Client                                               Server

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

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

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

               () Indicates messages protected using keys
                  derived from early_traffic_secret.

               {} Indicates messages protected using keys
                  derived from handshake_traffic_secret.

               [] Indicates messages protected using keys
                  derived from traffic_secret_N

Figure 4: Message flow for a zero round trip handshake

[[OPEN ISSUE: Should it be possible to combine 0-RTT with the server authenticating via a signature https://github.com/tlswg/tls13-spec/issues/443]]

IMPORTANT NOTE: The security properties for 0-RTT data (regardless of the cipher suite) are weaker than those for other kinds of TLS data. Specifically:

1. This data is not forward secret, because it is encrypted solely with the PSK.
2. There are no guarantees of non-replay between connections. Unless the server takes special measures outside those provided by TLS (See Section 6.3.2.7.2), the server has no guarantee that the same 0-RTT data was not transmitted on multiple 0-RTT connections. This is especially relevant if the data is authenticated either with TLS client authentication or inside the application layer protocol. However, 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.)

The contents and significance of each message will be presented in detail in the following sections.

6.3. Handshake Protocol

The TLS Handshake Protocol is one of the defined higher-level clients of the TLS Record Protocol. This protocol is used to negotiate the secure attributes of a session. 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 session state.

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

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

The TLS Handshake Protocol messages are presented below in the order they MUST be sent; sending handshake messages in an unexpected order results in an “unexpected_message” fatal error. Unneeded handshake messages can be omitted, however.

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

6.3.1. Key Exchange Messages

The key exchange messages are used to exchange security capabilities between the client and server and to establish the traffic keys used to protect the handshake and the data.

6.3.1.1. Client Hello

When this message will be sent:

• 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 ServerHello that selects cryptographic parameters that don’t match the client’s “key_share” extension. In that case, the client MUST send the same ClientHello (without modification) except including a new KeyShareEntry as the lowest priority share (i.e., appended to the list of shares in the “key_share” extension). If a server receives a ClientHello at any other time, it MUST send a fatal “unexpected_message” alert and close the connection.

Structure of this message:

• The ClientHello message includes a random structure, which is used later in the protocol.

The cipher suite list, passed from the client to the server in the ClientHello message, contains the combinations of cryptographic algorithms supported by the client in order of the client’s preference (favorite choice first). Each cipher suite defines a key exchange algorithm, a record protection algorithm (including secret key length) and a hash to be used with HKDF. The server will select a cipher suite or, if no acceptable choices are presented, return a “handshake_failure” alert and close the connection. If the list contains cipher suites the server does not recognize, support, or wish to use, the server MUST ignore those cipher suites, and process the remaining ones as usual.

   struct {
       opaque random_bytes[32];
   } Random;

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

   struct {
       ProtocolVersion client_version = { 3, 4 };    /* TLS v1.3 */
       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<0..2^16-1>;
   } ClientHello;

TLS allows extensions to follow the compression_methods field in an extensions block. The presence of extensions can be detected by determining whether there are bytes following the compression_methods 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. As of TLS 1.3, all clients and servers will send at least one extension (at least “key_share” or “pre_shared_key”).

client_version

The version of the TLS protocol by which the client wishes to communicate during this session. This SHOULD be the latest (highest valued) version supported by the client. For this version of the specification, the version will be { 3, 4 }. (See Appendix C for details about backward compatibility.)

random

32 bytes generated by a secure random number generator. See Appendix B 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 6.2.2). This field MUST be ignored by a server negotiating TLS 1.3 and SHOULD be set as a zero length vector (i.e., a single zero byte length field) by clients which do not have a cached session ID set by a pre-TLS 1.3 server.

cipher_suites

This is a list of the cryptographic options supported by the client, with the client’s first preference first. Values are defined in Appendix A.4.

legacy_compression_methods

Versions of TLS before 1.3 supported compression and the list of compression methods was supplied in this field. For any 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 generate a fatal “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.

extensions

Clients request extended functionality from servers by sending data in the extensions field. The actual “Extension” format is defined in Section 6.3.2.

