1.3 if a “supported_version” extension is present. If negotiating a version of TLS prior to 1.3, a server MUST check that the message either contains no data after legacy_compression_methods or that it contains a valid extensions block with no data following. If not, then it MUST abort the handshake with a “decode_error” alert.

In the event that a client requests additional functionality using extensions, and this functionality is not supplied by the server, the client MAY abort the handshake.

After sending the ClientHello message, the client waits for a ServerHello or HelloRetryRequest message. If early data is in use, the client may transmit early application data Section 2.3 while waiting for the next handshake message.

4.1.3. Server Hello

The server will send this message in response to a ClientHello message to proceed with the handshake if it is able to negotiate an acceptable set of handshake parameters based on the ClientHello.

Structure of this message:

   struct {
       ProtocolVersion version;
       Random random;
       CipherSuite cipher_suite;
       Extension extensions<6..2^16-1>;
   } ServerHello;

version
This field contains the version of TLS negotiated for this connection. Servers MUST select a version from the list in ClientHello’s supported_versions extension, or otherwise negotiate TLS 1.2 or previous. A client that receives a version that was not offered MUST abort the handshake. For this version of the specification, the version is 0x0304. (See Appendix D for details about backward compatibility.)
random
32 bytes generated by a secure random number generator. See Appendix C for additional information. The last eight bytes MUST be overwritten as described below if negotiating TLS 1.2 or TLS 1.1, but the remaining bytes MUST be random. This structure is generated by the server and MUST be generated independently of the ClientHello.random.
cipher_suite
The single cipher suite selected by the server from the list in ClientHello.cipher_suites. A client which receives a cipher suite that was not offered MUST abort the handshake with an “illegal_parameter” alert.
extensions
A list of extensions. The ServerHello MUST only include extensions which are required to establish the cryptographic context. Currently the only such extensions are “key_share” and “pre_shared_key”. All current TLS 1.3 ServerHello messages will contain one of these two extensions, or both when using a PSK with (EC)DHE key establishment. The remaining extensions are sent separately in the EncryptedExtensions message.

TLS 1.3 has a downgrade protection mechanism embedded in the server’s random value. TLS 1.3 servers which negotiate TLS 1.2 or below in response to a ClientHello MUST set the last eight bytes of their Random value specially.

If negotiating TLS 1.2, TLS 1.3 servers MUST set the last eight bytes of their Random value to the bytes:

  44 4F 57 4E 47 52 44 01

If negotiating TLS 1.1 or below, TLS 1.3 servers MUST and TLS 1.2 servers SHOULD set the last eight bytes of their Random value to the bytes:

  44 4F 57 4E 47 52 44 00

TLS 1.3 clients receiving a ServerHello indicating TLS 1.2 or below MUST check that the last eight bytes are not equal to either of these values. TLS 1.2 clients SHOULD also check that the last eight bytes are not equal to the second value if the ServerHello indicates TLS 1.1 or below. If a match is found, the client MUST abort the handshake with an “illegal_parameter” alert. This mechanism provides limited protection against downgrade attacks over and above what is provided by the Finished exchange: because the ServerKeyExchange, a message present in TLS 1.2 and below, includes a signature over both random values, it is not possible for an active attacker to modify the random values without detection as long as ephemeral ciphers are used. It does not provide downgrade protection when static RSA is used.

Note: This is a change from [RFC5246], so in practice many TLS 1.2 clients and servers will not behave as specified above.

A legacy TLS client performing renegotiation with TLS 1.2 or prior and which receives a TLS 1.3 ServerHello during renegotiation MUST abort the handshake with a “protocol_version” alert. Note that renegotiation is not possible when TLS 1.3 has been negotiated.

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH Implementations of draft versions (see Section 4.2.1.1) of this specification SHOULD NOT implement this mechanism on either client and server. A pre-RFC client connecting to RFC servers, or vice versa, will appear to downgrade to TLS 1.2. With the mechanism enabled, this will cause an interoperability failure.

4.1.4. Hello Retry Request

The server will send this message in response to a ClientHello message if it is able to find an acceptable set of parameters but the ClientHello does not contain sufficient information to proceed with the handshake.

Structure of this message:

   struct {
       ProtocolVersion server_version;
       CipherSuite cipher_suite;
       Extension extensions<2..2^16-1>;
   } HelloRetryRequest;

The version, cipher_suite, and extensions fields have the same meanings as their corresponding values in the ServerHello. The server SHOULD send only the extensions necessary for the client to generate a correct ClientHello pair. As with ServerHello, a HelloRetryRequest MUST NOT contain any extensions that were not first offered by the client in its ClientHello, with the exception of optionally the “cookie” (see Section 4.2.2) extension.

Upon receipt of a HelloRetryRequest, the client MUST verify that the extensions block is not empty and otherwise MUST abort the handshake with a “decode_error” alert. Clients MUST abort the handshake with an “illegal_parameter” alert if the HelloRetryRequest would not result in any change in the ClientHello. If a client receives a second HelloRetryRequest in the same connection (i.e., where the ClientHello was itself in response to a HelloRetryRequest), it MUST abort the handshake with an “unexpected_message” alert.

Otherwise, the client MUST process all extensions in the HelloRetryRequest and send a second updated ClientHello. The HelloRetryRequest extensions defined in this specification are:

In addition, in its updated ClientHello, the client SHOULD NOT offer any pre-shared keys associated with a hash other than that of the selected cipher suite. This allows the client to avoid having to compute partial hash transcripts for multiple hashes in the second ClientHello. A client which receives a cipher suite that was not offered MUST abort the handshake. Servers MUST ensure that they negotiate the same cipher suite when receiving a conformant updated ClientHello (if the server selects the cipher suite as the first step in the negotiation, then this will happen automatically). Upon receiving the ServerHello, clients MUST check that the cipher suite supplied in the ServerHello is the same as that in the HelloRetryRequest and otherwise abort the handshake with an “illegal_parameter” alert.

4.2. Extensions

A number of TLS messages contain tag-length-value encoded extensions structures.

   struct {
       ExtensionType extension_type;
       opaque extension_data<0..2^16-1>;
   } Extension;

   enum {
       server_name(0),                             /* RFC 6066 */
       max_fragment_length(1),                     /* RFC 6066 */
       status_request(5),                          /* RFC 6066 */
       supported_groups(10),                       /* RFC 4492, 7919 */
       signature_algorithms(13),                   /* RFC 5246 */
       use_srtp(14),                               /* RFC 5764 */
       heartbeat(15),                              /* RFC 6520 */
       application_layer_protocol_negotiation(16), /* RFC 7301 */
       signed_certificate_timestamp(18),           /* RFC 6962 */
       client_certificate_type(19),                /* RFC 7250 */
       server_certificate_type(20),                /* RFC 7250 */
       padding(21),                                /* RFC 7685 */
       key_share(40),                              /* [[this document]] */
       pre_shared_key(41),                         /* [[this document]] */
       early_data(42),                             /* [[this document]] */
       supported_versions(43),                     /* [[this document]] */
       cookie(44),                                 /* [[this document]] */
       psk_key_exchange_modes(45),                 /* [[this document]] */
       certificate_authorities(47),                /* [[this document]] */
       oid_filters(48),                            /* [[this document]] */
       post_handshake_auth(49),                    /* [[this document]] */
       (65535)
   } ExtensionType;

Here:

The list of extension types is maintained by IANA as described in Section 11.

Extensions are generally structured in a request/response fashion, though some extensions are just indications with no corresponding response. The client sends its extension requests in the ClientHello message and the server sends its extension responses in the ServerHello, EncryptedExtensions, HelloRetryRequest and Certificate messages. The server sends extension requests in the CertificateRequest message which a client MAY respond to with a Certificate message. The server MAY also send unsolicited extensions in the NewSessionTicket, though the client does not respond directly to these.

Implementations MUST NOT send extension responses if the remote endpoint did not send the corresponding extension requests, with the exception of the “cookie” extension in HelloRetryRequest. Upon receiving such an extension, an endpoint MUST abort the handshake with an “unsupported_extension” alert.

The table below indicates the messages where a given extension may appear, using the following notation: CH (ClientHello), SH (ServerHello), EE (EncryptedExtensions), CT (Certificate), CR (CertificateRequest), NST (NewSessionTicket) and HRR (HelloRetryRequest). If an implementation receives an extension which it recognizes and which is not specified for the message in which it appears it MUST abort the handshake with an “illegal_parameter” alert.

Extension TLS 1.3
server_name [RFC6066] CH, EE
max_fragment_length [RFC6066] CH, EE
status_request [RFC6066] CH, CR, CT
supported_groups [RFC7919] CH, EE
signature_algorithms [RFC5246] CH, CR
use_srtp [RFC5764] CH, EE
heartbeat [RFC6520] CH, EE
application_layer_protocol_negotiation [RFC7301] CH, EE
signed_certificate_timestamp [RFC6962] CH, CR, CT
client_certificate_type [RFC7250] CH, EE
server_certificate_type [RFC7250] CH, CT
padding [RFC7685] CH
key_share [[this document]] CH, SH, HRR
pre_shared_key [[this document]] CH, SH
psk_key_exchange_modes [[this document]] CH
early_data [[this document]] CH, EE, NST
cookie [[this document]] CH, HRR
supported_versions [[this document]] CH
certificate_authorities [[this document]] CH, CR
oid_filters [[this document]] CR
post_handshake_auth [[this document]] CH

When multiple extensions of different types are present, the extensions MAY appear in any order, with the exception of “pre_shared_key” Section 4.2.10 which MUST be the last extension in the ClientHello. There MUST NOT be more than one extension of the same type in a given extension block.

In TLS 1.3, unlike TLS 1.2, extensions are negotiated for each handshake even when in resumption-PSK mode. However, 0-RTT parameters are those negotiated in the previous handshake; mismatches may require rejecting 0-RTT (see Section 4.2.9).

There are subtle (and not so subtle) interactions that may occur in this protocol between new features and existing features which may result in a significant reduction in overall security. The following considerations should be taken into account when designing new extensions:

4.2.1. Supported Versions

   struct {
       ProtocolVersion versions<2..254>;
   } SupportedVersions;

The “supported_versions” extension is used by the client to indicate which versions of TLS it supports. The extension contains a list of supported versions in preference order, with the most preferred version first. Implementations of this specification MUST send this extension containing all versions of TLS which they are prepared to negotiate (for this specification, that means minimally 0x0304, but if previous versions of TLS are allowed to be negotiated, they MUST be present as well).

If this extension is not present, servers which are compliant with this specification MUST negotiate TLS 1.2 or prior as specified in [RFC5246], even if ClientHello.legacy_version is 0x0304 or later. Servers MAY abort the handshake upon receiving a ClientHello with legacy_version 0x0304 or later.

If this extension is present, servers MUST ignore the ClientHello.legacy_version value and MUST use only the “supported_versions” extension to determine client preferences. Servers MUST only select a version of TLS present in that extension and MUST ignore any unknown versions that are present in that extension. Note that this mechanism makes it possible to negotiate a version prior to TLS 1.2 if one side supports a sparse range. Implementations of TLS 1.3 which choose to support prior versions of TLS SHOULD support TLS 1.2. Servers should be prepared to receive ClientHellos that include this extension but do not include 0x0304 in the list of versions.

The server MUST NOT send the “supported_versions” extension. The server’s selected version is contained in the ServerHello.version field as in previous versions of TLS.

4.2.1.1. Draft Version Indicator

RFC EDITOR: PLEASE REMOVE THIS SECTION

While the eventual version indicator for the RFC version of TLS 1.3 will be 0x0304, implementations of draft versions of this specification SHOULD instead advertise 0x7f00 | draft_version in ServerHello.version, and HelloRetryRequest.server_version. For instance, draft-17 would be encoded as the 0x7f11. This allows pre-RFC implementations to safely negotiate with each other, even if they would otherwise be incompatible.

4.2.2. Cookie

   struct {
       opaque cookie<1..2^16-1>;
   } Cookie;

Cookies serve two primary purposes:

When sending a HelloRetryRequest, the server MAY provide a “cookie” extension to the client (this is an exception to the usual rule that the only extensions that may be sent are those that appear in the ClientHello). When sending the new ClientHello, the client MUST copy the contents of the extension received in the HelloRetryRequest into a “cookie” extension in the new ClientHello. Clients MUST NOT use cookies in subsequent connections.

4.2.3. Signature Algorithms

The client uses the “signature_algorithms” extension to indicate to the server which signature algorithms may be used in digital signatures. Clients which desire the server to authenticate itself via a certificate MUST send this extension. If a server is authenticating via a certificate and the client has not sent a “signature_algorithms” extension, then the server MUST abort the handshake with a “missing_extension” alert (see Section 9.2).

The “extension_data” field of this extension in a ClientHello contains a SignatureSchemeList value:

   enum {
       /* RSASSA-PKCS1-v1_5 algorithms */
       rsa_pkcs1_sha256(0x0401),
       rsa_pkcs1_sha384(0x0501),
       rsa_pkcs1_sha512(0x0601),

       /* ECDSA algorithms */
       ecdsa_secp256r1_sha256(0x0403),
       ecdsa_secp384r1_sha384(0x0503),
       ecdsa_secp521r1_sha512(0x0603),

       /* RSASSA-PSS algorithms */
       rsa_pss_sha256(0x0804),
       rsa_pss_sha384(0x0805),
       rsa_pss_sha512(0x0806),

       /* EdDSA algorithms */
       ed25519(0x0807),
       ed448(0x0808),

       /* Legacy algorithms */
       rsa_pkcs1_sha1(0x0201),
       ecdsa_sha1(0x0203),

       /* Reserved Code Points */
       private_use(0xFE00..0xFFFF),
       (0xFFFF)
   } SignatureScheme;

   struct {
       SignatureScheme supported_signature_algorithms<2..2^16-2>;
   } SignatureSchemeList;

Note: This enum is named “SignatureScheme” because there is already a “SignatureAlgorithm” type in TLS 1.2, which this replaces. We use the term “signature algorithm” throughout the text.

Each SignatureScheme value lists a single signature algorithm that the client is willing to verify. The values are indicated in descending order of preference. Note that a signature algorithm takes as input an arbitrary-length message, rather than a digest. Algorithms which traditionally act on a digest should be defined in TLS to first hash the input with a specified hash algorithm and then proceed as usual. The code point groups listed above have the following meanings:

RSASSA-PKCS1-v1_5 algorithms
Indicates a signature algorithm using RSASSA-PKCS1-v1_5 [RFC8017] with the corresponding hash algorithm as defined in [SHS]. These values refer solely to signatures which appear in certificates (see Section 4.4.2.2) and are not defined for use in signed TLS handshake messages.
ECDSA algorithms
Indicates a signature algorithm using ECDSA [ECDSA], the corresponding curve as defined in ANSI X9.62 [X962] and FIPS 186-4 [DSS], and the corresponding hash algorithm as defined in [SHS]. The signature is represented as a DER-encoded [X690] ECDSA-Sig-Value structure.
RSASSA-PSS algorithms
Indicates a signature algorithm using RSASSA-PSS [RFC8017] with mask generation function 1. The digest used in the mask generation function and the digest being signed are both the corresponding hash algorithm as defined in [SHS]. When used in signed TLS handshake messages, the length of the salt MUST be equal to the length of the digest output. This codepoint is new in this document and is also defined for use with TLS 1.2.
EdDSA algorithms
Indicates a signature algorithm using EdDSA as defined in [RFC8032] or its successors. Note that these correspond to the “PureEdDSA” algorithms and not the “prehash” variants.
Legacy algorithms
Indicates algorithms which are being deprecated because they use algorithms with known weaknesses, specifically SHA-1 which is used in this context with either with RSA using RSASSA-PKCS1-v1_5 or ECDSA. These values refer solely to signatures which appear in certificates (see Section 4.4.2.2) and are not defined for use in signed TLS handshake messages. Endpoints SHOULD NOT negotiate these algorithms but are permitted to do so solely for backward compatibility. Clients offering these values MUST list them as the lowest priority (listed after all other algorithms in SignatureSchemeList). TLS 1.3 servers MUST NOT offer a SHA-1 signed certificate unless no valid certificate chain can be produced without it (see Section 4.4.2.2).

The signatures on certificates that are self-signed or certificates that are trust anchors are not validated since they begin a certification path (see [RFC5280], Section 3.2). A certificate that begins a certification path MAY use a signature algorithm that is not advertised as being supported in the “signature_algorithms” extension.

Note that TLS 1.2 defines this extension differently. TLS 1.3 implementations willing to negotiate TLS 1.2 MUST behave in accordance with the requirements of [RFC5246] when negotiating that version. In particular:

4.2.4. Certificate Authorities

The “certificate_authorities” extension is used to indicate the certificate authorities which an endpoint supports and which SHOULD be used by the receiving endpoint to guide certificate selection.

The body of the “certificate_authorities” extension consists of a CertificateAuthoritiesExtension structure.

   opaque DistinguishedName<1..2^16-1>;

   struct {
       DistinguishedName authorities<3..2^16-1>;
   } CertificateAuthoritiesExtension;

authorities
A list of the distinguished names [X501] of acceptable certificate authorities, represented in DER-encoded [X690] format. These distinguished names specify a desired distinguished name for trust anchor or subordinate CA; thus, this message can be used to describe known trust anchors as well as a desired authorization space.

The client MAY send the “certificate_authorities” extension in the ClientHello message. The server MAY send it in the CertificateRequest message.

The “trusted_ca_keys” extension, which serves a similar purpose [RFC6066], but is more complicated, is not used in TLS 1.3 (although it may appear in ClientHello messages from clients which are offering prior versions of TLS).

4.2.4.1. OID Filters

The “oid_filters” extension allows servers to provide a set of OID/value pairs which it would like the client’s certificate to match. This extension, if provided by the server, MUST only be sent in the CertificateRequest message.

   struct {
       opaque certificate_extension_oid<1..2^8-1>;
       opaque certificate_extension_values<0..2^16-1>;
   } OIDFilter;

   struct {
       OIDFilter filters<0..2^16-1>;
   } OIDFilterExtension;

filters
A list of certificate extension OIDs [RFC5280] with their allowed values and represented in DER-encoded [X690] format. Some certificate extension OIDs allow multiple values (e.g., Extended Key Usage). If the server has included a non-empty filters list, the client certificate included in the response MUST contain all of the specified extension OIDs that the client recognizes. For each extension OID recognized by the client, all of the specified values MUST be present in the client certificate (but the certificate MAY have other values as well). However, the client MUST ignore and skip any unrecognized certificate extension OIDs. If the client ignored some of the required certificate extension OIDs and supplied a certificate that does not satisfy the request, the server MAY at its discretion either continue the connection without client authentication, or abort the handshake with an “unsupported_certificate” alert.