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. Note: TLS 1.3 ClientHello messages MUST always contain extensions, and a TLS 1.3 server MUST respond to any TLS 1.3 ClientHello without extensions with a fatal “decode_error” alert. TLS 1.3 servers may receive TLS 1.2 ClientHello messages without extensions. If negotiating TLS 1.2, a server MUST check that the amount of data in the message precisely matches one of these formats; if not, then it MUST send a fatal “decode_error” alert.

After sending the ClientHello message, the client waits for a ServerHello or HelloRetryRequest message.

6.3.1.2. Server Hello

When this message will be sent:

• The server will send this message in response to a ClientHello message when it was able to find an acceptable set of algorithms and the client’s “key_share” extension was acceptable. If the client proposed groups are not acceptable by the server, it will respond with a “handshake_failure” fatal alert.

Structure of this message:

   struct {
       ProtocolVersion server_version;
       Random random;
       CipherSuite cipher_suite;
       Extension extensions<0..2^16-1>;
   } ServerHello;

In prior versions of TLS, the extensions field could be omitted entirely if not needed, similar to ClientHello. As of TLS 1.3, all clients and servers will send at least one extension (at least “key_share” or “pre_shared_key”).

server_version

This field will contain the lower of that suggested by the client in the ClientHello and the highest supported by the server. For this version of the specification, the version is { 3, 4 }. (See Appendix C for details about backward compatibility.)

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. For resumed sessions, this field is the value from the state of the session being resumed. [[TODO: interaction with PSK.]]

extensions

A list of extensions. Note that only extensions offered by the client can appear in the server’s list. In TLS 1.3 as opposed to previous versions of TLS, the server’s extensions are split between the ServerHello and the EncryptedExtensions Section 6.3.3.1 message. The ServerHello MUST only include extensions which are required to establish the cryptographic context. Currently the only such extensions are “key_share”, “pre_shared_key”, and “early_data”. Clients MUST check the ServerHello for the presence of any forbidden extensions and if any are found MUST terminate the handshake with a “illegal_parameter” alert.

TLS 1.3 server implementations which respond to a ClientHello with a client_version indicating TLS 1.2 or below MUST set the first eight bytes of their Random value to the bytes:

  44 4F 57 4E 47 52 44 01

TLS 1.2 server implementations which respond to a ClientHello with a client_version indicating TLS 1.1 or below SHOULD set the first eight bytes of their Random value to the bytes:

  44 4F 57 4E 47 52 44 00

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

Note: This is an update to TLS 1.2 so in practice many TLS 1.2 clients and servers will not behave as specified above.

Note: Versions of TLS prior to TLS 1.3 used the top 32 bits of the Random value to encode the time since the UNIX epoch. The sentinel value above was selected to avoid conflicting with any valid TLS 1.2 Random value and to have a low (2^{-64}) probability of colliding with randomly selected Random values.

6.3.1.3. Hello Retry Request

When this message will be sent:

• Servers send this message in response to a ClientHello message when it was able to find an acceptable set of algorithms and groups that are mutually supported, but the client’s KeyShare did not contain an acceptable offer. If it cannot find such a match, it will respond with a fatal “handshake_failure” alert.

Structure of this message:

   struct {
       ProtocolVersion server_version;
       CipherSuite cipher_suite;
       NamedGroup selected_group;
       Extension extensions<0..2^16-1>;
   } HelloRetryRequest;

selected_group

The mutually supported group the server intends to negotiate and is requesting a retried ClientHello/KeyShare for.

The server_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.

Upon receipt of a HelloRetryRequest, the client MUST first verify that the selected_group field corresponds to a group which was provided in the “supported_groups” extension in the original ClientHello. It MUST then verify that 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 a fatal “handshake_failure” alert. Clients SHOULD also abort with “handshake_failure” in response to any second HelloRetryRequest which was sent in the same connection (i.e., where the ClientHello was itself in response to a HelloRetryRequest).

Otherwise, the client MUST send a ClientHello with an updated KeyShare extension to the server. The client MUST append a new KeyShareEntry for the group indicated in the selected_group field to the groups in its original KeyShare.

Upon re-sending the ClientHello and receiving the server’s ServerHello/KeyShare, the client MUST verify that the selected CipherSuite and NamedGroup match that supplied in the HelloRetryRequest. If either of these values differ, the client MUST abort the connection with a fatal “handshake_failure” alert.

6.3.2. Hello Extensions

The extension format is:

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

   enum {
       supported_groups(10),
       signature_algorithms(13),
       key_share(40),
       pre_shared_key(41),
       early_data(42),
       ticket_age(43),
       cookie (44),
       (65535)
   } ExtensionType;

Here:

• “extension_type” identifies the particular extension type.
• “extension_data” contains information specific to the particular extension type.

The initial set of extensions is defined in [RFC6066]. The list of extension types is maintained by IANA as described in Section 11.

An extension type MUST NOT appear in the ServerHello or HelloRetryRequest unless the same extension type appeared in the corresponding ClientHello. If a client receives an extension type in ServerHello or HelloRetryRequest that it did not request in the associated ClientHello, it MUST abort the handshake with an “unsupported_extension” fatal alert.

Nonetheless, “server-oriented” extensions may be provided in the future within this framework. Such an extension (say, of type x) would require the client to first send an extension of type x in a ClientHello with empty extension_data to indicate that it supports the extension type. In this case, the client is offering the capability to understand the extension type, and the server is taking the client up on its offer.

When multiple extensions of different types are present in the ClientHello or ServerHello messages, the extensions MAY appear in any order. There MUST NOT be more than one extension of the same type.

Finally, note that extensions can be sent both when starting a new session and when requesting session resumption or 0-RTT mode. Indeed, a client that requests session resumption does not in general know whether the server will accept this request, and therefore it SHOULD send the same extensions as it would send if it were not attempting resumption.

In general, the specification of each extension type needs to describe the effect of the extension both during full handshake and session resumption. Most current TLS extensions are relevant only when a session is initiated: when an older session is resumed, the server does not process these extensions in ClientHello, and does not include them in ServerHello. However, some extensions may specify different behavior during session resumption. [[TODO: update this and the previous paragraph to cover PSK-based resumption.]]

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:

• Some cases where a server does not agree to an extension are error conditions, and some are simply refusals to support particular features. In general, error alerts should be used for the former, and a field in the server extension response for the latter.
• Extensions should, as far as possible, be designed to prevent any attack that forces use (or non-use) of a particular feature by manipulation of handshake messages. This principle should be followed regardless of whether the feature is believed to cause a security problem. Often the fact that the extension fields are included in the inputs to the Finished message hashes will be sufficient, but extreme care is needed when the extension changes the meaning of messages sent in the handshake phase. Designers and implementors should be aware of the fact that until the handshake has been authenticated, active attackers can modify messages and insert, remove, or replace extensions.
• It would be technically possible to use extensions to change major aspects of the design of TLS; for example, the design of cipher suite negotiation. This is not recommended; it would be more appropriate to define a new version of TLS — particularly since the TLS handshake algorithms have specific protection against version rollback attacks based on the version number, and the possibility of version rollback should be a significant consideration in any major design change.

6.3.2.1. Cookie

   struct {
       opaque cookie<0..255>;
   } Cookie;

Cookies serve two primary purposes:

• Allowing the server to force the client to demonstrate reachability at their apparent network address (thus providing a measure of DoS protection). This is primarily useful for non-connection-oriented transports (see [RFC6347] for an example of this).
• Allowing the server to offload state to the client, thus allowing it to send a HelloRetryRequest without storing any state. The server does this by pickling that post-ClientHello hash state into the cookie (protected with some suitable integrity algorithm).

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 echo the value of the extension. Clients MUST NOT use cookies in subsequent connections.