PKIX RFCs define a variety of certificate extension OIDs and their corresponding value types. Depending on the type, matching certificate extension values are not necessarily bitwise-equal. It is expected that TLS implementations will rely on their PKI libraries to perform certificate selection using certificate extension OIDs.

This document defines matching rules for two standard certificate extensions defined in [RFC5280]:

Separate specifications may define matching rules for other certificate extensions.

4.2.5. Post-Handshake Client Authentication

The “post_handshake_auth” extension is used to indicate that a client is willing to perform post-handshake authentication Section 4.6.2. Servers MUST not send a post-handshake CertificateRequest to clients which do not offer this extension. Servers MUST NOT send this extension.

The “extension_data” field of the “post_handshake_auth” extension is zero length.

4.2.6. Negotiated Groups

When sent by the client, the “supported_groups” extension indicates the named groups which the client supports for key exchange, ordered from most preferred to least preferred.

Note: In versions of TLS prior to TLS 1.3, this extension was named “elliptic_curves” and only contained elliptic curve groups. See [RFC4492] and [RFC7919]. This extension was also used to negotiate ECDSA curves. Signature algorithms are now negotiated independently (see Section 4.2.3).

The “extension_data” field of this extension contains a “NamedGroupList” value:

   enum {
       /* Elliptic Curve Groups (ECDHE) */
       secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
       x25519(0x001D), x448(0x001E),

       /* Finite Field Groups (DHE) */
       ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096 (0x0102),
       ffdhe6144(0x0103), ffdhe8192(0x0104),

       /* Reserved Code Points */
       ffdhe_private_use(0x01FC..0x01FF),
       ecdhe_private_use(0xFE00..0xFEFF),
       (0xFFFF)
   } NamedGroup;

   struct {
       NamedGroup named_group_list<2..2^16-1>;
   } NamedGroupList;

Elliptic Curve Groups (ECDHE)
Indicates support for the corresponding named curve, defined either in FIPS 186-4 [DSS] or in [RFC7748]. Values 0xFE00 through 0xFEFF are reserved for private use.
Finite Field Groups (DHE)
Indicates support of the corresponding finite field group, defined in [RFC7919]. Values 0x01FC through 0x01FF are reserved for private use.

Items in named_group_list are ordered according to the client’s preferences (most preferred choice first).

As of TLS 1.3, servers are permitted to send the “supported_groups” extension to the client. Clients MUST NOT act upon any information found in “supported_groups” prior to successful completion of the handshake but MAY use the information learned from a successfully completed handshake to change what groups they use in their “key_share” extension in subsequent connections. If the server has a group it prefers to the ones in the “key_share” extension but is still willing to accept the ClientHello, it SHOULD send “supported_groups” to update the client’s view of its preferences; this extension SHOULD contain all groups the server supports, regardless of whether they are currently supported by the client.

4.2.7. Key Share

The “key_share” extension contains the endpoint’s cryptographic parameters.

Clients MAY send an empty client_shares vector in order to request group selection from the server at the cost of an additional round trip. (see Section 4.1.4)

   struct {
       NamedGroup group;
       opaque key_exchange<1..2^16-1>;
   } KeyShareEntry;

group
The named group for the key being exchanged. Finite Field Diffie-Hellman [DH] parameters are described in Section 4.2.7.1; Elliptic Curve Diffie-Hellman parameters are described in Section 4.2.7.2.
key_exchange
Key exchange information. The contents of this field are determined by the specified group and its corresponding definition.

The “extension_data” field of this extension contains a “KeyShare” value:

   struct {
       select (Handshake.msg_type) {
           case client_hello:
               KeyShareEntry client_shares<0..2^16-1>;

           case hello_retry_request:
               NamedGroup selected_group;

           case server_hello:
               KeyShareEntry server_share;
       };
   } KeyShare;

client_shares
A list of offered KeyShareEntry values in descending order of client preference. This vector MAY be empty if the client is requesting a HelloRetryRequest. Each KeyShareEntry value MUST correspond to a group offered in the “supported_groups” extension and MUST appear in the same order. However, the values MAY be a non-contiguous subset of the “supported_groups” extension and MAY omit the most preferred groups. Such a situation could arise if the most preferred groups are new and unlikely to be supported in enough places to make pregenerating key shares for them efficient.
selected_group
The mutually supported group the server intends to negotiate and is requesting a retried ClientHello/KeyShare for.
server_share
A single KeyShareEntry value that is in the same group as one of the client’s shares.

Clients can offer an arbitrary number of KeyShareEntry values, each representing a single set of key exchange parameters. For instance, a client might offer shares for several elliptic curves or multiple FFDHE groups. The key_exchange values for each KeyShareEntry MUST be generated independently. Clients MUST NOT offer multiple KeyShareEntry values for the same group. Clients MUST NOT offer any KeyShareEntry values for groups not listed in the client’s “supported_groups” extension. Servers MAY check for violations of these rules and abort the handshake with an “illegal_parameter” alert if one is violated.

Upon receipt of this extension in a HelloRetryRequest, the client MUST verify that (1) the selected_group field corresponds to a group which was provided in the “supported_groups” extension in the original ClientHello; and (2) the selected_group field does not correspond to a group which was provided in the “key_share” extension in the original ClientHello. If either of these checks fails, then the client MUST abort the handshake with an “illegal_parameter” alert. Otherwise, when sending the new ClientHello, the client MUST replace the original “key_share” extension with one containing only a new KeyShareEntry for the group indicated in the selected_group field of the triggering HelloRetryRequest.

If using (EC)DHE key establishment, servers offer exactly one KeyShareEntry in the ServerHello. This value MUST be in the same group as the KeyShareEntry value offered by the client that the server has selected for the negotiated key exchange. Servers MUST NOT send a KeyShareEntry for any group not indicated in the “supported_groups” extension and MUST NOT send a KeyShareEntry when using the “psk_ke” PskKeyExchangeMode. If a HelloRetryRequest was received by the client, the client MUST verify that the selected NamedGroup in the ServerHello is the same as that in the HelloRetryRequest. If this check fails, the client MUST abort the handshake with an “illegal_parameter” alert.

4.2.7.1. Diffie-Hellman Parameters

Diffie-Hellman [DH] parameters for both clients and servers are encoded in the opaque key_exchange field of a KeyShareEntry in a KeyShare structure. The opaque value contains the Diffie-Hellman public value (Y = g^X mod p) for the specified group (see [RFC7919] for group definitions) encoded as a big-endian integer and padded to the left with zeros to the size of p in bytes.

Note: For a given Diffie-Hellman group, the padding results in all public keys having the same length.

Peers MUST validate each other’s public key Y by ensuring that 1 < Y < p-1. This check ensures that the remote peer is properly behaved and isn’t forcing the local system into a small subgroup.

4.2.7.2. ECDHE Parameters

ECDHE parameters for both clients and servers are encoded in the the opaque key_exchange field of a KeyShareEntry in a KeyShare structure.

For secp256r1, secp384r1 and secp521r1, the contents are the serialized value of the following struct:

   struct {
       uint8                legacy_form = 4;
       opaque               X[coordinate_length];
       opaque               Y[coordinate_length];
    } UncompressedPointRepresentation;

X and Y respectively are the binary representations of the X and Y values in network byte order. There are no internal length markers, so each number representation occupies as many octets as implied by the curve parameters. For P-256 this means that each of X and Y use 32 octets, padded on the left by zeros if necessary. For P-384 they take 48 octets each, and for P-521 they take 66 octets each.

For the curves secp256r1, secp384r1 and secp521r1, peers MUST validate each other’s public value Y by ensuring that the point is a valid point on the elliptic curve. The appropriate validation procedures are defined in Section 4.3.7 of [X962] and alternatively in Section 5.6.2.6 of [KEYAGREEMENT]. This process consists of three steps: (1) verify that Y is not the point at infinity (O), (2) verify that for Y = (x, y) both integers are in the correct interval, (3) ensure that (x, y) is a correct solution to the elliptic curve equation. For these curves, implementers do not need to verify membership in the correct subgroup.

For X25519 and X448, the contents of the public value are the byte string inputs and outputs of the corresponding functions defined in [RFC7748], 32 bytes for X25519 and 56 bytes for X448.

Note: Versions of TLS prior to 1.3 permitted point format negotiation; TLS 1.3 removes this feature in favor of a single point format for each curve.

4.2.8. Pre-Shared Key Exchange Modes

In order to use PSKs, clients MUST also send a “psk_key_exchange_modes” extension. The semantics of this extension are that the client only supports the use of PSKs with these modes, which restricts both the use of PSKs offered in this ClientHello and those which the server might supply via NewSessionTicket.

A client MUST provide a “psk_key_exchange_modes” extension if it offers a “pre_shared_key” extension. If clients offer “pre_shared_key” without a “psk_key_exchange_modes” extension, servers MUST abort the handshake. Servers MUST NOT select a key exchange mode that is not listed by the client. This extension also restricts the modes for use with PSK resumption; servers SHOULD NOT send NewSessionTicket with tickets that are not compatible with the advertised modes; however, if a server does so, the impact will just be that the client’s attempts at resumption fail.

The server MUST NOT send a “psk_key_exchange_modes” extension.

   enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;

   struct {
       PskKeyExchangeMode ke_modes<1..255>;
   } PskKeyExchangeModes;

psk_ke
PSK-only key establishment. In this mode, the server MUST NOT supply a “key_share” value.
psk_dhe_ke
PSK with (EC)DHE key establishment. In this mode, the client and servers MUST supply “key_share” values as described in Section 4.2.7.

4.2.9. Early Data Indication

When a PSK is used, the client can send application data in its first flight of messages. If the client opts to do so, it MUST supply both the “early_data” extension as well as the “pre_shared_key” extension.

The “extension_data” field of this extension contains an “EarlyDataIndication” value.

   struct {} Empty;

   struct {
       select (Handshake.msg_type) {
           case new_session_ticket:   uint32 max_early_data_size;
           case client_hello:         Empty;
           case encrypted_extensions: Empty;
       };
   } EarlyDataIndication;

See Section 4.6.1 for the use of the max_early_data_size field.

The parameters for the 0-RTT data (symmetric cipher suite, ALPN protocol, etc.) are the same as those which were negotiated in the connection which established the PSK. The PSK used to encrypt the early data MUST be the first PSK listed in the client’s “pre_shared_key” extension.

For PSKs provisioned via NewSessionTicket, a server MUST validate that the ticket age for the selected PSK identity (computed by subtracting ticket_age_add from PskIdentity.obfuscated_ticket_age modulo 2^32) is within a small tolerance of the time since the ticket was issued (see Section 8). If it is not, the server SHOULD proceed with the handshake but reject 0-RTT, and SHOULD NOT take any other action that assumes that this ClientHello is fresh.

0-RTT messages sent in the first flight have the same (encrypted) content types as their corresponding messages sent in other flights (handshake and application_data) but are protected under different keys. After receiving the server’s Finished message, if the server has accepted early data, an EndOfEarlyData message will be sent to indicate the key change. This message will be encrypted with the 0-RTT traffic keys.

A server which receives an “early_data” extension MUST behave in one of three ways:

In order to accept early data, the server MUST have accepted a PSK cipher suite and selected the first key offered in the client’s “pre_shared_key” extension. In addition, it MUST verify that the following values are consistent with those negotiated in the connection during which the ticket was established.

Future extensions MUST define their interaction with 0-RTT.

If any of these checks fail, the server MUST NOT respond with the extension and must discard all the first flight data using one of the first two mechanisms listed above (thus falling back to 1-RTT or 2-RTT). If the client attempts a 0-RTT handshake but the server rejects it, the server will generally not have the 0-RTT record protection keys and must instead use trial decryption (either with the 1-RTT handshake keys or by looking for a cleartext ClientHello in the case of HelloRetryRequest) to find the first non-0RTT message.

If the server chooses to accept the “early_data” extension, then it MUST comply with the same error handling requirements specified for all records when processing early data records. Specifically, if the server fails to decrypt any 0-RTT record following an accepted “early_data” extension it MUST terminate the connection with a “bad_record_mac” alert as per Section 5.2.

If the server rejects the “early_data” extension, the client application MAY opt to retransmit early data once the handshake has been completed. Note that automatic re-transmission of early data could result in assumptions about the status of the connection being incorrect. For instance, when the negotiated connection selects a different ALPN protocol from what was used for the early data, an application might need to construct different messages. Similarly, if early data assumes anything about the connection state, it might be sent in error after the handshake completes.

A TLS implementation SHOULD NOT automatically re-send early data; applications are in a better position to decide when re-transmission is appropriate. A TLS implementation MUST NOT automatically re-send early data unless the negotiated connection selects the same ALPN protocol.

4.2.10. Pre-Shared Key Extension

The “pre_shared_key” extension is used to indicate the identity of the pre-shared key to be used with a given handshake in association with PSK key establishment.

The “extension_data” field of this extension contains a “PreSharedKeyExtension” value:

   struct {
       opaque identity<1..2^16-1>;
       uint32 obfuscated_ticket_age;
   } PskIdentity;

   opaque PskBinderEntry<32..255>;

   struct {
       select (Handshake.msg_type) {
           case client_hello:
               PskIdentity identities<7..2^16-1>;
               PskBinderEntry binders<33..2^16-1>;

           case server_hello:
               uint16 selected_identity;
       };

   } PreSharedKeyExtension;

identity
A label for a key. For instance, a ticket defined in Appendix B.3.4 or a label for a pre-shared key established externally.
obfuscated_ticket_age
An obfuscated version of the age of the key. Section 4.2.10.1 describes how to form this value for identities established via the NewSessionTicket message. For identities established externally an obfuscated_ticket_age of 0 SHOULD be used, and servers MUST ignore the value.
identities
A list of the identities that the client is willing to negotiate with the server. If sent alongside the “early_data” extension (see Section 4.2.9), the first identity is the one used for 0-RTT data.
binders
A series of HMAC values, one for each PSK offered in the “pre_shared_keys” extension and in the same order, computed as described below.
selected_identity
The server’s chosen identity expressed as a (0-based) index into the identities in the client’s list.

Each PSK is associated with a single Hash algorithm. For PSKs established via the ticket mechanism (Section 4.6.1), this is the KDF Hash algorithm on the connection where the ticket was established. For externally established PSKs, the Hash algorithm MUST be set when the PSK is established, or default to SHA-256 if no such algorithm is defined. The server must ensure that it selects a compatible PSK (if any) and cipher suite.

Implementor’s note: the most straightforward way to implement the PSK/cipher suite matching requirements is to negotiate the cipher suite first and then exclude any incompatible PSKs. Any unknown PSKs (e.g., they are not in the PSK database or are encrypted with an unknown key) SHOULD simply be ignored. If no acceptable PSKs are found, the server SHOULD perform a non-PSK handshake if possible.

Prior to accepting PSK key establishment, the server MUST validate the corresponding binder value (see Section 4.2.10.2 below). If this value is not present or does not validate, the server MUST abort the handshake. Servers SHOULD NOT attempt to validate multiple binders; rather they SHOULD select a single PSK and validate solely the binder that corresponds to that PSK. In order to accept PSK key establishment, the server sends a “pre_shared_key” extension indicating the selected identity.

Clients MUST verify that the server’s selected_identity is within the range supplied by the client, that the server selected a cipher suite indicating a Hash associated with the PSK and that a server “key_share” extension is present if required by the ClientHello “psk_key_exchange_modes”. If these values are not consistent the client MUST abort the handshake with an “illegal_parameter” alert.

If the server supplies an “early_data” extension, the client MUST verify that the server’s selected_identity is 0. If any other value is returned, the client MUST abort the handshake with an “illegal_parameter” alert.

This extension MUST be the last extension in the ClientHello (this facilitates implementation as described below). Servers MUST check that it is the last extension and otherwise fail the handshake with an “illegal_parameter” alert.

4.2.10.1. Ticket Age

The client’s view of the age of a ticket is the time since the receipt of the NewSessionTicket message. Clients MUST NOT attempt to use tickets which have ages greater than the “ticket_lifetime” value which was provided with the ticket. The “obfuscated_ticket_age” field of each PskIdentity contains an obfuscated version of the ticket age formed by taking the age in milliseconds and adding the “ticket_age_add” value that was included with the ticket, see Section 4.6.1 modulo 2^32. This addition prevents passive observers from correlating connections unless tickets are reused. Note that the “ticket_lifetime” field in the NewSessionTicket message is in seconds but the “obfuscated_ticket_age” is in milliseconds. Because ticket lifetimes are restricted to a week, 32 bits is enough to represent any plausible age, even in milliseconds.

4.2.10.2. PSK Binder

The PSK binder value forms a binding between a PSK and the current handshake, as well as between the handshake in which the PSK was generated (if via a NewSessionTicket message) and the handshake where it was used. Each entry in the binders list is computed as an HMAC over a transcript hash (see Section 4.4.1) containing a partial ClientHello up to and including the PreSharedKeyExtension.identities field. That is, it includes all of the ClientHello but not the binders list itself. The length fields for the message (including the overall length, the length of the extensions block, and the length of the “pre_shared_key” extension) are all set as if binders of the correct lengths were present.

The PskBinderEntry is computed in the same way as the Finished message (Section 4.4.4) but with the BaseKey being the binder_key derived via the key schedule from the corresponding PSK which is being offered (see Section 7.1).

If the handshake includes a HelloRetryRequest, the initial ClientHello and HelloRetryRequest are included in the transcript along with the new ClientHello. For instance, if the client sends ClientHello1, its binder will be computed over:

   Transcript-Hash(ClientHello1[truncated])

If the server responds with HelloRetryRequest, and the client then sends ClientHello2, its binder will be computed over:

   Transcript-Hash(ClientHello1,
                   HelloRetryRequest,
                   ClientHello2[truncated])

The full ClientHello1 is included in all other handshake hash computations. Note that in the first flight, ClientHello1[truncated] is hashed directly, but in the second flight, ClientHello1 is hashed and then reinjected as a “handshake_hash” message, as described in Section 4.4.1.

4.2.10.3. Processing Order

Clients are permitted to “stream” 0-RTT data until they receive the server’s Finished, only then sending the EndOfEarlyData message, followed by the rest of the handshake. In order to avoid deadlocks, when accepting “early_data”, servers MUST process the client’s ClientHello and then immediately send the ServerHello, rather than waiting for the client’s EndOfEarlyData message.