6.3.2.2. 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 offer one or more cipher suites which use certificate authentication (i.e., any non-PSK cipher suite) MUST send the “signature_algorithms” extension. If this extension is not provided and no alternative cipher suite is available, the server MUST close the connection with a fatal “missing_extension” alert. (see Section 8.2)

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

   enum {
       /* RSASSA-PKCS-v1_5 algorithms */
       rsa_pkcs1_sha1 (0x0201),
       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 (0x0700),
       rsa_pss_sha384 (0x0701),
       rsa_pss_sha512 (0x0702),

       /* EdDSA algorithms */
       ed25519 (0x0703),
       ed448 (0x0704),

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

   SignatureScheme supported_signature_algorithms<2..2^16-2>;

Note: This production is named “SignatureScheme” because there is already a SignatureAlgorithm type in TLS 1.2. 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 function and then proceed as usual. The code point groups listed above have the following meanings:

RSASSA-PKCS-v1_5 algorithms

Indicates a signature algorithm using RSASSA-PKCS1-v1_5 [RFC3447] with the corresponding hash algorithm as defined in [SHS]. These values refer solely to signatures which appear in certificates (see Section 6.3.4.1.1) and are not defined for use in signed TLS handshake messages (see Section 4.8.1).

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 [RFC3447] with MGF1. 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 (see Section 4.8.1), the length of the salt MUST be equal to the length of the digest output.

EdDSA algorithms

Indicates a signature algorithm using EdDSA as defined in [I-D.irtf-cfrg-eddsa] or its successors. Note that these correspond to the “PureEdDSA” algorithms and not the “prehash” variants.

The semantics of this extension are somewhat complicated because the cipher suite adds additional constraints on signature algorithms. Section 6.3.4.1.1 describes the appropriate rules.

rsa_pkcs1_sha1 and dsa_sha1 SHOULD NOT be offered. Clients offering these values for backwards compatibility MUST list them as the lowest priority (listed after all other algorithms in the supported_signature_algorithms vector). TLS 1.3 servers MUST NOT offer a SHA-1 signed certificate unless no valid certificate chain can be produced without it (see Section 6.3.4.1.1).

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:

• TLS 1.2 ClientHellos may omit this extension.
• In TLS 1.2, the extension contained hash/signature pairs. The pairs are encoded in two octets, so SignatureScheme values have been allocated to align with TLS 1.2’s encoding. Some legacy pairs are left unallocated. These algorithms are deprecated as of TLS 1.3. They MUST NOT be offered or negotiated by any implementation. In particular, MD5 [SLOTH] and SHA-224 MUST NOT be used.
• ecdsa_secp256r1_sha256, etc., align with TLS 1.2’s ECDSA hash/signature pairs. However, the old semantics did not constrain the signing curve.

6.3.2.3. Negotiated Groups

When sent by the client, the “supported_groups” extension indicates the named groups which the client supports, 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 [I-D.ietf-tls-negotiated-ff-dhe]. This extension was also used to negotiate ECDSA curves. Signature algorithms are now negotiated independently (see Section 6.3.2.2).

Clients which offer one or more (EC)DHE cipher suites MUST send at least one supported NamedGroup value and servers MUST NOT negotiate any of these cipher suites unless a supported value was provided. If this extension is not provided and no alternative cipher suite is available, the server MUST close the connection with a fatal “missing_extension” alert. (see Section 8.2) If the extension is provided, but no compatible group is offered, the server MUST NOT negotiate a cipher suite of the relevant type. For instance, if a client supplies only ECDHE groups, the server MUST NOT negotiate finite field Diffie-Hellman. If no acceptable group can be selected across all cipher suites, then the server MUST generate a fatal “handshake_failure” alert.

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

   enum {
       /* Elliptic Curve Groups (ECDHE) */
       secp256r1 (23), secp384r1 (24), secp521r1 (25),
       x25519 (29), x448 (30),

       /* Finite Field Groups (DHE) */
       ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
       ffdhe6144 (259), ffdhe8192 (260),

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

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

Elliptic Curve Groups (ECDHE)

Indicates support of the corresponding named curve. Note that some curves are also recommended in ANSI X9.62 [X962] and FIPS 186-4 [DSS]. Others are recommended 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 [I-D.ietf-tls-negotiated-ff-dhe]. 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. 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. 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 offer to a server in subsequent connections.

[[TODO: IANA Considerations.]]

6.3.2.4. Key Share

The “key_share” extension contains the endpoint’s cryptographic parameters for non-PSK key establishment methods (currently DHE or ECDHE).

Clients which offer one or more (EC)DHE cipher suites MUST send this extension and SHOULD send at least one supported KeyShareEntry value. Servers MUST NOT negotiate any of these cipher suites unless a supported value was provided. If this extension is not provided in a ServerHello or ClientHello, and the peer is offering (EC)DHE cipher suites, then the endpoint MUST close the connection with a fatal “missing_extension” alert. (see Section 8.2) 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 6.3.1.3)

   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 6.3.2.4.1; Elliptic Curve Diffie-Hellman parameters are described in Section 6.3.2.4.2.

key_exchange

Key exchange information. The contents of this field are determined by the specified group and its corresponding definition. Endpoints MUST NOT send empty or otherwise invalid key_exchange values for any reason.

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

   struct {
       select (role) {
           case client:
               KeyShareEntry client_shares<0..2^16-1>;

           case server:
               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. The ordering of values here SHOULD match that of the ordering of offered support in the “supported_groups” extension.

server_share

A single KeyShareEntry value for the negotiated cipher suite.

Servers offer exactly one KeyShareEntry value, which corresponds to the key exchange used for the negotiated cipher suite.

Clients 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 by generated independently. Clients MUST NOT offer multiple KeyShareEntry values for the same group and servers receiving multiple KeyShareEntry values for the same group MUST abort the connection with a fatal “illegal_parameter” alert. Clients and servers MUST NOT offer or accept any KeyShareEntry values for groups not listed in the client’s “supported_groups” extension. Servers MUST NOT offer a KeyShareEntry value for a group not offered by the client in its corresponding KeyShare.

If the server selects an (EC)DHE cipher suite and no mutually supported group is available between the two endpoints’ KeyShare offers, yet there is a mutually supported group that can be found via the “supported_groups” extension, then the server MUST reply with a HelloRetryRequest. If there is no mutually supported group at all, the server MUST NOT negotiate an (EC)DHE cipher suite.

[[TODO: Recommendation about what the client offers. Presumably which integer DH groups and which curves.]]

6.3.2.4.1. 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), encoded as a big-endian integer, padded with zeros to the size of p.

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

6.3.2.4.2. 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 byte string representation of an elliptic curve public value following the conversion routine in Section 4.3.6 of ANSI X9.62 [X962].

Although X9.62 supports multiple point formats, any given curve MUST specify only a single point format. All curves currently specified in this document MUST only be used with the uncompressed point format (the format for all ECDH functions is considered uncompressed).

For x25519 and x448, the contents 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 negotiation; TLS 1.3 removes this feature in favor of a single point format for each curve.

6.3.2.5. 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 a PSK or (EC)DHE-PSK cipher suite (see [RFC4279] for background).

Clients which offer one or more PSK cipher suites MUST send at least one supported psk_identity value and servers MUST NOT negotiate any of these cipher suites unless a supported value was provided. If this extension is not provided and no alternative cipher suite is available, the server MUST close the connection with a fatal “missing_extension” alert. (see Section 8.2)

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

   opaque psk_identity<0..2^16-1>;

   struct {
       select (Role) {
           case client:
               psk_identity identities<2..2^16-1>;

           case server:
               uint16 selected_identity;
       }
   } PreSharedKeyExtension;

identities

A list of the identities (labels for keys) that the client is willing to negotiate with the server.

selected_identity

The server’s chosen identity expressed as a (0-based) index into the identies in the client’s list.

If no suitable identity is provided, the server MUST NOT negotiate a PSK cipher suite and MAY respond with an “unknown_psk_identity” alert message. Sending this alert is OPTIONAL; servers MAY instead choose to send a “decrypt_error” alert to merely indicate an invalid PSK identity or instead negotiate use of a non-PSK cipher suite, if available.