4.3. Server Parameters

The next two messages from the server, EncryptedExtensions and CertificateRequest, contain information from the server that determines the rest of the handshake. These messages are encrypted with keys derived from the server_handshake_traffic_secret.

4.3.1. Encrypted Extensions

In all handshakes, the server MUST send the EncryptedExtensions message immediately after the ServerHello message. This is the first message that is encrypted under keys derived from the server_handshake_traffic_secret.

The EncryptedExtensions message contains extensions that can be protected, i.e., any which are not needed to establish the cryptographic context, but which are not associated with individual certificates. The client MUST check EncryptedExtensions for the presence of any forbidden extensions and if any are found MUST abort the handshake with an “illegal_parameter” alert.

Structure of this message:

   struct {
       Extension extensions<0..2^16-1>;
   } EncryptedExtensions;

extensions
A list of extensions. For more information, see the table in Section 4.2.

4.3.2. Certificate Request

A server which is authenticating with a certificate MAY optionally request a certificate from the client. This message, if sent, MUST follow EncryptedExtensions.

Structure of this message:

   struct {
       opaque certificate_request_context<0..2^8-1>;
       Extension extensions<2..2^16-1>;
   } CertificateRequest;

certificate_request_context
An opaque string which identifies the certificate request and which will be echoed in the client’s Certificate message. The certificate_request_context MUST be unique within the scope of this connection (thus preventing replay of client CertificateVerify messages). This field SHALL be zero length unless used for the post-handshake authentication exchanges described in Section 4.6.2. When requesting post-handshake authentication, the server SHOULD make the context unpredictable to the client (e.g., by randomly generating it) in order to prevent an attacker who has temporary access to the client’s private key from pre-computing valid CertificateVerify messages.
extensions
A set of extensions describing the parameters of the certificate being requested. The “signature_algorithms” extension MUST be specified, and other extensions may optionally be included if defined for this message. Clients MUST ignore unrecognized extensions.

In prior versions of TLS, the CertificateRequest message carried a list of signature algorithms and certificate authorities which the server would accept. In TLS 1.3 the former is expressed by sending the “signature_algorithms” extension. The latter is expressed by sending the “certificate_authorities” extension (see Section 4.2.4).

Servers which are authenticating with a PSK MUST NOT send the CertificateRequest message in the main handshake, though they MAY send it in post-handshake authentication (see Section 4.6.2) provided that the client has sent the “post_handshake_auth” extension (see Section 4.2.5).

4.4. Authentication Messages

As discussed in Section 2, TLS generally uses a common set of messages for authentication, key confirmation, and handshake integrity: Certificate, CertificateVerify, and Finished. (The PreSharedKey binders also perform key confirmation, in a similar fashion.) These three messages are always sent as the last messages in their handshake flight. The Certificate and CertificateVerify messages are only sent under certain circumstances, as defined below. The Finished message is always sent as part of the Authentication block. These messages are encrypted under keys derived from [sender]_handshake_traffic_secret.

The computations for the Authentication messages all uniformly take the following inputs:

Based on these inputs, the messages then contain:

Certificate
The certificate to be used for authentication, and any supporting certificates in the chain. Note that certificate-based client authentication is not available in 0-RTT mode.
CertificateVerify
A signature over the value Transcript-Hash(Handshake Context, Certificate)
Finished
A MAC over the value Transcript-Hash(Handshake Context, Certificate, CertificateVerify) using a MAC key derived from the base key.

The following table defines the Handshake Context and MAC Base Key for each scenario:

Mode Handshake Context Base Key
Server ClientHello … later of EncryptedExtensions/CertificateRequest server_handshake_traffic_secret
Client ClientHello … later of server Finished/EndOfEarlyData client_handshake_traffic_secret
Post-Handshake ClientHello … client Finished + CertificateRequest client_application_traffic_secret_N

4.4.1. The Transcript Hash

Many of the cryptographic computations in TLS make use of a transcript hash. This value is computed by hashing the concatenation of each included handshake message, including the handshake message header carrying the handshake message type and length fields, but not including record layer headers. I.e.,

 Transcript-Hash(M1, M2, ... MN) = Hash(M1 || M2 ... MN)

As an exception to this general rule, when the server responds to a ClientHello with a HelloRetryRequest, the value of ClientHello1 is replaced with a special synthetic handshake message of handshake type “message_hash” containing Hash(ClientHello1). I.e.,

 Transcript-Hash(ClientHello1, HelloRetryRequest, ... MN) =
     Hash(message_hash ||        // Handshake type
          00 00 Hash.length ||   // Handshake message length
          Hash(ClientHello1) ||  // Hash of ClientHello1
          HelloRetryRequest ... MN)

The reason for this construction is to allow the server to do a stateless HelloRetryRequest by storing just the hash of ClientHello1 in the cookie, rather than requiring it to export the entire intermediate hash state (see Section 4.2.2).

For concreteness, the transcript hash is always taken from the following sequence of handshake messages, starting at the first ClientHello and including only those messages that were sent: ClientHello, HelloRetryRequest, ClientHello, ServerHello, EncryptedExtensions, server CertificateRequest, server Certificate, server CertificateVerify, server Finished, EndOfEarlyData, client Certificate, client CertificateVerify, client Finished.

In general, implementations can implement the transcript by keeping a running transcript hash value based on the negotiated hash. Note, however, that subsequent post-handshake authentications do not include each other, just the messages through the end of the main handshake.

4.4.2. Certificate

This message conveys the endpoint’s certificate chain to the peer.

The server MUST send a Certificate message whenever the agreed-upon key exchange method uses certificates for authentication (this includes all key exchange methods defined in this document except PSK).

The client MUST send a Certificate message if and only if the server has requested client authentication via a CertificateRequest message (Section 4.3.2). If the server requests client authentication but no suitable certificate is available, the client MUST send a Certificate message containing no certificates (i.e., with the “certificate_list” field having length 0).

Structure of this message:

   struct {
       select(certificate_type){
           case RawPublicKey:
             // From RFC 7250 ASN.1_subjectPublicKeyInfo
             opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;

           case X509:
             opaque cert_data<1..2^24-1>;
       };
       Extension extensions<0..2^16-1>;
   } CertificateEntry;

   struct {
       opaque certificate_request_context<0..2^8-1>;
       CertificateEntry certificate_list<0..2^24-1>;
   } Certificate;

certificate_request_context
If this message is in response to a CertificateRequest, the value of certificate_request_context in that message. Otherwise (in the case of server authentication), this field SHALL be zero length.
certificate_list
This is a sequence (chain) of CertificateEntry structures, each containing a single certificate and set of extensions.
extensions:
A set of extension values for the CertificateEntry. The “Extension” format is defined in Section 4.2. Valid extensions include OCSP Status extensions ([RFC6066] and [RFC6961]) and SignedCertificateTimestamps ([RFC6962]). An extension MUST only be present in a Certificate message if the corresponding ClientHello extension was presented in the initial handshake. If an extension applies to the entire chain, it SHOULD be included in the first CertificateEntry.

If the corresponding certificate type extension (“server_certificate_type” or “client_certificate_type”) was not used or the X.509 certificate type was negotiated, then each CertificateEntry contains an X.509 certificate. The sender’s certificate MUST come in the first CertificateEntry in the list. Each following certificate SHOULD directly certify one preceding it. Because certificate validation requires that trust anchors be distributed independently, a certificate that specifies a trust anchor MAY be omitted from the chain, provided that supported peers are known to possess any omitted certificates.

Note: Prior to TLS 1.3, “certificate_list” ordering required each certificate to certify the one immediately preceding it; however, some implementations allowed some flexibility. Servers sometimes send both a current and deprecated intermediate for transitional purposes, and others are simply configured incorrectly, but these cases can nonetheless be validated properly. For maximum compatibility, all implementations SHOULD be prepared to handle potentially extraneous certificates and arbitrary orderings from any TLS version, with the exception of the end-entity certificate which MUST be first.

If the RawPublicKey certificate type was negotiated, then the certificate_list MUST contain no more than one CertificateEntry, which contains an ASN1_subjectPublicKeyInfo value as defined in [RFC7250], Section 3.

The OpenPGP certificate type [RFC6091] MUST NOT be used with TLS 1.3.

The server’s certificate_list MUST always be non-empty. A client will send an empty certificate_list if it does not have an appropriate certificate to send in response to the server’s authentication request.

4.4.2.1. OCSP Status and SCT Extensions

[RFC6066] and [RFC6961] provide extensions to negotiate the server sending OCSP responses to the client. In TLS 1.2 and below, the server replies with an empty extension to indicate negotiation of this extension and the OCSP information is carried in a CertificateStatus message. In TLS 1.3, the server’s OCSP information is carried in an extension in the CertificateEntry containing the associated certificate. Specifically: The body of the “status_request” extension from the server MUST be a CertificateStatus structure as defined in [RFC6066], which is interpreted as defined in [RFC6960].

A server MAY request that a client present an OCSP response with its certificate by sending an empty “status_request” extension in its CertificateRequest message. If the client opts to send an OCSP response, the body of its “status_request” extension MUST be a CertificateStatus structure as defined in [RFC6066].

Similarly, [RFC6962] provides a mechanism for a server to send a Signed Certificate Timestamp (SCT) as an extension in the ServerHello in TLS 1.2 and below. In TLS 1.3, the server’s SCT information is carried in an extension in CertificateEntry.

4.4.2.2. Server Certificate Selection

The following rules apply to the certificates sent by the server:

All certificates provided by the server MUST be signed by a signature algorithm that appears in the “signature_algorithms” extension provided by the client, if they are able to provide such a chain (see Section 4.2.3). Certificates that are self-signed or certificates that are expected to be trust anchors are not validated as part of the chain and therefore MAY be signed with any algorithm.

If the server cannot produce a certificate chain that is signed only via the indicated supported algorithms, then it SHOULD continue the handshake by sending the client a certificate chain of its choice that may include algorithms that are not known to be supported by the client. This fallback chain SHOULD NOT use the deprecated SHA-1 hash algorithm in general, but MAY do so if the “signature_algorithms” extension provided by the client permits it, and MUST NOT do so otherwise.

If the client cannot construct an acceptable chain using the provided certificates and decides to abort the handshake, then it MUST abort the handshake with an appropriate certificate-related alert (by default, “unsupported_certificate”; see Section 6.2 for more).

If the server has multiple certificates, it chooses one of them based on the above-mentioned criteria (in addition to other criteria, such as transport layer endpoint, local configuration and preferences).

4.4.2.3. Client Certificate Selection

The following rules apply to certificates sent by the client:

Note that, as with the server certificate, there are certificates that use algorithm combinations that cannot be currently used with TLS.

4.4.2.4. Receiving a Certificate Message

In general, detailed certificate validation procedures are out of scope for TLS (see [RFC5280]). This section provides TLS-specific requirements.

If the server supplies an empty Certificate message, the client MUST abort the handshake with a “decode_error” alert.

If the client does not send any certificates, the server MAY at its discretion either continue the handshake without client authentication, or abort the handshake with a “certificate_required” alert. Also, if some aspect of the certificate chain was unacceptable (e.g., it was not signed by a known, trusted CA), the server MAY at its discretion either continue the handshake (considering the client unauthenticated) or abort the handshake.

Any endpoint receiving any certificate which it would need to validate using any signature algorithm using an MD5 hash MUST abort the handshake with a “bad_certificate” alert. SHA-1 is deprecated and it is RECOMMENDED that any endpoint receiving any certificate which it would need to validate using any signature algorithm using a SHA-1 hash abort the handshake with a “bad_certificate” alert. For clarity, this means that endpoints MAY accept these algorithms for certificates that are self-signed or are trust anchors.

All endpoints are RECOMMENDED to transition to SHA-256 or better as soon as possible to maintain interoperability with implementations currently in the process of phasing out SHA-1 support.

Note that a certificate containing a key for one signature algorithm MAY be signed using a different signature algorithm (for instance, an RSA key signed with an ECDSA key).

4.4.3. Certificate Verify

This message is used to provide explicit proof that an endpoint possesses the private key corresponding to its certificate. The CertificateVerify message also provides integrity for the handshake up to this point. Servers MUST send this message when authenticating via a certificate. Clients MUST send this message whenever authenticating via a certificate (i.e., when the Certificate message is non-empty). When sent, this message MUST appear immediately after the Certificate message and immediately prior to the Finished message.

Structure of this message:

   struct {
       SignatureScheme algorithm;
       opaque signature<0..2^16-1>;
   } CertificateVerify;

The algorithm field specifies the signature algorithm used (see Section 4.2.3 for the definition of this field). The signature is a digital signature using that algorithm. The content that is covered under the signature is the hash output as described in Section 4.4, namely:

   Transcript-Hash(Handshake Context, Certificate)

The digital signature is then computed over the concatenation of:

This structure is intended to prevent an attack on previous versions of TLS in which the ServerKeyExchange format meant that attackers could obtain a signature of a message with a chosen 32-byte prefix (ClientHello.random). The initial 64-byte pad clears that prefix along with the server-controlled ServerHello.random.

The context string for a server signature is “TLS 1.3, server CertificateVerify” and for a client signature is “TLS 1.3, client CertificateVerify”. It is used to provide separation between signatures made in different contexts, helping against potential cross-protocol attacks.

For example, if the transcript hash was 32 bytes of 01 (this length would make sense for SHA-256), the content covered by the digital signature for a server CertificateVerify would be:

   2020202020202020202020202020202020202020202020202020202020202020
   2020202020202020202020202020202020202020202020202020202020202020
   544c5320312e332c207365727665722043657274696669636174655665726966
   79
   00
   0101010101010101010101010101010101010101010101010101010101010101

On the sender side the process for computing the signature field of the CertificateVerify message takes as input:

If the CertificateVerify message is sent by a server, the signature algorithm MUST be one offered in the client’s “signature_algorithms” extension unless no valid certificate chain can be produced without unsupported algorithms (see Section 4.2.3).

If sent by a client, the signature algorithm used in the signature MUST be one of those present in the supported_signature_algorithms field of the “signature_algorithms” extension in the CertificateRequest message.

In addition, the signature algorithm MUST be compatible with the key in the sender’s end-entity certificate. RSA signatures MUST use an RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS1-v1_5 algorithms appear in “signature_algorithms”. The SHA-1 algorithm MUST NOT be used in any signatures of CertificateVerify messages. All SHA-1 signature algorithms in this specification are defined solely for use in legacy certificates and are not valid for CertificateVerify signatures.

The receiver of a CertificateVerify message MUST verify the signature field. The verification process takes as input:

If the verification fails, the receiver MUST terminate the handshake with a “decrypt_error” alert.

4.4.4. Finished

The Finished message is the final message in the authentication block. It is essential for providing authentication of the handshake and of the computed keys.

Recipients of Finished messages MUST verify that the contents are correct and if incorrect MUST terminate the connection with a “decrypt_error” alert.

Once a side has sent its Finished message and received and validated the Finished message from its peer, it may begin to send and receive application data over the connection. There are two settings in which it is permitted to send data prior to receiving the peer’s Finished:

  1. Clients ending 0-RTT data as described in Section 4.2.9.
  2. Servers MAY send data after sending their first flight, but because the handshake is not yet complete, they have no assurance of either the peer’s identity or of its liveness (i.e., the ClientHello might have been replayed).

The key used to compute the finished message is computed from the Base key defined in Section 4.4 using HKDF (see Section 7.1). Specifically:

finished_key =
    HKDF-Expand-Label(BaseKey, "finished", "", Hash.length)

Structure of this message:

   struct {
       opaque verify_data[Hash.length];
   } Finished;

The verify_data value is computed as follows:

   verify_data =
       HMAC(finished_key,
            Transcript-Hash(Handshake Context,
                            Certificate*, CertificateVerify*))

   * Only included if present.

HMAC [RFC2104] uses the Hash algorithm for the handshake. As noted above, the HMAC input can generally be implemented by a running hash, i.e., just the handshake hash at this point.

In previous versions of TLS, the verify_data was always 12 octets long. In TLS 1.3, it is the size of the HMAC output for the Hash used for the handshake.

Note: Alerts and any other record types are not handshake messages and are not included in the hash computations.

Any records following a 1-RTT Finished message MUST be encrypted under the appropriate application traffic key as described in Section 7.2. In particular, this includes any alerts sent by the server in response to client Certificate and CertificateVerify messages.

4.5. End of Early Data

   struct {} EndOfEarlyData;

If the server sent an “early_data” extension, the client MUST send an EndOfEarlyData message after receiving the server Finished. If the server does not send an “early_data” extension, then the client MUST NOT send an EndOfEarlyData message. This message indicates that all 0-RTT application_data messages, if any, have been transmitted and that the following records are protected under handshake traffic keys. Servers MUST NOT send this message and clients receiving it MUST terminate the connection with an “unexpected_message” alert. This message is encrypted under keys derived from the client_early_traffic_secret.

4.6. Post-Handshake Messages

TLS also allows other messages to be sent after the main handshake. These messages use a handshake content type and are encrypted under the appropriate application traffic key.

4.6.1. New Session Ticket Message

At any time after the server has received the client Finished message, it MAY send a NewSessionTicket message. This message creates a unique association between the ticket value and a secret PSK derived from the resumption master secret.

The client MAY use this PSK for future handshakes by including the ticket value in the “pre_shared_key” extension in its ClientHello (Section 4.2.10). Servers MAY send multiple tickets on a single connection, either immediately after each other or after specific events. For instance, the server might send a new ticket after post-handshake authentication in order to encapsulate the additional client authentication state. Clients SHOULD attempt to use each ticket no more than once, with more recent tickets being used first.

Any ticket MUST only be resumed with a cipher suite that has the same KDF hash algorithm as that used to establish the original connection, and only if the client provides the same SNI value as in the original connection, as described in Section 3 of [RFC6066].

Note: Although the resumption master secret depends on the client’s second flight, servers which do not request client authentication MAY compute the remainder of the transcript independently and then send a NewSessionTicket immediately upon sending its Finished rather than waiting for the client Finished. This might be appropriate in cases where the client is expected to open multiple TLS connections in parallel and would benefit from the reduced overhead of a resumption handshake, for example.

   struct {
       uint32 ticket_lifetime;
       uint32 ticket_age_add;
       opaque ticket_nonce<1..255>;
       opaque ticket<1..2^16-1>;
       Extension extensions<0..2^16-2>;
   } NewSessionTicket;

ticket_lifetime
Indicates the lifetime in seconds as a 32-bit unsigned integer in network byte order from the time of ticket issuance. Servers MUST NOT use any value greater than 604800 seconds (7 days). The value of zero indicates that the ticket should be discarded immediately. Clients MUST NOT cache tickets for longer than 7 days, regardless of the ticket_lifetime, and MAY delete the ticket earlier based on local policy. A server MAY treat a ticket as valid for a shorter period of time than what is stated in the ticket_lifetime.
ticket_age_add
A securely generated, random 32-bit value that is used to obscure the age of the ticket that the client includes in the “pre_shared_key” extension. The client-side ticket age is added to this value modulo 2^32 to obtain the value that is transmitted by the client. The server MUST generate a fresh value for each ticket it sends.
ticket_nonce
A unique per-ticket value.
ticket
The value of the ticket to be used as the PSK identity. The ticket itself is an opaque label. It MAY either be a database lookup key or a self-encrypted and self-authenticated value. Section 4 of [RFC5077] describes a recommended ticket construction mechanism.
extensions
A set of extension values for the ticket. The “Extension” format is defined in Section 4.2. Clients MUST ignore unrecognized extensions.

The sole extension currently defined for NewSessionTicket is “early_data”, indicating that the ticket may be used to send 0-RTT data (Section 4.2.9)). It contains the following value:

max_early_data_size
The maximum amount of 0-RTT data that the client is allowed to send when using this ticket, in bytes. Only Application Data payload (i.e., plaintext but not padding or the inner content type byte) is counted. A server receiving more than max_early_data_size bytes of 0-RTT data SHOULD terminate the connection with an “unexpected_message” alert. Note that servers that reject early data due to lack of cryptographic material will be unable to differentiate padding from content, so clients SHOULD NOT depend on being able to send large quantities of padding in early data records.

The PSK associated with the ticket is computed as:

    HKDF-Expand-Label(resumption_master_secret,
                     "resumption", ticket_nonce, Hash.length)

Because the ticket_nonce value is distinct for each NewSessionTicket message, a different PSK will be derived for each ticket.

Note that in principle it is possible to continue issuing new tickets which indefinitely extend the lifetime of the keying material originally derived from an initial non-PSK handshake (which was most likely tied to the peer’s certificate). It is RECOMMENDED that implementations place limits on the total lifetime of such keying material; these limits should take into account the lifetime of the peer’s certificate, the likelihood of intervening revocation, and the time since the peer’s online CertificateVerify signature.

4.6.2. Post-Handshake Authentication

When the client has sent the “post_handshake_auth” extension (see Section 4.2.5), a server MAY request client authentication at any time after the handshake has completed by sending a CertificateRequest message. The client MUST respond with the appropriate Authentication messages (see Section 4.4). If the client chooses to authenticate, it MUST send Certificate, CertificateVerify, and Finished. If it declines, it MUST send a Certificate message containing no certificates followed by Finished. All of the client’s messages for a given response MUST appear consecutively on the wire with no intervening messages of other types.

A client that receives a CertificateRequest message without having sent the “post_handshake_auth” extension MUST send an “unexpected_message” fatal alert.

Note: Because client authentication could involve prompting the user, servers MUST be prepared for some delay, including receiving an arbitrary number of other messages between sending the CertificateRequest and receiving a response. In addition, clients which receive multiple CertificateRequests in close succession MAY respond to them in a different order than they were received (the certificate_request_context value allows the server to disambiguate the responses).

4.6.3. Key and IV Update

   enum {
       update_not_requested(0), update_requested(1), (255)
   } KeyUpdateRequest;

   struct {
       KeyUpdateRequest request_update;
   } KeyUpdate;

request_update
Indicates whether the recipient of the KeyUpdate should respond with its own KeyUpdate. If an implementation receives any other value, it MUST terminate the connection with an “illegal_parameter” alert.

The KeyUpdate handshake message is used to indicate that the sender is updating its sending cryptographic keys. This message can be sent by either peer after it has sent a Finished message. Implementations that receive a KeyUpdate message prior to receiving a Finished message MUST terminate the connection with an “unexpected_message” alert. After sending a KeyUpdate message, the sender SHALL send all its traffic using the next generation of keys, computed as described in Section 7.2. Upon receiving a KeyUpdate, the receiver MUST update its receiving keys.

If the request_update field is set to “update_requested” then the receiver MUST send a KeyUpdate of its own with request_update set to “update_not_requested” prior to sending its next application data record. This mechanism allows either side to force an update to the entire connection, but causes an implementation which receives multiple KeyUpdates while it is silent to respond with a single update. Note that implementations may receive an arbitrary number of messages between sending a KeyUpdate with request_update set to update_requested and receiving the peer’s KeyUpdate, because those messages may already be in flight. However, because send and receive keys are derived from independent traffic secrets, retaining the receive traffic secret does not threaten the forward secrecy of data sent before the sender changed keys.

If implementations independently send their own KeyUpdates with request_update set to “update_requested”, and they cross in flight, then each side will also send a response, with the result that each side increments by two generations.

Both sender and receiver MUST encrypt their KeyUpdate messages with the old keys. Additionally, both sides MUST enforce that a KeyUpdate with the old key is received before accepting any messages encrypted with the new key. Failure to do so may allow message truncation attacks.

5. Record Protocol

The TLS record protocol takes messages to be transmitted, fragments the data into manageable blocks, protects the records, and transmits the result. Received data is verified, decrypted, reassembled, and then delivered to higher-level clients.

TLS records are typed, which allows multiple higher-level protocols to be multiplexed over the same record layer. This document specifies three content types: handshake, application data, and alert. Implementations MUST NOT send record types not defined in this document unless negotiated by some extension. If a TLS implementation receives an unexpected record type, it MUST terminate the connection with an “unexpected_message” alert. New record content type values are assigned by IANA in the TLS Content Type Registry as described in Section 11.

5.1. Record Layer

The record layer fragments information blocks into TLSPlaintext records carrying data in chunks of 2^14 bytes or less. Message boundaries are handled differently depending on the underlying ContentType. Any future content types MUST specify appropriate rules. Note that these rules are stricter than what was enforced in TLS 1.2.

Handshake messages MAY be coalesced into a single TLSPlaintext record or fragmented across several records, provided that:

Implementations MUST NOT send zero-length fragments of Handshake types, even if those fragments contain padding.

Alert messages (Section 6) MUST NOT be fragmented across records and multiple Alert messages MUST NOT be coalesced into a single TLSPlaintext record. In other words, a record with an Alert type MUST contain exactly one message.

Application Data messages contain data that is opaque to TLS. Application Data messages are always protected. Zero-length fragments of Application Data MAY be sent as they are potentially useful as a traffic analysis countermeasure.

   enum {
       invalid(0),
       alert(21),
       handshake(22),
       application_data(23),
       (255)
   } ContentType;

   struct {
       ContentType type;
       ProtocolVersion legacy_record_version;
       uint16 length;
       opaque fragment[TLSPlaintext.length];
   } TLSPlaintext;

type
The higher-level protocol used to process the enclosed fragment.
legacy_record_version
This value MUST be set to 0x0301 for all records generated by a TLS 1.3 implementation. This field is deprecated and MUST be ignored for all purposes. Previous versions of TLS would use other values in this field under some circumstances.
length
The length (in bytes) of the following TLSPlaintext.fragment. The length MUST NOT exceed 2^14 bytes. An endpoint that receives a record that exceeds this length MUST terminate the connection with a “record_overflow” alert.
fragment
The data being transmitted. This value is transparent and is treated as an independent block to be dealt with by the higher-level protocol specified by the type field.

This document describes TLS 1.3, which uses the version 0x0304. This version value is historical, deriving from the use of 0x0301 for TLS 1.0 and 0x0300 for SSL 3.0. In order to maximize backwards compatibility, the record layer version identifies as simply TLS 1.0. Endpoints supporting multiple versions negotiate the version to use by following the procedure and requirements in Appendix D.

When record protection has not yet been engaged, TLSPlaintext structures are written directly onto the wire. Once record protection has started, TLSPlaintext records are protected and sent as described in the following section.

5.2. Record Payload Protection

The record protection functions translate a TLSPlaintext structure into a TLSCiphertext. The deprotection functions reverse the process. In TLS 1.3, as opposed to previous versions of TLS, all ciphers are modeled as “Authenticated Encryption with Additional Data” (AEAD) [RFC5116]. AEAD functions provide an unified encryption and authentication operation which turns plaintext into authenticated ciphertext and back again. Each encrypted record consists of a plaintext header followed by an encrypted body, which itself contains a type and optional padding.

   struct {
       opaque content[TLSPlaintext.length];
       ContentType type;
       uint8 zeros[length_of_padding];
   } TLSInnerPlaintext;

   struct {
       ContentType opaque_type = 23; /* application_data */
       ProtocolVersion legacy_record_version = 0x0301; /* TLS v1.x */
       uint16 length;
       opaque encrypted_record[length];
   } TLSCiphertext;

content
The byte encoding of a handshake or an alert message, or the raw bytes of the application’s data to send.
type
The content type of the record.
zeros
An arbitrary-length run of zero-valued bytes may appear in the cleartext after the type field. This provides an opportunity for senders to pad any TLS record by a chosen amount as long as the total stays within record size limits. See Section 5.4 for more details.
opaque_type
The outer opaque_type field of a TLSCiphertext record is always set to the value 23 (application_data) for outward compatibility with middleboxes accustomed to parsing previous versions of TLS. The actual content type of the record is found in TLSInnerPlaintext.type after decryption.
legacy_record_version
The legacy_record_version field is always 0x0301. TLS 1.3 TLSCiphertexts are not generated until after TLS 1.3 has been negotiated, so there are no historical compatibility concerns where other values might be received. Implementations MAY verify that the legacy_record_version field is 0x0301 and abort the connection if it is not. Note that the handshake protocol including the ClientHello and ServerHello messages authenticates the protocol version, so this value is redundant.
length
The length (in bytes) of the following TLSCiphertext.encrypted_record, which is the sum of the lengths of the content and the padding, plus one for the inner content type, plus any expansion added by the AEAD algorithm. The length MUST NOT exceed 2^14 + 256 bytes. An endpoint that receives a record that exceeds this length MUST terminate the connection with a “record_overflow” alert.
encrypted_record
The AEAD-encrypted form of the serialized TLSInnerPlaintext structure.

AEAD algorithms take as input a single key, a nonce, a plaintext, and “additional data” to be included in the authentication check, as described in Section 2.1 of [RFC5116]. The key is either the client_write_key or the server_write_key, the nonce is derived from the sequence number (see Section 5.3) and the client_write_iv or server_write_iv, and the additional data input is empty (zero length). Derivation of traffic keys is defined in Section 7.3.

The plaintext input to the AEAD algorithm is the encoded TLSInnerPlaintext structure.

The AEAD output consists of the ciphertext output from the AEAD encryption operation. The length of the plaintext is greater than the corresponding TLSPlaintext.length due to the inclusion of TLSInnerPlaintext.type and any padding supplied by the sender. The length of the AEAD output will generally be larger than the plaintext, but by an amount that varies with the AEAD algorithm. Since the ciphers might incorporate padding, the amount of overhead could vary with different lengths of plaintext. Symbolically,

   AEADEncrypted =
       AEAD-Encrypt(write_key, nonce, plaintext)

In order to decrypt and verify, the cipher takes as input the key, nonce, and the AEADEncrypted value. The output is either the plaintext or an error indicating that the decryption failed. There is no separate integrity check. That is:

   plaintext of encrypted_record =
       AEAD-Decrypt(peer_write_key, nonce, AEADEncrypted)

If the decryption fails, the receiver MUST terminate the connection with a “bad_record_mac” alert.

An AEAD algorithm used in TLS 1.3 MUST NOT produce an expansion greater than 255 octets. An endpoint that receives a record from its peer with TLSCiphertext.length larger than 2^14 + 256 octets MUST terminate the connection with a “record_overflow” alert. This limit is derived from the maximum TLSPlaintext length of 2^14 octets + 1 octet for ContentType + the maximum AEAD expansion of 255 octets.

5.3. Per-Record Nonce

A 64-bit sequence number is maintained separately for reading and writing records. Each sequence number is set to zero at the beginning of a connection and whenever the key is changed.

The appropriate sequence number is incremented by one after reading or writing each record. The first record transmitted under a particular traffic key MUST use sequence number 0.

Because the size of sequence numbers is 64-bit, they should not wrap. If a TLS implementation would need to wrap a sequence number, it MUST either re-key (Section 4.6.3) or terminate the connection.

Each AEAD algorithm will specify a range of possible lengths for the per-record nonce, from N_MIN bytes to N_MAX bytes of input ([RFC5116]). The length of the TLS per-record nonce (iv_length) is set to the larger of 8 bytes and N_MIN for the AEAD algorithm (see [RFC5116] Section 4). An AEAD algorithm where N_MAX is less than 8 bytes MUST NOT be used with TLS. The per-record nonce for the AEAD construction is formed as follows:

  1. The 64-bit record sequence number is encoded in network byte order and padded to the left with zeros to iv_length.
  2. The padded sequence number is XORed with the static client_write_iv or server_write_iv, depending on the role.

The resulting quantity (of length iv_length) is used as the per-record nonce.

Note: This is a different construction from that in TLS 1.2, which specified a partially explicit nonce.

5.4. Record Padding

All encrypted TLS records can be padded to inflate the size of the TLSCiphertext. This allows the sender to hide the size of the traffic from an observer.

When generating a TLSCiphertext record, implementations MAY choose to pad. An unpadded record is just a record with a padding length of zero. Padding is a string of zero-valued bytes appended to the ContentType field before encryption. Implementations MUST set the padding octets to all zeros before encrypting.

Application Data records may contain a zero-length TLSInnerPlaintext.content if the sender desires. This permits generation of plausibly-sized cover traffic in contexts where the presence or absence of activity may be sensitive. Implementations MUST NOT send Handshake or Alert records that have a zero-length TLSInnerPlaintext.content; if such a message is received, the receiving implementation MUST terminate the connection with an “unexpected_message” alert.

The padding sent is automatically verified by the record protection mechanism; upon successful decryption of a TLSCiphertext.encrypted_record, the receiving implementation scans the field from the end toward the beginning until it finds a non-zero octet. This non-zero octet is the content type of the message. This padding scheme was selected because it allows padding of any encrypted TLS record by an arbitrary size (from zero up to TLS record size limits) without introducing new content types. The design also enforces all-zero padding octets, which allows for quick detection of padding errors.

Implementations MUST limit their scanning to the cleartext returned from the AEAD decryption. If a receiving implementation does not find a non-zero octet in the cleartext, it MUST terminate the connection with an “unexpected_message” alert.

The presence of padding does not change the overall record size limitations - the full encoded TLSInnerPlaintext MUST not exceed 2^14 octets. If the maximum fragment length is reduced, as for example by the max_fragment_length extension from [RFC6066], then the reduced limit applies to the full plaintext, including the padding.

Selecting a padding policy that suggests when and how much to pad is a complex topic and is beyond the scope of this specification. If the application layer protocol on top of TLS has its own padding, it may be preferable to pad application_data TLS records within the application layer. Padding for encrypted handshake and alert TLS records must still be handled at the TLS layer, though. Later documents may define padding selection algorithms or define a padding policy request mechanism through TLS extensions or some other means.

5.5. Limits on Key Usage

There are cryptographic limits on the amount of plaintext which can be safely encrypted under a given set of keys. [AEAD-LIMITS] provides an analysis of these limits under the assumption that the underlying primitive (AES or ChaCha20) has no weaknesses. Implementations SHOULD do a key update as described in Section 4.6.3 prior to reaching these limits.

For AES-GCM, up to 2^24.5 full-size records (about 24 million) may be encrypted on a given connection while keeping a safety margin of approximately 2^-57 for Authenticated Encryption (AE) security. For ChaCha20/Poly1305, the record sequence number would wrap before the safety limit is reached.

6. Alert Protocol

One of the content types supported by the TLS record layer is the alert type. Like other messages, alert messages are encrypted as specified by the current connection state.

Alert messages convey a description of the alert and a legacy field that conveyed the severity of the message in previous versions of TLS. In TLS 1.3, the severity is implicit in the type of alert being sent, and the ‘level’ field can safely be ignored. The “close_notify” alert is used to indicate orderly closure of the connection. Upon receiving such an alert, the TLS implementation SHOULD indicate end-of-data to the application.

Error alerts indicate abortive closure of the connection (see Section 6.2). Upon receiving an error alert, the TLS implementation SHOULD indicate an error to the application and MUST NOT allow any further data to be sent or received on the connection. Servers and clients MUST forget keys and secrets associated with a failed connection. Stateful implementations of tickets (as in many clients) SHOULD discard tickets associated with failed connections.

All the alerts listed in Section 6.2 MUST be sent as fatal and MUST be treated as fatal regardless of the AlertLevel in the message. Unknown alert types MUST be treated as fatal.

Note: TLS defines two generic alerts (see Section 6) to use upon failure to parse a message. Peers which receive a message which cannot be parsed according to the syntax (e.g., have a length extending beyond the message boundary or contain an out-of-range length) MUST terminate the connection with a “decode_error” alert. Peers which receive a message which is syntactically correct but semantically invalid (e.g., a DHE share of p - 1, or an invalid enum) MUST terminate the connection with an “illegal_parameter” alert.

   enum { warning(1), fatal(2), (255) } AlertLevel;

   enum {
       close_notify(0),
       unexpected_message(10),
       bad_record_mac(20),
       record_overflow(22),
       handshake_failure(40),
       bad_certificate(42),
       unsupported_certificate(43),
       certificate_revoked(44),
       certificate_expired(45),
       certificate_unknown(46),
       illegal_parameter(47),
       unknown_ca(48),
       access_denied(49),
       decode_error(50),
       decrypt_error(51),
       protocol_version(70),
       insufficient_security(71),
       internal_error(80),
       inappropriate_fallback(86),
       user_canceled(90),
       missing_extension(109),
       unsupported_extension(110),
       certificate_unobtainable(111),
       unrecognized_name(112),
       bad_certificate_status_response(113),
       bad_certificate_hash_value(114),
       unknown_psk_identity(115),
       certificate_required(116),
       no_application_protocol(120),
       (255)
   } AlertDescription;

   struct {
       AlertLevel level;
       AlertDescription description;
   } Alert;

6.1. Closure Alerts

The client and the server must share knowledge that the connection is ending in order to avoid a truncation attack.

close_notify
This alert notifies the recipient that the sender will not send any more messages on this connection. Any data received after a closure MUST be ignored.
user_canceled
This alert notifies the recipient that the sender is canceling the handshake for some reason unrelated to a protocol failure. If a user cancels an operation after the handshake is complete, just closing the connection by sending a “close_notify” is more appropriate. This alert SHOULD be followed by a “close_notify”. This alert is generally a warning.