If the server selects a PSK cipher suite, it MUST send a “pre_shared_key” extension with the identity that it selected. The client MUST verify that the server’s selected_identity is within the range supplied by the client. If any other value is returned, the client MUST generate a fatal “unknown_psk_identity” alert and close the connection.

6.3.2.6. OCSP Status Extensions

[RFC6066] and [RFC6961] provide extensions to negotiate the server sending OCSP responses to the client. In TLS 1.2 and below, the server sends 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 EncryptedExtensions. Specifically: The body of the “status_request” or “status_request_v2” extension from the server MUST be a CertificateStatus structure as defined in [RFC6066] and [RFC6961] respectively.

Note: this means that the certificate status appears prior to the certificates it applies to. This is slightly anomalous but matches the existing behavior for SignedCertificateTimestamps [RFC6962], and is more easily extensible in the handshake state machine.

6.3.2.7. Early Data Indication

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

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

   struct {
       select (Role) {
           case client:
               opaque context<0..255>;

           case server:
              struct {};
       }
   } EarlyDataIndication;

context

An optional context value that can be used for anti-replay (see below).

All of the parameters for the 0-RTT data (symmetric cipher suite, ALPN, etc.) MUST be 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.

0-RTT messages sent in the first flight have the same content types as their corresponding messages sent in other flights (handshake, application_data, and alert respectively) but are protected under different keys. After all the 0-RTT application data messages (if any) have been sent, a “end_of_early_data” alert of type “warning” is sent to indicate the end of the flight. 0-RTT MUST always be followed by an “end_of_early_data” alert.

A server which receives an “early_data” extension can behave in one of two ways:

• Ignore the extension and return no response. This indicates that the server has ignored any early data and an ordinary 1-RTT handshake is required.
• Return an empty extension, indicating that it intends to process the early data. It is not possible for the server to accept only a subset of the early data messages.

[[OPEN ISSUE: are the rules below correct? https://github.com/tlswg/tls13-spec/issues/451]] Prior to accepting the “early_data” extension, the server MUST validate that the session ticket parameters are consistent with its current configuration. It MUST also validate that the extensions negotiated in the previous connection are identical to those being negotiated in the ServerHello, with the exception of the following extensions:

• The use of “signed_certificate_timestamp” [RFC6962] MUST be identical but the server’s SCT extension value may differ.
• The “padding” extension [RFC7685] MUST be ignored for this purpose.
• The values of “key_share”, “pre_shared_key”, and “early_data”, which MUST be as defined in this document.

In addition, it MUST validate that the ticket_age is within a small tolerance of the time since the ticket was issued (see Section 6.3.2.7.2).

If any of these checks fail, the server MUST NOT respond with the extension and must discard all the remaining first flight data (thus falling back to 1-RTT). If the client attempts a 0-RTT handshake but the server rejects it, it will generally not have the 0-RTT record protection keys and must instead trial decrypt each record with the 1-RTT handshake keys until it finds one that decrypts properly, and then pick up the handshake from that point.

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, decryption failure of any 0-RTT record following an accepted “early_data” extension MUST produce a fatal “bad_record_mac” alert as per Section 5.2.2. Implementations SHOULD determine the security parameters for the 1-RTT phase of the connection entirely before processing the EncryptedExtensions and Finished, using those values solely to determine whether to accept or reject 0-RTT data.

[[TODO: How does the client behave if the indication is rejected.]]

6.3.2.7.1. Processing Order

Clients are permitted to “stream” 0-RTT data until they receive the server’s Finished, only then sending the “end_of_early_data” alert. In order to avoid deadlock, when accepting “early_data”, servers MUST process the client’s Finished and then immediately send the ServerHello, rather than waiting for the client’s “end_of_early_data” alert.

6.3.2.7.2. Replay Properties

As noted in Section 6.2.3, TLS provides only a limited inter-connection mechanism for replay protection for data sent by the client in the first flight.

The “ticket_age” extension sent by the client SHOULD be used by servers to limit the time over which the first flight might be replayed. A server can store the time at which it sends a server configuration to a client, or encode the time in a ticket. Then, each time it receives an early_data extension, it can check to see if the value used by the client matches its expectations.