Either party MAY initiate a close by sending a “close_notify” alert. Any data received after a closure alert MUST be ignored. If a transport-level close is received prior to a “close_notify”, the receiver cannot know that all the data that was sent has been received.

Each party MUST send a “close_notify” alert before closing the write side of the connection, unless some other fatal alert has been transmitted. The other party MUST respond with a “close_notify” alert of its own and close down the connection immediately, discarding any pending writes. The initiator of the close need not wait for the responding “close_notify” alert before closing the read side of the connection.

If the application protocol using TLS provides that any data may be carried over the underlying transport after the TLS connection is closed, the TLS implementation MUST receive the responding “close_notify” alert before indicating to the application layer that the TLS connection has ended. If the application protocol will not transfer any additional data but will only close the underlying transport connection, then the implementation MAY choose to close the transport without waiting for the responding “close_notify”. No part of this standard should be taken to dictate the manner in which a usage profile for TLS manages its data transport, including when connections are opened or closed.

Note: It is assumed that closing a connection reliably delivers pending data before destroying the transport.

6.2. Error Alerts

Error handling in the TLS Handshake Protocol is very simple. When an error is detected, the detecting party sends a message to its peer. Upon transmission or receipt of a fatal alert message, both parties MUST immediately close the connection.

Whenever an implementation encounters a fatal error condition, it SHOULD send an appropriate fatal alert and MUST close the connection without sending or receiving any additional data. In the rest of this specification, when the phrases “terminate the connection” and “abort the handshake” are used without a specific alert it means that the implementation SHOULD send the alert indicated by the descriptions below. The phrases “terminate the connection with a X alert” and “abort the handshake with a X alert” mean that the implementation MUST send alert X if it sends any alert. All alerts defined in this section below, as well as all unknown alerts, are universally considered fatal as of TLS 1.3 (see Section 6). The implementation SHOULD provide a way to facilitate logging the sending and receiving of alerts.

unexpected_message
An inappropriate message (e.g., the wrong handshake message, premature application data, etc.) was received. This alert should never be observed in communication between proper implementations.
bad_record_mac
This alert is returned if a record is received which cannot be deprotected. Because AEAD algorithms combine decryption and verification, and also to avoid side channel attacks, this alert is used for all deprotection failures. This alert should never be observed in communication between proper implementations, except when messages were corrupted in the network.
record_overflow
A TLSCiphertext record was received that had a length more than 2^14 + 256 bytes, or a record decrypted to a TLSPlaintext record with more than 2^14 bytes. This alert should never be observed in communication between proper implementations, except when messages were corrupted in the network.
handshake_failure
Receipt of a “handshake_failure” alert message indicates that the sender was unable to negotiate an acceptable set of security parameters given the options available.
bad_certificate
A certificate was corrupt, contained signatures that did not verify correctly, etc.
unsupported_certificate
A certificate was of an unsupported type.
certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not currently valid.
certificate_unknown
Some other (unspecified) issue arose in processing the certificate, rendering it unacceptable.
illegal_parameter
A field in the handshake was incorrect or inconsistent with other fields. This alert is used for errors which conform to the formal protocol syntax but are otherwise incorrect.
unknown_ca
A valid certificate chain or partial chain was received, but the certificate was not accepted because the CA certificate could not be located or could not be matched with a known trust anchor.
access_denied
A valid certificate or PSK was received, but when access control was applied, the sender decided not to proceed with negotiation.
decode_error
A message could not be decoded because some field was out of the specified range or the length of the message was incorrect. This alert is used for errors where the message does not conform to the formal protocol syntax. This alert should never be observed in communication between proper implementations, except when messages were corrupted in the network.
decrypt_error
A handshake (not record-layer) cryptographic operation failed, including being unable to correctly verify a signature or validate a Finished message or a PSK binder.
protocol_version
The protocol version the peer has attempted to negotiate is recognized but not supported. (see Appendix D)
insufficient_security
Returned instead of “handshake_failure” when a negotiation has failed specifically because the server requires parameters more secure than those supported by the client.
internal_error
An internal error unrelated to the peer or the correctness of the protocol (such as a memory allocation failure) makes it impossible to continue.
inappropriate_fallback
Sent by a server in response to an invalid connection retry attempt from a client (see [RFC7507]).
missing_extension
Sent by endpoints that receive a hello message not containing an extension that is mandatory to send for the offered TLS version or other negotiated parameters.
unsupported_extension
Sent by endpoints receiving any hello message containing an extension known to be prohibited for inclusion in the given hello message, or including any extensions in a ServerHello or Certificate not first offered in the corresponding ClientHello.
certificate_unobtainable
Sent by servers when unable to obtain a certificate from a URL provided by the client via the “client_certificate_url” extension (see [RFC6066]).
unrecognized_name
Sent by servers when no server exists identified by the name provided by the client via the “server_name” extension (see [RFC6066]).
bad_certificate_status_response
Sent by clients when an invalid or unacceptable OCSP response is provided by the server via the “status_request” extension (see [RFC6066]).
bad_certificate_hash_value
Sent by servers when a retrieved object does not have the correct hash provided by the client via the “client_certificate_url” extension (see [RFC6066]).
unknown_psk_identity
Sent by servers when PSK key establishment is desired but no acceptable PSK identity is provided by the client. Sending this alert is OPTIONAL; servers MAY instead choose to send a “decrypt_error” alert to merely indicate an invalid PSK identity.
certificate_required
Sent by servers when a client certificate is desired but none was provided by the client.
no_application_protocol
Sent by servers when a client “application_layer_protocol_negotiation” extension advertises protocols that the server does not support (see [RFC7301]).

New Alert values are assigned by IANA as described in Section 11.

7. Cryptographic Computations

The TLS handshake establishes one or more input secrets which are combined to create the actual working keying material, as detailed below. The key derivation process incorporates both the input secrets and the handshake transcript. Note that because the handshake transcript includes the random values from the Hello messages, any given handshake will have different traffic secrets, even if the same input secrets are used, as is the case when the same PSK is used for multiple connections

7.1. Key Schedule

The key derivation process makes use of the HKDF-Extract and HKDF-Expand functions as defined for HKDF [RFC5869], as well as the functions defined below:

    HKDF-Expand-Label(Secret, Label, HashValue, Length) =
         HKDF-Expand(Secret, HkdfLabel, Length)

    Where HkdfLabel is specified as:

    struct {
        uint16 length = Length;
        opaque label<7..255> = "tls13 " + Label;
        opaque hash_value<0..255> = HashValue;
    } HkdfLabel;

    Derive-Secret(Secret, Label, Messages) =
         HKDF-Expand-Label(Secret, Label,
                           Transcript-Hash(Messages), Hash.length)

The Hash function used by Transcript-Hash and HKDF is the cipher suite hash algorithm. Hash.length is its output length in bytes. Messages are the concatenation of the indicated handshake messages, including the handshake message type and length fields, but not including record layer headers. Note that in some cases a zero-length HashValue (indicated by “”) is passed to HKDF-Expand-Label.

Note: with common hash functions, any label longer than 12 characters requires an additional iteration of the hash function to compute. The labels in this specification have all been chosen to fit within this limit.

Given a set of n InputSecrets, the final “master secret” is computed by iteratively invoking HKDF-Extract with InputSecret_1, InputSecret_2, etc. The initial secret is simply a string of Hash.length zero bytes. Concretely, for the present version of TLS 1.3, secrets are added in the following order:

This produces a full key derivation schedule shown in the diagram below. In this diagram, the following formatting conventions apply:

                 0
                 |
                 v
   PSK ->  HKDF-Extract = Early Secret
                 |
                 +-----> Derive-Secret(.,
                 |                     "ext binder" |
                 |                     "res binder",
                 |                     "")
                 |                     = binder_key
                 |
                 +-----> Derive-Secret(., "c e traffic",
                 |                     ClientHello)
                 |                     = client_early_traffic_secret
                 |
                 +-----> Derive-Secret(., "e exp master",
                 |                     ClientHello)
                 |                     = early_exporter_master_secret
                 v
           Derive-Secret(., "derived", "")
                 |
                 v
(EC)DHE -> HKDF-Extract = Handshake Secret
                 |
                 +-----> Derive-Secret(., "c hs traffic",
                 |                     ClientHello...ServerHello)
                 |                     = client_handshake_traffic_secret
                 |
                 +-----> Derive-Secret(., "s hs traffic",
                 |                     ClientHello...ServerHello)
                 |                     = server_handshake_traffic_secret
                 v
           Derive-Secret(., "derived", "")
                 |
                 v
      0 -> HKDF-Extract = Master Secret
                 |
                 +-----> Derive-Secret(., "c ap traffic",
                 |                     ClientHello...server Finished)
                 |                     = client_application_traffic_secret_0
                 |
                 +-----> Derive-Secret(., "s ap traffic",
                 |                     ClientHello...server Finished)
                 |                     = server_application_traffic_secret_0
                 |
                 +-----> Derive-Secret(., "exp master",
                 |                     ClientHello...server Finished)
                 |                     = exporter_master_secret
                 |
                 +-----> Derive-Secret(., "res master",
                                       ClientHello...client Finished)
                                       = resumption_master_secret

The general pattern here is that the secrets shown down the left side of the diagram are just raw entropy without context, whereas the secrets down the right side include handshake context and therefore can be used to derive working keys without additional context. Note that the different calls to Derive-Secret may take different Messages arguments, even with the same secret. In a 0-RTT exchange, Derive-Secret is called with four distinct transcripts; in a 1-RTT-only exchange with three distinct transcripts.

If a given secret is not available, then the 0-value consisting of a string of Hash.length zero bytes is used. Note that this does not mean skipping rounds, so if PSK is not in use Early Secret will still be HKDF-Extract(0, 0). For the computation of the binder_secret, the label is “ext binder” for external PSKs (those provisioned outside of TLS) and “res binder” for resumption PSKs (those provisioned as the resumption master secret of a previous handshake). The different labels prevent the substitution of one type of PSK for the other.

There are multiple potential Early Secret values depending on which PSK the server ultimately selects. The client will need to compute one for each potential PSK; if no PSK is selected, it will then need to compute the early secret corresponding to the zero PSK.

Once all the values which are to be derived from a given secret have been computed, that secret SHOULD be erased.

7.2. Updating Traffic Keys and IVs

Once the handshake is complete, it is possible for either side to update its sending traffic keys using the KeyUpdate handshake message defined in Section 4.6.3. The next generation of traffic keys is computed by generating client_/server_application_traffic_secret_N+1 from client_/server_application_traffic_secret_N as described in this section then re-deriving the traffic keys as described in Section 7.3.

The next-generation application_traffic_secret is computed as:

    application_traffic_secret_N+1 =
        HKDF-Expand-Label(application_traffic_secret_N,
                          "traffic upd", "", Hash.length)

Once client/server_application_traffic_secret_N+1 and its associated traffic keys have been computed, implementations SHOULD delete client_/server_application_traffic_secret_N and its associated traffic keys.

7.3. Traffic Key Calculation

The traffic keying material is generated from the following input values:

The traffic keying material is generated from an input traffic secret value using:

    [sender]_write_key = HKDF-Expand-Label(Secret, "key", "", key_length)
    [sender]_write_iv  = HKDF-Expand-Label(Secret, "iv" , "", iv_length)

[sender] denotes the sending side. The Secret value for each record type is shown in the table below.

Record Type Secret
0-RTT Application client_early_traffic_secret
Handshake [sender]_handshake_traffic_secret
Application Data [sender]_application_traffic_secret_N

All the traffic keying material is recomputed whenever the underlying Secret changes (e.g., when changing from the handshake to application data keys or upon a key update).

7.4. (EC)DHE Shared Secret Calculation

7.4.1. Finite Field Diffie-Hellman

For finite field groups, a conventional Diffie-Hellman computation is performed. The negotiated key (Z) is converted to a byte string by encoding in big-endian and padded with zeros up to the size of the prime. This byte string is used as the shared secret in the key schedule as specified above.

Note that this construction differs from previous versions of TLS which remove leading zeros.

7.4.2. Elliptic Curve Diffie-Hellman

For secp256r1, secp384r1 and secp521r1, ECDH calculations (including parameter and key generation as well as the shared secret calculation) are performed according to [IEEE1363] using the ECKAS-DH1 scheme with the identity map as key derivation function (KDF), so that the shared secret is the x-coordinate of the ECDH shared secret elliptic curve point represented as an octet string. Note that this octet string (Z in IEEE 1363 terminology) as output by FE2OSP, the Field Element to Octet String Conversion Primitive, has constant length for any given field; leading zeros found in this octet string MUST NOT be truncated.

(Note that this use of the identity KDF is a technicality. The complete picture is that ECDH is employed with a non-trivial KDF because TLS does not directly use this secret for anything other than for computing other secrets.)

ECDH functions are used as follows:

For X25519 and X448, implementations SHOULD use the approach specified in [RFC7748] to calculate the Diffie-Hellman shared secret. Implementations MUST check whether the computed Diffie-Hellman shared secret is the all-zero value and abort if so, as described in Section 6 of [RFC7748]. If implementers use an alternative implementation of these elliptic curves, they SHOULD perform the additional checks specified in Section 7 of [RFC7748].

7.5. Exporters

[RFC5705] defines keying material exporters for TLS in terms of the TLS pseudorandom function (PRF). This document replaces the PRF with HKDF, thus requiring a new construction. The exporter interface remains the same.

The exporter value is computed as:

HKDF-Expand-Label(Derive-Secret(Secret, label, ""),
                  "exporter", Hash(context_value), key_length)

Where Secret is either the early_exporter_master_secret or the exporter_master_secret. Implementations MUST use the exporter_master_secret unless explicitly specified by the application. The early_exporter_master_secret is defined for use in settings where an exporter is needed for 0-RTT data. A separate interface for the early exporter is RECOMMENDED, especially on a server where a single interface can make the early exporter inaccessible.

If no context is provided, the context_value is zero-length. Consequently, providing no context computes the same value as providing an empty context. This is a change from previous versions of TLS where an empty context produced a different output to an absent context. As of this document’s publication, no allocated exporter label is used both with and without a context. Future specifications MUST NOT define a use of exporters that permit both an empty context and no context with the same label. New uses of exporters SHOULD provide a context in all exporter computations, though the value could be empty.

Requirements for the format of exporter labels are defined in section 4 of [RFC5705].

8. 0-RTT and Anti-Replay

As noted in Section 2.3 and Appendix E.5, TLS does not provide inherent replay protections for 0-RTT data. There are two potential threats to be concerned with:

The first class of attack can be prevented by the mechanism described in this section. Servers need not permit 0-RTT at all, but those which do SHOULD implement either the single-use tickets or ClientHello recording techniques described in the following two sections.

The second class of attack cannot be prevented at the TLS layer and MUST be dealt with by any application. Note that any application whose clients implement any kind of retry behavior already needs to implement some sort of anti-replay defense.

In normal operation, clients will not know which, if any, of these mechanisms servers actually implement and therefore MUST only send early data which they are willing to have subject to the attacks described in Appendix E.5.

8.1. Single-Use Tickets

The simplest form of anti-replay defense is for the server to only allow each session ticket to be used once. For instance, the server can maintain a database of all outstanding valid tickets; deleting each ticket from the database as it is used. If an unknown ticket is provided, the server would then fall back to a full handshake.

If the tickets are not self-contained but rather are database keys, and the corresponding PSKs are deleted upon use, then connections established using one PSK enjoy forward security. This improves security for all 0-RTT data and PSK usage when PSK is used without (EC)DHE.

Because this mechanism requires sharing the session database between server nodes in environments with multiple distributed servers, it may be hard to achieve high rates of successful PSK 0-RTT connections when compared to self-encrypted tickets. Unlike session databases, session tickets can successfully do PSK-based session establishment even without consistent storage, though when 0-RTT is allowed they still require consistent storage for anti-replay of 0-RTT data, as detailed in the following section.

8.2. Client Hello Recording

An alternative form of anti-replay is to record a unique value derived from the ClientHello (generally either the random value or the PSK binder) and reject duplicates. Recording all ClientHellos causes state to grow without bound, but a server can instead record ClientHellos within a given time window and use the “obfuscated_ticket_age” to ensure that tickets aren’t reused outside that window.

In order to implement this, when a ClientHello is received, the server first verifies the PSK binder as described Section 4.2.10. It then computes the expected_arrival_time as described in the next section and rejects 0-RTT if it is outside the recording window, falling back to the 1-RTT handshake.

If the expected arrival time is in the window, then the server checks to see if it has recorded a matching ClientHello. If one is found, it either aborts the handshake with an “illegal_parameter” alert or accepts the PSK but reject 0-RTT. If no matching ClientHello is found, then it accepts 0-RTT and then stores the ClientHello for as long as the expected_arrival_time is inside the window. Servers MAY also implement data stores with false positives, such as Bloom filters, in which case they MUST respond to apparent replay by rejecting 0-RTT but MUST NOT abort the handshake.

The server MUST derive the storage key only from validated sections of the ClientHello. If the ClientHello contains multiple PSK identities, then an attacker can create multiple ClientHellos with different binder values for the less-preferred identity on the assumption that the server will not verify it, as recommended by Section 4.2.10. I.e., if the client sends PSKs A and B but the server prefers A, then the attacker can change the binder for B without affecting the binder for A. This will cause the ClientHello to be accepted, and may casue side effects such as replay cache pollution, although any 0-RTT data will not be decryptable because it will use different keys. If the validated binder or the ClientHello.random are used as the storage key, then this attack is not possible.

Because this mechanism does not require storing all outstanding tickets, it may be easier to implement in distributed systems with high rates of resumption and 0-RTT, at the cost of potentially weaker anti-replay defense because of the difficulty of reliably storing and retrieving the received ClientHello messages. In many such systems, it is impractical to have globally consistent storage of all the received ClientHellos. In this case, the best anti-replay protection is provided by having a single storage zone be authoritative for a given ticket and refusing 0-RTT for that ticket in any other zone. This approach prevents simple replay by the attacker because only one zone will accept 0-RTT data. A weaker design is to implement separate storage for each zone but allow 0-RTT in any zone. This approach limits the number of replays to once per zone. Application message duplication of course remains possible with either design.

When implementations are freshly started, they SHOULD reject 0-RTT as long as any portion of their recording window overlaps the startup time. Otherwise, they run the risk of accepting replays which were originally sent during that period.

Note: If the client’s clock is running much faster than the server’s then a ClientHello may be received that is outside the window in the future, in which case it might be accepted for 1-RTT, causing a client retry, and then acceptable later for 0-RTT. This is another variant of the second form of attack described above.

8.3. Freshness Checks

Because the ClientHello indicates the time at which the client sent it, it is possible to efficiently determine whether a ClientHello was likely sent reasonably recently and only accept 0-RTT for such a ClientHello, otherwise falling back to a 1-RTT handshake. This is necessary for the ClientHello storage mechanism described in Section 8.2 because otherwise the server needs to store an unlimited number of ClientHellos and is a useful optimization for single-use tickets because it allows efficient rejection of ClientHellos which cannot be used for 0-RTT.

In order to implement this mechanism, a server needs to store the time that the server generated the session ticket, offset by an estimate of the round trip time between client and server. I.e.,

    adjusted_creation_time = creation_time + estimated_RTT

This value can be encoded in the ticket, thus avoiding the need to keep state for each outstanding ticket. The server can determine the client’s view of the age of the ticket by subtracting the ticket’s “ticket_age_add value” from the “obfuscated_ticket_age” parameter in the client’s “pre_shared_key” extension. The server can determine the “expected arrival time” of the ClientHello as:

    expected_arrival_time = adjusted_creation_time + clients_ticket_age

When a new ClientHello is received, the expected_arrival_time is then compared against the current server wall clock time and if they differ by more than a certain amount, 0-RTT is rejected, though the 1-RTT handshake can be allowed to complete.

There are several potential sources of error that might cause mismatches between the expected arrival time and the measured time. Variations in client and server clock rates are likely to be minimal, though potentially with gross time corrections. Network propagation delays are the most likely causes of a mismatch in legitimate values for elapsed time. Both the NewSessionTicket and ClientHello messages might be retransmitted and therefore delayed, which might be hidden by TCP. For clients on the Internet, this implies windows on the order of ten seconds to account for errors in clocks and variations in measurements; other deployment scenarios may have different needs. Clock skew distributions are not symmetric, so the optimal tradeoff may involve an asymmetric range of permissible mismatch values.

Note that freshness checking alone is not sufficient to prevent replays because it does not detect them during the error window, which, depending on bandwidth and system capacity could include billions of replays in real-world settings. In addition, this freshness checking is only done at the time the ClientHello is received, and not when later early application data records are received. After early data is accepted, records may continue to be streamed to the server over a longer time period.

9. Compliance Requirements

9.1. Mandatory-to-Implement Cipher Suites

In the absence of an application profile standard specifying otherwise, a TLS-compliant application MUST implement the TLS_AES_128_GCM_SHA256 [GCM] cipher suite and SHOULD implement the TLS_AES_256_GCM_SHA384 [GCM] and TLS_CHACHA20_POLY1305_SHA256 [RFC7539] cipher suites. (see Appendix B.4)

A TLS-compliant application MUST support digital signatures with rsa_pkcs1_sha256 (for certificates), rsa_pss_sha256 (for CertificateVerify and certificates), and ecdsa_secp256r1_sha256. A TLS-compliant application MUST support key exchange with secp256r1 (NIST P-256) and SHOULD support key exchange with X25519 [RFC7748].

9.2. Mandatory-to-Implement Extensions

In the absence of an application profile standard specifying otherwise, a TLS-compliant application MUST implement the following TLS extensions:

All implementations MUST send and use these extensions when offering applicable features:

A client is considered to be attempting to negotiate using this specification if the ClientHello contains a “supported_versions” extension 0x0304 the highest version number contained in its body. Such a ClientHello message MUST meet the following requirements:

Servers receiving a ClientHello which does not conform to these requirements MUST abort the handshake with a “missing_extension” alert.

Additionally, all implementations MUST support use of the “server_name” extension with applications capable of using it. Servers MAY require clients to send a valid “server_name” extension. Servers requiring this extension SHOULD respond to a ClientHello lacking a “server_name” extension by terminating the connection with a “missing_extension” alert.

10. Security Considerations

Security issues are discussed throughout this memo, especially in Appendix C, Appendix D, and Appendix E.

11. IANA Considerations

This document uses several registries that were originally created in [RFC4346]. IANA has updated these to reference this document. The registries and their allocation policies are below:

This document also uses the TLS ExtensionType Registry originally created in [RFC4366]. IANA has updated it to reference this document. The registry and its allocation policy is listed below:

In addition, this document defines a new registry to be maintained by IANA:

12. References

12.1. Normative References

[DH] Diffie, W. and M. Hellman, "New Directions in Cryptography", IEEE Transactions on Information Theory, V.IT-22 n.6 , June 1977.
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", NIST Special Publication 800-38D, November 2007.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, DOI 10.17487/RFC5226, May 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, March 2010.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for Transport Layer Security (TLS)", RFC 6655, DOI 10.17487/RFC6655, July 2012.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S. and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, June 2013.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) Multiple Certificate Status Request Extension", RFC 6961, DOI 10.17487/RFC6961, June 2013.
[RFC6962] Laurie, B., Langley, A. and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013.
[RFC7301] Friedl, S., Popov, A., Langley, A. and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, July 2014.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015.
[RFC7748] Langley, A., Hamburg, M. and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016.
[RFC7919] Gillmor, D., "Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)", RFC 7919, DOI 10.17487/RFC7919, August 2016.
[RFC8017] Moriarty, K., Kaliski, B., Jonsson, J. and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017.
[SHS] National Institute of Standards and Technology, U.S. Department of Commerce, "Secure Hash Standard", NIST FIPS PUB 180-4, March 2012.
[X690] ITU-T, "Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ISO/IEC 8825-1:2002, 2002.
[X962] ANSI, "Public Key Cryptography For The Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI X9.62, 1998.

12.2. Informative References

[AEAD-LIMITS] Luykx, A. and K. Paterson, "Limits on Authenticated Encryption Use in TLS", 2016.
[BBFKZG16] Bhargavan, K., Brzuska, C., Fournet, C., Kohlweiss, M., Zanella-Beguelin, S. and M. Green, "Downgrade Resilience in Key-Exchange Protocols", Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 , 2016.
[BBK17] Bhargavan, K., Blanchet, B. and N. Kobeissi, "Verified Models and Reference Implementations for the TLS 1.3 Standard Candidate", Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2017 , 2017.
[BDFKPPRSZZ16] Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Kohlweiss, M., Pan, J., Protzenko, J., Rastogi, A., Swamy, N., Zanella-Beguelin, S. and J. Zinzindohoue, "Implementing and Proving the TLS 1.3 Record Layer", Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2017 , December 2016.
[BMMT15] Badertscher, C., Matt, C., Maurer, U. and B. Tackmann, "Augmented Secure Channels and the Goal of the TLS 1.3 Record Layer", ProvSec 2015 , September 2015.
[BT16] Bellare, M. and B. Tackmann, "The Multi-User Security of Authenticated Encryption: AES-GCM in TLS 1.3", Proceedings of CRYPTO 2016 , 2016.
[CCG16] Cohn-Gordon, K., Cremers, C. and L. Garratt, "On Post-Compromise Security", IEEE Computer Security Foundations Symposium , 2015.
[CHHSV17] Cremers, C., Horvat, M., Hoyland, J., van der Merwe, T. and S. Scott, "Awkward Handshake: Possible mismatch of client/server view on client authentication in post-handshake mode in Revision 18", 2017.
[CHSV16] Cremers, C., Horvat, M., Scott, S. and T. van der Merwe, "Automated Analysis and Verification of TLS 1.3: 0-RTT, Resumption and Delayed Authentication", Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 , 2016.
[CK01] Canetti, R. and H. Krawczyk, "Analysis of Key-Exchange Protocols and Their Use for Building Secure Channels", Proceedings of Eurocrypt 2001 , 2001.
[CLINIC] Miller, B., Huang, L., Joseph, A. and J. Tygar, "I Know Why You Went to the Clinic: Risks and Realization of HTTPS Traffic Analysis", Privacy Enhancing Technologies pp. 143-163, DOI 10.1007/978-3-319-08506-7_8, 2014.
[DFGS15] Dowling, B., Fischlin, M., Guenther, F. and D. Stebila, "A Cryptographic Analysis of the TLS 1.3 draft-10 Full and Pre-shared Key Handshake Protocol", Proceedings of ACM CCS 2015 , 2015.
[DFGS16] Dowling, B., Fischlin, M., Guenther, F. and D. Stebila, "A Cryptographic Analysis of the TLS 1.3 draft-10 Full and Pre-shared Key Handshake Protocol", TRON 2016 , 2016.
[DOW92] Diffie, W., van Oorschot, P. and M. Wiener, "“Authentication and authenticated key exchanges”", Designs, Codes and Cryptography , 1992.
[DSS] National Institute of Standards and Technology, U.S. Department of Commerce, "Digital Signature Standard, version 4", NIST FIPS PUB 186-4, 2013.
[ECDSA] American National Standards Institute, "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI ANS X9.62-2005, November 2005.
[FG17] Fischlin, M. and F. Guenther, "Replay Attacks on Zero Round-Trip Time: The Case of the TLS 1.3 Handshake Candidates", Proceedings of Euro S"P 2017 , 2017.
[FGSW16] Fischlin, M., Guenther, F., Schmidt, B. and B. Warinschi, "Key Confirmation in Key Exchange: A Formal Treatment and Implications for TLS 1.3", Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 , 2016.
[FW15] Florian Weimer, ., "Factoring RSA Keys With TLS Perfect Forward Secrecy", September 2015.
[HCJ16] Husák, M., Čermák, M., Jirsík, T. and P. Čeleda, "HTTPS traffic analysis and client identification using passive SSL/TLS fingerprinting", EURASIP Journal on Information Security Vol. 2016, DOI 10.1186/s13635-016-0030-7, February 2016.
[HGFS15] Hlauschek, C., Gruber, M., Fankhauser, F. and C. Schanes, "Prying Open Pandora's Box: KCI Attacks against TLS", Proceedings of USENIX Workshop on Offensive Technologies , 2015.
[I-D.ietf-tls-iana-registry-updates] Salowey, J. and S. Turner, "D/TLS IANA Registry Updates", Internet-Draft draft-ietf-tls-iana-registry-updates-01, April 2017.
[I-D.ietf-tls-tls13-vectors] Thomson, M., "Example Handshake Traces for TLS 1.3", Internet-Draft draft-ietf-tls-tls13-vectors-01, June 2017.
[IEEE1363] IEEE, "Standard Specifications for Public Key Cryptography", IEEE 1363 , 2000.
[KEYAGREEMENT] Barker, E., Lily Chen, ., Roginsky, A. and M. Smid, "Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography", NIST Special Publication 800-38D, May 2013.
[Kraw10] Krawczyk, H., "Cryptographic Extraction and Key Derivation: The HKDF Scheme", Proceedings of CRYPTO 2010 , 2010.
[Kraw16] Krawczyk, H., "A Unilateral-to-Mutual Authentication Compiler for Key Exchange (with Applications to Client Authentication in TLS 1.3", Proceedings of ACM CCS 2016 , 2016.
[KW16] Krawczyk, H. and H. Wee, "The OPTLS Protocol and TLS 1.3", Proceedings of Euro S"P 2016 , 2016.
[LXZFH16] Li, X., Xu, J., Feng, D., Zhang, Z. and H. Hu, "Multiple Handshakes Security of TLS 1.3 Candidates", Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 , 2016.
[Mac17] MacCarthaigh, C., "Security Review of TLS1.3 0-RTT", 2017.
[PSK-FINISHED] Cremers, C., Horvat, M., van der Merwe, T. and S. Scott, "Revision 10: possible attack if client authentication is allowed during PSK", 2015.
[REKEY] Abdalla, M. and M. Bellare, "Increasing the Lifetime of a Key: A Comparative Analysis of the Security of Re-keying Techniques", ASIACRYPT2000 , October 2000.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, DOI 10.17487/RFC4346, April 2006.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C. and B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, DOI 10.17487/RFC4492, May 2006.
[RFC4681] Santesson, S., Medvinsky, A. and J. Ball, "TLS User Mapping Extension", RFC 4681, DOI 10.17487/RFC4681, October 2006.
[RFC5077] Salowey, J., Zhou, H., Eronen, P. and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, DOI 10.17487/RFC5077, January 2008.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)", RFC 5764, DOI 10.17487/RFC5764, May 2010.
[RFC5929] Altman, J., Williams, N. and L. Zhu, "Channel Bindings for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010.
[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys for Transport Layer Security (TLS) Authentication", RFC 6091, DOI 10.17487/RFC6091, February 2011.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC6520] Seggelmann, R., Tuexen, M. and M. Williams, "Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Heartbeat Extension", RFC 6520, DOI 10.17487/RFC6520, February 2012.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S. and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, June 2014.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, DOI 10.17487/RFC7465, February 2015.
[RFC7568] Barnes, R., Thomson, M., Pironti, A. and A. Langley, "Deprecating Secure Sockets Layer Version 3.0", RFC 7568, DOI 10.17487/RFC7568, June 2015.
[RFC7627] Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley, A. and M. Ray, "Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension", RFC 7627, DOI 10.17487/RFC7627, September 2015.
[RFC7685] Langley, A., "A Transport Layer Security (TLS) ClientHello Padding Extension", RFC 7685, DOI 10.17487/RFC7685, October 2015.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", RFC 7924, DOI 10.17487/RFC7924, July 2016.
[RSA] Rivest, R., Shamir, A. and L. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems", Communications of the ACM v. 21, n. 2, pp. 120-126., February 1978.
[SIGMA] Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' approach to authenticated Diffie-Hellman and its use in the IKE protocols", Proceedings of CRYPTO 2003 , 2003.
[SLOTH] Bhargavan, K. and G. Leurent, "Transcript Collision Attacks: Breaking Authentication in TLS, IKE, and SSH", Network and Distributed System Security Symposium (NDSS 2016) , 2016.
[SSL2] Hickman, K., "The SSL Protocol", February 1995.
[SSL3] Freier, A., Karlton, P. and P. Kocher, "The SSL 3.0 Protocol", November 1996.
[TIMING] Boneh, D. and D. Brumley, "Remote timing attacks are practical", USENIX Security Symposium, 2003.
[X501] "Information Technology - Open Systems Interconnection - The Directory: Models", ITU-T X.501, 1993.

Appendix A. State Machine

This section provides a summary of the legal state transitions for the client and server handshakes. State names (in all capitals, e.g., START) have no formal meaning but are provided for ease of comprehension. Actions which are taken only in certain circumstances are indicated in []. The notation “K_{send,recv} = foo” means “set the send/recv key to the given key”.

A.1. Client

                           START <----+
            Send ClientHello |        | Recv HelloRetryRequest
       [K_send = early data] |        |
                             v        |
        /                 WAIT_SH ----+
        |                    | Recv ServerHello
        |                    | K_recv = handshake
    Can |                    V
   send |                 WAIT_EE
  early |                    | Recv EncryptedExtensions
   data |           +--------+--------+
        |     Using |                 | Using certificate
        |       PSK |                 v
        |           |            WAIT_CERT_CR
        |           |        Recv |       | Recv CertificateRequest
        |           | Certificate |       v
        |           |             |    WAIT_CERT
        |           |             |       | Recv Certificate
        |           |             v       v
        |           |              WAIT_CV
        |           |                 | Recv CertificateVerify
        |           +> WAIT_FINISHED <+
        |                  | Recv Finished
        \                  | [Send EndOfEarlyData]
                           | K_send = handshake
                           | [Send Certificate [+ CertificateVerify]]
 Can send                  | Send Finished
 app data   -->            | K_send = K_recv = application
 after here                v
                       CONNECTED

Note that with the transitions as shown above, clients may send alerts that derive from post-ServerHello messages in the clear or with the early data keys. If clients need to send such alerts, they SHOULD first rekey to the handshake keys if possible.

A.2. Server

                             START <-----+
              Recv ClientHello |         | Send HelloRetryRequest
                               v         |
                            RECVD_CH ----+
                               | Select parameters
                               v
                            NEGOTIATED
                               | Send ServerHello
                               | K_send = handshake
                               | Send EncryptedExtensions
                               | [Send CertificateRequest]
Can send                       | [Send Certificate + CertificateVerify]
app data                       | Send Finished
after   -->                    | K_send = application
here                  +--------+--------+
             No 0-RTT |                 | 0-RTT
   K_recv = handshake |                 | K_recv = early_data
[Skip decrypt errors] |             WAIT_EOED <---+
                      |            Recv |   |     | Recv
                      |  EndOfEarlyData |   |     | Early data
                      |        K_recv = |   +-----+
                      |       handshake |
                      |                 |
                      +> WAIT_FLIGHT2 <-+
                               |
                      +--------+--------+
              No auth |                 | Client auth
                      |                 |
                      |                 v
                      |             WAIT_CERT
                      |        Recv |       | Recv Certificate
                      |       empty |       v
                      | Certificate |    WAIT_CV
                      |             |       | Recv
                      |             v       | CertificateVerify
                      +-> WAIT_FINISHED <---+
                               | Recv Finished
                               | K_recv = application
                               v
                           CONNECTED

Appendix B. Protocol Data Structures and Constant Values

This section describes protocol types and constants. Values listed as _RESERVED were used in previous versions of TLS and are listed here for completeness. TLS 1.3 implementations MUST NOT send them but might receive them from older TLS implementations.