The “ticket_age” value provided by the client will be shorter than the actual time elapsed on the server by a single round trip time. This difference is comprised of the delay in sending the NewSessionTicket message to the client, plus the time taken to send the ClientHello to the server. For this reason, a server SHOULD measure the round trip time prior to sending the NewSessionTicket message and account for that in the value it saves.

There are several potential sources of error that make an exact measurement of time difficult. Variations in client and server clocks are likely to be minimal, outside of gross time corrections. Network propagation delays are 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.

A small allowance for errors in clocks and variations in measurements is advisable. However, any allowance also increases the opportunity for replay. In this case, it is better to reject early data than to risk greater exposure to replay attacks.

6.3.2.8. Ticket Age

   struct {
       uint32 ticket_age;
   } TicketAge;

When the client sends the “early_data” extension, it MUST also send a “ticket_age” extension in its EncryptedExtensions block. This value contains the time elapsed since the client learned about the server configuration that it is using, in milliseconds. This value can be used by the server to limit the time over which early data can be replayed. Note: because ticket lifetimes are restricted to a week, 32 bits is enough to represent any plausible age, even in milliseconds.

6.3.3. Server Parameters

6.3.3.1. Encrypted Extensions

When this message will be sent:

• 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 handshake_traffic_secret. If the client indicates “early_data” in its ClientHello, it MUST also send EncryptedExtensions immediately following the ClientHello and immediately prior to the Finished.

Meaning of this message:

• The EncryptedExtensions message contains any extensions which should be protected, i.e., any which are not needed to establish the cryptographic context.

The same extension types MUST NOT appear in both the ServerHello and EncryptedExtensions. If the same extension appears in both locations, the client MUST rely only on the value in the EncryptedExtensions block. All server-sent extensions other than those explicitly listed in Section 6.3.1.2 or designated in the IANA registry MUST only appear in EncryptedExtensions. Extensions which are designated to appear in ServerHello MUST NOT appear in EncryptedExtensions. Clients MUST check EncryptedExtensions for the presence of any forbidden extensions and if any are found MUST terminate the handshake with an “illegal_parameter” alert.

The client’s EncryptedExtensions apply only to the early data with which they appear. Servers MUST NOT use them to negotiate the rest of the handshake. Only those extensions explicitly designated as being included in 0-RTT Encrypted Extensions in the IANA registry can be sent in the client’s EncryptedExtensions.

Structure of this message:

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

extensions

A list of extensions.

6.3.3.2. Certificate Request

When this message will be sent:

• A non-anonymous server can optionally request a certificate from the client, if appropriate for the selected cipher suite. This message, if sent, will follow EncryptedExtensions.

Structure of this message:

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

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

   struct {
       opaque certificate_request_context<0..2^8-1>;
       SignatureScheme
         supported_signature_algorithms<2..2^16-2>;
       DistinguishedName certificate_authorities<0..2^16-1>;
       CertificateExtension certificate_extensions<0..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).

supported_signature_algorithms

A list of the signature algorithms that the server is able to verify, listed in descending order of preference. Any certificates provided by the client MUST be signed using a signature algorithm found in supported_signature_algorithms.

certificate_authorities

A list of the distinguished names [X501] of acceptable certificate_authorities, represented in DER-encoded [X690] format. These distinguished names may specify a desired distinguished name for a root CA or for a subordinate CA; thus, this message can be used to describe known roots as well as a desired authorization space. If the certificate_authorities list is empty, then the client MAY send any certificate that meets the rest of the selection criteria in the CertificateRequest, unless there is some external arrangement to the contrary.

certificate_extensions

A list of certificate extension OIDs [RFC5280] with their allowed values, 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 certificate_extensions list, the client certificate 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 has 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 session without client authentication, or terminate the session with a fatal 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]:

• The Key Usage extension in a certificate matches the request when all key usage bits asserted in the request are also asserted in the Key Usage certificate extension.
• The Extended Key Usage extension in a certificate matches the request when all key purpose OIDs present in the request are also found in the Extended Key Usage certificate extension. The special anyExtendedKeyUsage OID MUST NOT be used in the request.