B.1. Record Layer

   enum {
       invalid(0),
       change_cipher_spec_RESERVED(20),
       alert(21),
       handshake(22),
       application_data(23),
       (255)
   } ContentType;

   struct {
       ContentType type;
       ProtocolVersion legacy_record_version;
       uint16 length;
       opaque fragment[TLSPlaintext.length];
   } TLSPlaintext;

   struct {
       opaque content[TLSPlaintext.length];
       ContentType type;
       uint8 zeros[length_of_padding];
   } TLSInnerPlaintext;

   struct {
       ContentType opaque_type = 23; /* application_data */
       ProtocolVersion legacy_record_version = 0x0301; /* TLS v1.x */
       uint16 length;
       opaque encrypted_record[length];
   } TLSCiphertext;

B.2. Alert Messages

   enum { warning(1), fatal(2), (255) } AlertLevel;

   enum {
       close_notify(0),
       unexpected_message(10),
       bad_record_mac(20),
       decryption_failed_RESERVED(21),
       record_overflow(22),
       decompression_failure_RESERVED(30),
       handshake_failure(40),
       no_certificate_RESERVED(41),
       bad_certificate(42),
       unsupported_certificate(43),
       certificate_revoked(44),
       certificate_expired(45),
       certificate_unknown(46),
       illegal_parameter(47),
       unknown_ca(48),
       access_denied(49),
       decode_error(50),
       decrypt_error(51),
       export_restriction_RESERVED(60),
       protocol_version(70),
       insufficient_security(71),
       internal_error(80),
       inappropriate_fallback(86),
       user_canceled(90),
       no_renegotiation_RESERVED(100),
       missing_extension(109),
       unsupported_extension(110),
       certificate_unobtainable(111),
       unrecognized_name(112),
       bad_certificate_status_response(113),
       bad_certificate_hash_value(114),
       unknown_psk_identity(115),
       certificate_required(116),
       no_application_protocol(120),
       (255)
   } AlertDescription;

   struct {
       AlertLevel level;
       AlertDescription description;
   } Alert;

B.3. Handshake Protocol

   enum {
       hello_request_RESERVED(0),
       client_hello(1),
       server_hello(2),
       hello_verify_request_RESERVED(3),
       new_session_ticket(4),
       end_of_early_data(5),
       hello_retry_request(6),
       encrypted_extensions(8),
       certificate(11),
       server_key_exchange_RESERVED(12),
       certificate_request(13),
       server_hello_done_RESERVED(14),
       certificate_verify(15),
       client_key_exchange_RESERVED(16),
       finished(20),
       key_update(24),
       message_hash(254),
       (255)
   } HandshakeType;

   struct {
       HandshakeType msg_type;    /* handshake type */
       uint24 length;             /* bytes in message */
       select (Handshake.msg_type) {
           case client_hello:          ClientHello;
           case server_hello:          ServerHello;
           case end_of_early_data:     EndOfEarlyData;
           case hello_retry_request:   HelloRetryRequest;
           case encrypted_extensions:  EncryptedExtensions;
           case certificate_request:   CertificateRequest;
           case certificate:           Certificate;
           case certificate_verify:    CertificateVerify;
           case finished:              Finished;
           case new_session_ticket:    NewSessionTicket;
           case key_update:            KeyUpdate;
       } body;
   } Handshake;

B.3.1. Key Exchange Messages

   uint16 ProtocolVersion;
   opaque Random[32];

   uint8 CipherSuite[2];    /* Cryptographic suite selector */

   struct {
       ProtocolVersion legacy_version = 0x0303;    /* TLS v1.2 */
       Random random;
       opaque legacy_session_id<0..32>;
       CipherSuite cipher_suites<2..2^16-2>;
       opaque legacy_compression_methods<1..2^8-1>;
       Extension extensions<8..2^16-1>;
   } ClientHello;

   struct {
       ProtocolVersion version;
       Random random;
       CipherSuite cipher_suite;
       Extension extensions<6..2^16-1>;
   } ServerHello;

   struct {
       ProtocolVersion server_version;
       CipherSuite cipher_suite;
       Extension extensions<2..2^16-1>;
   } HelloRetryRequest;

   struct {
       ExtensionType extension_type;
       opaque extension_data<0..2^16-1>;
   } Extension;

   enum {
       server_name(0),                             /* RFC 6066 */
       max_fragment_length(1),                     /* RFC 6066 */
       status_request(5),                          /* RFC 6066 */
       supported_groups(10),                       /* RFC 4492, 7919 */
       signature_algorithms(13),                   /* RFC 5246 */
       use_srtp(14),                               /* RFC 5764 */
       heartbeat(15),                              /* RFC 6520 */
       application_layer_protocol_negotiation(16), /* RFC 7301 */
       signed_certificate_timestamp(18),           /* RFC 6962 */
       client_certificate_type(19),                /* RFC 7250 */
       server_certificate_type(20),                /* RFC 7250 */
       padding(21),                                /* RFC 7685 */
       key_share(40),                              /* [[this document]] */
       pre_shared_key(41),                         /* [[this document]] */
       early_data(42),                             /* [[this document]] */
       supported_versions(43),                     /* [[this document]] */
       cookie(44),                                 /* [[this document]] */
       psk_key_exchange_modes(45),                 /* [[this document]] */
       certificate_authorities(47),                /* [[this document]] */
       oid_filters(48),                            /* [[this document]] */
       post_handshake_auth(49),                    /* [[this document]] */
       (65535)
   } ExtensionType;

   struct {
       NamedGroup group;
       opaque key_exchange<1..2^16-1>;
   } KeyShareEntry;

   struct {
       select (Handshake.msg_type) {
           case client_hello:
               KeyShareEntry client_shares<0..2^16-1>;

           case hello_retry_request:
               NamedGroup selected_group;

           case server_hello:
               KeyShareEntry server_share;
       };
   } KeyShare;

   enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;

   struct {
       PskKeyExchangeMode ke_modes<1..255>;
   } PskKeyExchangeModes;

   struct {} Empty;

   struct {
       select (Handshake.msg_type) {
           case new_session_ticket:   uint32 max_early_data_size;
           case client_hello:         Empty;
           case encrypted_extensions: Empty;
       };
   } EarlyDataIndication;

   struct {
       opaque identity<1..2^16-1>;
       uint32 obfuscated_ticket_age;
   } PskIdentity;

   opaque PskBinderEntry<32..255>;

   struct {
       select (Handshake.msg_type) {
           case client_hello:
               PskIdentity identities<7..2^16-1>;
               PskBinderEntry binders<33..2^16-1>;

           case server_hello:
               uint16 selected_identity;
       };

   } PreSharedKeyExtension;

B.3.1.1. Version Extension

   struct {
       ProtocolVersion versions<2..254>;
   } SupportedVersions;

B.3.1.2. Cookie Extension

   struct {
       opaque cookie<1..2^16-1>;
   } Cookie;

B.3.1.3. Signature Algorithm Extension

   enum {
       /* RSASSA-PKCS1-v1_5 algorithms */
       rsa_pkcs1_sha256(0x0401),
       rsa_pkcs1_sha384(0x0501),
       rsa_pkcs1_sha512(0x0601),

       /* ECDSA algorithms */
       ecdsa_secp256r1_sha256(0x0403),
       ecdsa_secp384r1_sha384(0x0503),
       ecdsa_secp521r1_sha512(0x0603),

       /* RSASSA-PSS algorithms */
       rsa_pss_sha256(0x0804),
       rsa_pss_sha384(0x0805),
       rsa_pss_sha512(0x0806),

       /* EdDSA algorithms */
       ed25519(0x0807),
       ed448(0x0808),

       /* Legacy algorithms */
       rsa_pkcs1_sha1(0x0201),
       ecdsa_sha1(0x0203),

       /* Reserved Code Points */
       obsolete_RESERVED(0x0000..0x0200),
       dsa_sha1_RESERVED(0x0202),
       obsolete_RESERVED(0x0204..0x0400),
       dsa_sha256_RESERVED(0x0402),
       obsolete_RESERVED(0x0404..0x0500),
       dsa_sha384_RESERVED(0x0502),
       obsolete_RESERVED(0x0504..0x0600),
       dsa_sha512_RESERVED(0x0602),
       obsolete_RESERVED(0x0604..0x06FF),
       private_use(0xFE00..0xFFFF),
       (0xFFFF)
   } SignatureScheme;

   struct {
       SignatureScheme supported_signature_algorithms<2..2^16-2>;
   } SignatureSchemeList;

B.3.1.4. Supported Groups Extension

   enum {
       /* Elliptic Curve Groups (ECDHE) */
       obsolete_RESERVED(0x0001..0x0016),
       secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
       obsolete_RESERVED(0x001A..0x001C),
       x25519(0x001D), x448(0x001E),

       /* Finite Field Groups (DHE) */
       ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096 (0x0102),
       ffdhe6144(0x0103), ffdhe8192(0x0104),

       /* Reserved Code Points */
       ffdhe_private_use(0x01FC..0x01FF),
       ecdhe_private_use(0xFE00..0xFEFF),
       obsolete_RESERVED(0xFF01..0xFF02),
       (0xFFFF)
   } NamedGroup;

   struct {
       NamedGroup named_group_list<2..2^16-1>;
   } NamedGroupList;

Values within “obsolete_RESERVED” ranges are used in previous versions of TLS and MUST NOT be offered or negotiated by TLS 1.3 implementations. The obsolete curves have various known/theoretical weaknesses or have had very little usage, in some cases only due to unintentional server configuration issues. They are no longer considered appropriate for general use and should be assumed to be potentially unsafe. The set of curves specified here is sufficient for interoperability with all currently deployed and properly configured TLS implementations.

B.3.2. Server Parameters Messages

   opaque DistinguishedName<1..2^16-1>;

   struct {
       DistinguishedName authorities<3..2^16-1>;
   } CertificateAuthoritiesExtension;

   struct {
       opaque certificate_extension_oid<1..2^8-1>;
       opaque certificate_extension_values<0..2^16-1>;
   } OIDFilter;

   struct {
       OIDFilter filters<0..2^16-1>;
   } OIDFilterExtension;

   struct {
       Extension extensions<0..2^16-1>;
   } EncryptedExtensions;

   struct {
       opaque certificate_request_context<0..2^8-1>;
       Extension extensions<2..2^16-1>;
   } CertificateRequest;

B.3.3. Authentication Messages

   struct {
       select(certificate_type){
           case RawPublicKey:
             // From RFC 7250 ASN.1_subjectPublicKeyInfo
             opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;

           case X509:
             opaque cert_data<1..2^24-1>;
       };
       Extension extensions<0..2^16-1>;
   } CertificateEntry;

   struct {
       opaque certificate_request_context<0..2^8-1>;
       CertificateEntry certificate_list<0..2^24-1>;
   } Certificate;

   struct {
       SignatureScheme algorithm;
       opaque signature<0..2^16-1>;
   } CertificateVerify;

   struct {
       opaque verify_data[Hash.length];
   } Finished;

B.3.4. Ticket Establishment

   struct {
       uint32 ticket_lifetime;
       uint32 ticket_age_add;
       opaque ticket_nonce<1..255>;
       opaque ticket<1..2^16-1>;
       Extension extensions<0..2^16-2>;
   } NewSessionTicket;

B.3.5. Updating Keys

   struct {} EndOfEarlyData;

   enum {
       update_not_requested(0), update_requested(1), (255)
   } KeyUpdateRequest;

   struct {
       KeyUpdateRequest request_update;
   } KeyUpdate;

B.4. Cipher Suites

A symmetric cipher suite defines the pair of the AEAD algorithm and hash algorithm to be used with HKDF. Cipher suite names follow the naming convention:

   CipherSuite TLS_AEAD_HASH = VALUE;
Component Contents
TLS The string “TLS”
AEAD The AEAD algorithm used for record protection
HASH The hash algorithm used with HKDF
VALUE The two byte ID assigned for this cipher suite

This specification defines the following cipher suites for use with TLS 1.3.

Description Value
TLS_AES_128_GCM_SHA256 {0x13,0x01}
TLS_AES_256_GCM_SHA384 {0x13,0x02}
TLS_CHACHA20_POLY1305_SHA256 {0x13,0x03}
TLS_AES_128_CCM_SHA256 {0x13,0x04}
TLS_AES_128_CCM_8_SHA256 {0x13,0x05}

The corresponding AEAD algorithms AEAD_AES_128_GCM, AEAD_AES_256_GCM, and AEAD_AES_128_CCM are defined in [RFC5116]. AEAD_CHACHA20_POLY1305 is defined in [RFC7539]. AEAD_AES_128_CCM_8 is defined in [RFC6655]. The corresponding hash algorithms are defined in [SHS].

Although TLS 1.3 uses the same cipher suite space as previous versions of TLS, TLS 1.3 cipher suites are defined differently, only specifying the symmetric ciphers, and cannot be used for TLS 1.2. Similarly, TLS 1.2 and lower cipher suites cannot be used with TLS 1.3.

New cipher suite values are assigned by IANA as described in Section 11.

Appendix C. Implementation Notes

The TLS protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementors. [I-D.ietf-tls-tls13-vectors] provides test vectors for TLS 1.3 handshakes.

C.1. Random Number Generation and Seeding

TLS requires a cryptographically secure pseudorandom number generator (CSPRNG). In most cases, the operating system provides an appropriate facility such as /dev/urandom, which should be used absent other (performance) concerns. It is RECOMMENDED to use an existing CSPRNG implementation in preference to crafting a new one. Many adequate cryptographic libraries are already available under favorable license terms. Should those prove unsatisfactory, [RFC4086] provides guidance on the generation of random values.

C.2. Certificates and Authentication

Implementations are responsible for verifying the integrity of certificates and should generally support certificate revocation messages. Absent a specific indication from an application profile, Certificates should always be verified to ensure proper signing by a trusted Certificate Authority (CA). The selection and addition of trust anchors should be done very carefully. Users should be able to view information about the certificate and trust anchor. Applications SHOULD also enforce minimum and maximum key sizes. For example, certification paths containing keys or signatures weaker than 2048-bit RSA or 224-bit ECDSA are not appropriate for secure applications.

C.3. Implementation Pitfalls

Implementation experience has shown that certain parts of earlier TLS specifications are not easy to understand and have been a source of interoperability and security problems. Many of these areas have been clarified in this document but this appendix contains a short list of the most important things that require special attention from implementors.

TLS protocol issues:

Cryptographic details:

C.4. Client Tracking Prevention

Clients SHOULD NOT reuse a ticket for multiple connections. Reuse of a ticket allows passive observers to correlate different connections. Servers that issue tickets SHOULD offer at least as many tickets as the number of connections that a client might use; for example, a web browser using HTTP/1.1 [RFC7230] might open six connections to a server. Servers SHOULD issue new tickets with every connection. This ensures that clients are always able to use a new ticket when creating a new connection.

C.5. Unauthenticated Operation

Previous versions of TLS offered explicitly unauthenticated cipher suites based on anonymous Diffie-Hellman. These modes have been deprecated in TLS 1.3. However, it is still possible to negotiate parameters that do not provide verifiable server authentication by several methods, including:

Either technique used alone is vulnerable to man-in-the-middle attacks and therefore unsafe for general use. However, it is also possible to bind such connections to an external authentication mechanism via out-of-band validation of the server’s public key, trust on first use, or a mechanism such as channel bindings (though the channel bindings described in [RFC5929] are not defined for TLS 1.3). If no such mechanism is used, then the connection has no protection against active man-in-the-middle attack; applications MUST NOT use TLS in such a way absent explicit configuration or a specific application profile.

Appendix D. Backward Compatibility

The TLS protocol provides a built-in mechanism for version negotiation between endpoints potentially supporting different versions of TLS.

TLS 1.x and SSL 3.0 use compatible ClientHello messages. Servers can also handle clients trying to use future versions of TLS as long as the ClientHello format remains compatible and the client supports the highest protocol version available in the server.

Prior versions of TLS used the record layer version number for various purposes. (TLSPlaintext.legacy_record_version and TLSCiphertext.legacy_record_version) As of TLS 1.3, this field is deprecated. The value of TLSPlaintext.legacy_record_version MUST be ignored by all implementations. The value of TLSCiphertext.legacy_record_version MAY be ignored, or MAY be validated to match the fixed constant value. Version negotiation is performed using only the handshake versions (ClientHello.legacy_version, ClientHello “supported_versions” extension, and ServerHello.version). In order to maximize interoperability with older endpoints, implementations that negotiate the use of TLS 1.0-1.2 SHOULD set the record layer version number to the negotiated version for the ServerHello and all records thereafter.

For maximum compatibility with previously non-standard behavior and misconfigured deployments, all implementations SHOULD support validation of certification paths based on the expectations in this document, even when handling prior TLS versions’ handshakes. (see Section 4.4.2.2)

TLS 1.2 and prior supported an “Extended Master Secret” [RFC7627] extension which digested large parts of the handshake transcript into the master secret. Because TLS 1.3 always hashes in the transcript up to the server CertificateVerify, implementations which support both TLS 1.3 and earlier versions SHOULD indicate the use of the Extended Master Secret extension in their APIs whenever TLS 1.3 is used.

D.1. Negotiating with an older server

A TLS 1.3 client who wishes to negotiate with servers that do not support TLS 1.3 will send a normal TLS 1.3 ClientHello containing 0x0303 (TLS 1.2) in ClientHello.legacy_version but with the correct version in the “supported_versions” extension. If the server does not support TLS 1.3 it will respond with a ServerHello containing an older version number. If the client agrees to use this version, the negotiation will proceed as appropriate for the negotiated protocol. A client using a ticket for resumption SHOULD initiate the connection using the version that was previously negotiated.

Note that 0-RTT data is not compatible with older servers and SHOULD NOT be sent absent knowledge that the server supports TLS 1.3. See Appendix D.3.

If the version chosen by the server is not supported by the client (or not acceptable), the client MUST abort the handshake with a “protocol_version” alert.

Some legacy server implementations are known to not implement the TLS specification properly and might abort connections upon encountering TLS extensions or versions which they are not aware of. Interoperability with buggy servers is a complex topic beyond the scope of this document. Multiple connection attempts may be required in order to negotiate a backwards compatible connection; however, this practice is vulnerable to downgrade attacks and is NOT RECOMMENDED.

D.2. Negotiating with an older client

A TLS server can also receive a ClientHello indicating a version number smaller than its highest supported version. If the “supported_versions” extension is present, the server MUST negotiate using that extension as described in Section 4.2.1. If the “supported_versions” extension is not present, the server MUST negotiate the minimum of ClientHello.legacy_version and TLS 1.2. For example, if the server supports TLS 1.0, 1.1, and 1.2, and legacy_version is TLS 1.0, the server will proceed with a TLS 1.0 ServerHello. If the “supported_versions” extension is absent and the server only supports versions greater than ClientHello.legacy_version, the server MUST abort the handshake with a “protocol_version” alert.

Note that earlier versions of TLS did not clearly specify the record layer version number value in all cases (TLSPlaintext.legacy_record_version). Servers will receive various TLS 1.x versions in this field, but its value MUST always be ignored.

D.3. Zero-RTT backwards compatibility

0-RTT data is not compatible with older servers. An older server will respond to the ClientHello with an older ServerHello, but it will not correctly skip the 0-RTT data and will fail to complete the handshake. This can cause issues when a client attempts to use 0-RTT, particularly against multi-server deployments. For example, a deployment could deploy TLS 1.3 gradually with some servers implementing TLS 1.3 and some implementing TLS 1.2, or a TLS 1.3 deployment could be downgraded to TLS 1.2.

A client that attempts to send 0-RTT data MUST fail a connection if it receives a ServerHello with TLS 1.2 or older. A client that attempts to repair this error SHOULD NOT send a TLS 1.2 ClientHello, but instead send a TLS 1.3 ClientHello without 0-RTT data.

To avoid this error condition, multi-server deployments SHOULD ensure a uniform and stable deployment of TLS 1.3 without 0-RTT prior to enabling 0-RTT.

D.4. Backwards Compatibility Security Restrictions

Implementations negotiating use of older versions of TLS SHOULD prefer forward secret and AEAD cipher suites, when available.

The security of RC4 cipher suites is considered insufficient for the reasons cited in [RFC7465]. Implementations MUST NOT offer or negotiate RC4 cipher suites for any version of TLS for any reason.

Old versions of TLS permitted the use of very low strength ciphers. Ciphers with a strength less than 112 bits MUST NOT be offered or negotiated for any version of TLS for any reason.

The security of SSL 3.0 [SSL3] is considered insufficient for the reasons enumerated in [RFC7568], and MUST NOT be negotiated for any reason.

The security of SSL 2.0 [SSL2] is considered insufficient for the reasons enumerated in [RFC6176], and MUST NOT be negotiated for any reason.

Implementations MUST NOT send an SSL version 2.0 compatible CLIENT-HELLO. Implementations MUST NOT negotiate TLS 1.3 or later using an SSL version 2.0 compatible CLIENT-HELLO. Implementations are NOT RECOMMENDED to accept an SSL version 2.0 compatible CLIENT-HELLO in order to negotiate older versions of TLS.

Implementations MUST NOT send a ClientHello.legacy_version or ServerHello.version set to 0x0300 or less. Any endpoint receiving a Hello message with ClientHello.legacy_version or ServerHello.version set to 0x0300 MUST abort the handshake with a “protocol_version” alert.

Implementations MUST NOT send any records with a version less than 0x0300. Implementations SHOULD NOT accept any records with a version less than 0x0300 (but may inadvertently do so if the record version number is ignored completely).

Implementations MUST NOT use the Truncated HMAC extension, defined in Section 7 of [RFC6066], as it is not applicable to AEAD algorithms and has been shown to be insecure in some scenarios.

Appendix E. Overview of Security Properties

A complete security analysis of TLS is outside the scope of this document. In this section, we provide an informal description the desired properties as well as references to more detailed work in the research literature which provides more formal definitions.

We cover properties of the handshake separately from those of the record layer.

E.1. Handshake

The TLS handshake is an Authenticated Key Exchange (AKE) protocol which is intended to provide both one-way authenticated (server-only) and mutually authenticated (client and server) functionality. At the completion of the handshake, each side outputs its view of the following values:

We assume the attacker to be an active network attacker, which means it has complete control over the network used to communicate between the parties [RFC3552]. Even under these conditions, the handshake should provide the properties listed below. Note that these properties are not necessarily independent, but reflect the protocol consumers’ needs.

Establishing the same session keys.
The handshake needs to output the same set of session keys on both sides of the handshake, provided that it completes successfully on each endpoint (See [CK01]; defn 1, part 1).
Secrecy of the session keys.
The shared session keys should be known only to the communicating parties and not to the attacker (See [CK01]; defn 1, part 2). Note that in a unilaterally authenticated connection, the attacker can establish its own session keys with the server, but those session keys are distinct from those established by the client.
Peer Authentication.
The client’s view of the peer identity should reflect the server’s identity. If the client is authenticated, the server’s view of the peer identity should match the client’s identity.
Uniqueness of the session keys:
Any two distinct handshakes should produce distinct, unrelated session keys. Individual session keys produced by a handshake should also be distinct and unrelated.
Downgrade protection.
The cryptographic parameters should be the same on both sides and should be the same as if the peers had been communicating in the absence of an attack (See [BBFKZG16]; defns 8 and 9}).
Forward secret with respect to long-term keys
If the long-term keying material (in this case the signature keys in certificate-based authentication modes or the external/resumption PSK in PSK with (EC)DHE modes) is compromised after the handshake is complete, this does not compromise the security of the session key (See [DOW92]), as long as the session key itself has been erased. The forward secrecy property is not satisfied when PSK is used in the “psk_ke” PskKeyExchangeMode.
Key Compromise Impersonation (KCI) resistance
In a mutually-authenticated connection with certificates, peer authentication should hold even if the local long-term secret was compromised before the connection was established (see [HGFS15]). For example, if a client’s signature key is compromised, it should not be possible to impersonate arbitrary servers to that client in subsequent handshakes.
Protection of endpoint identities.
The server’s identity (certificate) should be protected against passive attackers. The client’s identity should be protected against both passive and active attackers.

Informally, the signature-based modes of TLS 1.3 provide for the establishment of a unique, secret, shared key established by an (EC)DHE key exchange and authenticated by the server’s signature over the handshake transcript, as well as tied to the server’s identity by a MAC. If the client is authenticated by a certificate, it also signs over the handshake transcript and provides a MAC tied to both identities. [SIGMA] describes the design and analysis of this type of key exchange protocol. If fresh (EC)DHE keys are used for each connection, then the output keys are forward secret.

The external PSK and resumption PSK bootstrap from a long-term shared secret into a unique per-connection set of short-term session keys. This secret may have been established in a previous handshake. If PSK with (EC)DHE key establishment is used, these session keys will also be forward secret. The resumption PSK has been designed so that the resumption master secret computed by connection N and needed to form connection N+1 is separate from the traffic keys used by connection N, thus providing forward secrecy between the connections. In addition, if multiple tickets are established on the same connection, they are associated with different keys, so compromise of the PSK associated with one ticket does not lead to the compromise of connections established with PSKs associated with other tickets. This property is most interesting if tickets are stored in a database (and so can be deleted) rather than if they are self-encrypted.

The PSK binder value forms a binding between a PSK and the current handshake, as well as between the session where the PSK was established and the session where it was used. This binding transitively includes the original handshake transcript, because that transcript is digested into the values which produce the Resumption Master Secret. This requires that both the KDF used to produce the resumption master secret and the MAC used to compute the binder be collision resistant. See Appendix E.1.1 for more on this. Note: The binder does not cover the binder values from other PSKs, though they are included in the Finished MAC.

Note: TLS does not currently permit the server to send a certificate_request message in non-certificate-based handshakes (e.g., PSK). If this restriction were to be relaxed in future, the client’s signature would not cover the server’s certificate directly. However, if the PSK was established through a NewSessionTicket, the client’s signature would transitively cover the server’s certificate through the PSK binder. [PSK-FINISHED] describes a concrete attack on constructions that do not bind to the server’s certificate (see also [Kraw16]). It is unsafe to use certificate-based client authentication when the client might potentially share the same PSK/key-id pair with two different endpoints. Implementations MUST NOT combine external PSKs with certificate-based authentication of either the client or the server.

If an exporter is used, then it produces values which are unique and secret (because they are generated from a unique session key). Exporters computed with different labels and contexts are computationally independent, so it is not feasible to compute one from another or the session secret from the exported value. Note: exporters can produce arbitrary-length values. If exporters are to be used as channel bindings, the exported value MUST be large enough to provide collision resistance. The exporters provided in TLS 1.3 are derived from the same handshake contexts as the early traffic keys and the application traffic keys respectively, and thus have similar security properties. Note that they do not include the client’s certificate; future applications which wish to bind to the client’s certificate may need to define a new exporter that includes the full handshake transcript.

For all handshake modes, the Finished MAC (and where present, the signature), prevents downgrade attacks. In addition, the use of certain bytes in the random nonces as described in Section 4.1.3 allows the detection of downgrade to previous TLS versions. See [BBFKZG16] for more detail on TLS 1.3 and downgrade.

As soon as the client and the server have exchanged enough information to establish shared keys, the remainder of the handshake is encrypted, thus providing protection against passive attackers, even if the computed shared key is not authenticated. Because the server authenticates before the client, the client can ensure that if it authenticates to the server, it only reveals its identity to an authenticated server. Note that implementations must use the provided record padding mechanism during the handshake to avoid leaking information about the identities due to length. The client’s proposed PSK identities are not encrypted, nor is the one that the server selects.

E.1.1. Key Derivation and HKDF

Key derivation in TLS 1.3 uses the HKDF function defined in [RFC5869] and its two components, HKDF-Extract and HKDF-Expand. The full rationale for the HKDF construction can be found in [Kraw10] and the rationale for the way it is used in TLS 1.3 in [KW16]. Throughout this document, each application of HKDF-Extract is followed by one or more invocations of HKDF-Expand. This ordering should always be followed (including in future revisions of this document), in particular, one SHOULD NOT use an output of HKDF-Extract as an input to another application of HKDF-Extract without an HKDF-Expand in between. Consecutive applications of HKDF-Expand are allowed as long as these are differentiated via the key and/or the labels.

Note that HKDF-Expand implements a pseudorandom function (PRF) with both inputs and outputs of variable length. In some of the uses of HKDF in this document (e.g., for generating exporters and the resumption_master_secret), it is necessary that the application of HKDF-Expand be collision-resistant, namely, it should be infeasible to find two different inputs to HKDF-Expand that output the same value. This requires the underlying hash function to be collision resistant and the output length from HKDF-Expand to be of size at least 256 bits (or as much as needed for the hash function to prevent finding collisions).

E.1.2. Client Authentication

A client that has sent authentication data to a server, either during the handshake or in post-handshake authentication, cannot be sure if the server afterwards considers the client to be authenticated or not. If the client needs to determine if the server considers the connection to be unilaterally or mutually authenticated, this has to be provisioned by the application layer. See [CHHSV17] for details. In addition, the analysis of post-handshake authentication from [Kraw16] shows that the client identified by the certificate sent in the post-handshake phase possesses the traffic key. This party is therefore the client that participated in the original handshake or one to whom the original client delegated the traffic key (assuming that the traffic key has not been compromised).

E.1.3. 0-RTT

The 0-RTT mode of operation generally provides similar security properties as 1-RTT data, with the two exceptions that the 0-RTT encryption keys do not provide full forward secrecy and that the server is not able to guarantee uniqueness of the handshake (non-replayability) without keeping potentially undue amounts of state. See Section 4.2.9 for one mechanism to limit the exposure to replay.

E.1.4. Exporter Independence

The exporter_master_secret and early_exporter_master_secret are derived to be independent of the traffic keys and therefore do not represent a threat to the security of traffic encrypted with those keys. However, because these secrets can be used to compute any exporter value, they SHOULD be erased as soon as possible. If the total set of exporter labels is known, then implementations SHOULD pre-compute the inner Derive-Secret stage of the exporter computation for all those labels, then erase the [early_]exporter_master_secret, followed by each inner values as soon as it is known that it will not be needed again.

E.1.5. Post-Compromise Security

TLS does not provide security for handshakes which take place after the peer’s long-term secret (signature key or external PSK) is compromised. It therefore does not provide post-compromise security [CCG16], sometimes also referred to as backwards or future secrecy. This is in contrast to KCI resistance, which describes the security guarantees that a party has after its own long-term secret has been compromised.

E.1.6. External References

The reader should refer to the following references for analysis of the TLS handshake: [DFGS15] [CHSV16] [DFGS16] [KW16] [Kraw16] [FGSW16] [LXZFH16] [FG17] [BBK17].

E.2. Record Layer

The record layer depends on the handshake producing strong traffic secrets which can be used to derive bidirectional encryption keys and nonces. Assuming that is true, and the keys are used for no more data than indicated in Section 5.5 then the record layer should provide the following guarantees:

Confidentiality.
An attacker should not be able to determine the plaintext contents of a given record.
Integrity.
An attacker should not be able to craft a new record which is different from an existing record which will be accepted by the receiver.
Order protection/non-replayability
An attacker should not be able to cause the receiver to accept a record which it has already accepted or cause the receiver to accept record N+1 without having first processed record N.
Length concealment.
Given a record with a given external length, the attacker should not be able to determine the amount of the record that is content versus padding.
Forward secrecy after key change.
If the traffic key update mechanism described in Section 4.6.3 has been used and the previous generation key is deleted, an attacker who compromises the endpoint should not be able to decrypt traffic encrypted with the old key.

Informally, TLS 1.3 provides these properties by AEAD-protecting the plaintext with a strong key. AEAD encryption [RFC5116] provides confidentiality and integrity for the data. Non-replayability is provided by using a separate nonce for each record, with the nonce being derived from the record sequence number (Section 5.3), with the sequence number being maintained independently at both sides thus records which are delivered out of order result in AEAD deprotection failures. In order to prevent mass cryptanalysis when the same plaintext is repeatedly encrypted by different users under the same key (as is commonly the case for HTTP), the nonce is formed by mixing the sequence number with a secret per-connection initialization vector derived along with the traffic keys. See [BT16] for analysis of this construction.

The re-keying technique in TLS 1.3 (see Section 7.2) follows the construction of the serial generator in [REKEY], which shows that re-keying can allow keys to be used for a larger number of encryptions than without re-keying. This relies on the security of the HKDF-Expand-Label function as a pseudorandom function (PRF). In addition, as long as this function is truly one way, it is not possible to compute traffic keys from prior to a key change (forward secrecy).

TLS does not provide security for data which is communicated on a connection after a traffic secret of that connection is compromised. That is, TLS does not provide post-compromise security/future secrecy/backward secrecy with respect to the traffic secret. Indeed, an attacker who learns a traffic secret can compute all future traffic secrets on that connection. Systems which want such guarantees need to do a fresh handshake and establish a new connection with an (EC)DHE exchange.

E.2.1. External References

The reader should refer to the following references for analysis of the TLS record layer: [BMMT15] [BT16] [BDFKPPRSZZ16] [BBK17].

E.3. Traffic Analysis

TLS is susceptible to a variety of traffic analysis attacks based on observing the length and timing of encrypted packets [CLINIC] [HCJ16]. This is particularly easy when there is a small set of possible messages to be distinguished, such as for a video server hosting a fixed corpus of content, but still provides usable information even in more complicated scenarios.

TLS does not provide any specific defenses against this form of attack but does include a padding mechanism for use by applications: The plaintext protected by the AEAD function consists of content plus variable-length padding, which allows the application to produce arbitrary length encrypted records as well as padding-only cover traffic to conceal the difference between periods of transmission and periods of silence. Because the padding is encrypted alongside the actual content, an attacker cannot directly determine the length of the padding, but may be able to measure it indirectly by the use of timing channels exposed during record processing (i.e., seeing how long it takes to process a record or trickling in records to see which ones elicit a response from the server). In general, it is not known how to remove all of these channels because even a constant time padding removal function will then feed the content into data-dependent functions.

Note: Robust traffic analysis defences will likely lead to inferior performance due to delay in transmitting packets and increased traffic volume.

E.4. Side Channel Attacks

In general, TLS does not have specific defenses against side-channel attacks (i.e., those which attack the communications via secondary channels such as timing) leaving those to the implementation of the relevant cryptographic primitives. However, certain features of TLS are designed to make it easier to write side-channel resistant code:

Information leakage through side channels can occur at layers above TLS, in application protocols and the applications that use them. Resistance to side-channel attacks depends on applications and application protocols separately ensuring that confidential information is not inadvertently leaked.

E.5. Replay Attacks on 0-RTT

Replayable 0-RTT data presents a number of security threats to TLS-using applications, unless those applications are specifically engineered to be safe under replay (minimally, this means idempotent, but in many cases may also require other stronger conditions, such as constant-time response). Potential attacks include:

If data can be replayed a large number of times, additional attacks become possible, such as making repeated measurements of the the speed of cryptographic operations. In addition, they may be able to overload rate-limiting systems. For further description of these attacks, see [Mac17].

Ultimately, servers have the responsibility to protect themselves against attacks employing 0-RTT data replication. The mechanisms described in Section 8 are intended to prevent replay at the TLS layer do not provide complete protection against receiving multiple copies of client data. TLS 1.3 falls back to the 1-RTT handshake when the server does not have any information about the client, e.g., because it is in a different cluster which does not share state or because the ticket has been deleted as described in Section 8.1. If the application layer protocol retransmits data in this setting, then it is possible for an attacker to induce message duplication by sending the ClientHello to both the original cluster (which processes the data immediately) and another cluster which will fall back to 1-RTT and process the data upon application layer replay. The scale of this attack is limited by the client’s willingness to retry transactions and therefore only allows a limited amount of duplication, with each copy appearing as a new connection at the server.

If implemented correctly, the mechanisms described in Section 8.1 and Section 8.2 prevent a replayed ClientHello and its associated 0-RTT data from being accepted multiple times by any cluster with consistent state; for servers which limit the use of 0-RTT to one cluster for a single ticket, then a given ClientHello and its associated 0-RTT data will only be accepted once. However, if state is not completely consistent, then an attacker might be able to have multiple copies of the data be accepted during the replication window. Because clients do not know the exact details of server behavior, they MUST NOT send messages in early data which are not safe to have replayed and which they would not be willing to retry across multiple 1-RTT connections.

Application protocols MUST NOT use 0-RTT data without a profile that defines its use. That profile needs to identify which messages or interactions are safe to use with 0-RTT and how to handle the situation when the server rejects 0-RTT and falls back to 1-RTT.

In addition, to avoid accidental misuse, TLS implementations MUST NOT enable 0-RTT (either sending or accepting) unless specifically requested by the application and MUST NOT automatically resend 0-RTT data if it is rejected by the server unless instructed by the application. Server-side applications may wish to implement special processing for 0-RTT data for some kinds of application traffic (e.g., abort the connection, request that data be resent at the application layer, or delay processing until the handshake completes). In order to allow applications to implement this kind of processing, TLS implementations MUST provide a way for the application to determine if the handshake has completed.

E.5.1. Replay and Exporters

Replays of the ClientHello produce the same early exporter, thus requiring additional care by applications which use these exporters. In particular, if these exporters are used as an authentication channel binding (e.g., by signing the output of the exporter) an attacker who compromises the PSK can transplant authenticators between connections without compromising the authentication key.

In addition, the early exporter SHOULD NOT be used to generate server-to-client encryption keys because that would entail the reuse of those keys. This parallels the use of the early application traffic keys only in the client-to-server direction.

Appendix F. Working Group Information

The discussion list for the IETF TLS working group is located at the e-mail address tls@ietf.org. Information on the group and information on how to subscribe to the list is at https://www.ietf.org/mailman/listinfo/tls

Archives of the list can be found at: https://www.ietf.org/mail-archive/web/tls/current/index.html

Appendix G. Contributors

Author's Address

Eric Rescorla RTFM, Inc. EMail: ekr@rtfm.com
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