Separate specifications may define matching rules for other certificate extensions.

Note: It is a fatal “handshake_failure” alert for an anonymous server to request client authentication.

6.3.4. Authentication Messages

As discussed in Section 6.2, TLS uses a common set of messages for authentication, key confirmation, and handshake integrity: Certificate, CertificateVerify, and Finished. These 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.

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

• The certificate and signing key to be used.
• A Handshake Context based on the hash of the handshake messages
• A base key to be used to compute a MAC key.

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 the 0-RTT case.

CertificateVerify

A signature over the value Hash(Handshake Context + Certificate) + Hash(resumption_context) See Section 6.3.5.1 for the definition of resumption_context.

Finished

A MAC over the value Hash(Handshake Context + Certificate + CertificateVerify) + Hash(resumption_context) using a MAC key derived from the base key.

Because the CertificateVerify signs the Handshake Context + Certificate and the Finished MACs the Handshake Context + Certificate + CertificateVerify, this is mostly equivalent to keeping a running hash of the handshake messages (exactly so in the pure 1-RTT cases). Note, however, that subsequent post-handshake authentications do not include each other, just the messages through the end of the main handshake.

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

Mode	Handshake Context	Base Key
0-RTT	ClientHello	early_traffic_secret
1-RTT (Server)	ClientHello … later of EncryptedExtensions/CertificateRequest	handshake_traffic_secret
1-RTT (Client)	ClientHello … ServerFinished	handshake_traffic_secret
Post-Handshake	ClientHello … ClientFinished + CertificateRequest	traffic_secret_0

Note: The Handshake Context for the last three rows does not include any 0-RTT handshake messages, regardless of whether 0-RTT is used.

6.3.4.1. Certificate

When this message will be sent:

• 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 server has requested client authentication via a CertificateRequest message (Section 6.3.3.2). 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).

Meaning of this message:

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

• The certificate MUST be appropriate for the negotiated cipher suite’s key exchange algorithm and any negotiated extensions.

Structure of this message:

   opaque ASN1Cert<1..2^24-1>;

   struct {
       opaque certificate_request_context<0..2^8-1>;
       ASN1Cert 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 or client authentication in 0-RTT, this field SHALL be zero length.

certificate_list

This is a sequence (chain) of certificates. The sender’s certificate MUST come first 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.

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.

6.3.4.1.1. Server Certificate Selection

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

• The certificate type MUST be X.509v3 [RFC5280], unless explicitly negotiated otherwise (e.g., [RFC5081]).
• The server’s end-entity certificate’s public key (and associated restrictions) MUST be compatible with the selected key exchange algorithm.

Key Exchange Alg.	Certificate Key Type
DHE_RSA or ECDHE_RSA	RSA public key
ECDHE_ECDSA	ECDSA or EdDSA public key

• The certificate MUST allow the key to be used for signing (i.e., the digitalSignature bit MUST be set if the Key Usage extension is present) with a signature scheme indicated in the client’s “signature_algorithms” extension.
• The “server_name” and “trusted_ca_keys” extensions [RFC6066] are used to guide certificate selection. As servers MAY require the presence of the “server_name” extension, clients SHOULD send this extension.

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 6.3.2.2). 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 MAY use the deprecated SHA-1 hash algorithm only if the “signature_algorithms” extension provided by the client permits it. If the client cannot construct an acceptable chain using the provided certificates and decides to abort the handshake, then it MUST send an “unsupported_certificate” alert message and close the connection.

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).

As cipher suites that specify new key exchange methods are specified for the TLS protocol, they will imply the certificate format and the required encoded keying information.

6.3.4.1.2. Client Certificate Selection

The following rules apply to certificates sent by the client:

In particular:

• The certificate type MUST be X.509v3 [RFC5280], unless explicitly negotiated otherwise (e.g., [RFC5081]).
• If the certificate_authorities list in the certificate request message was non-empty, one of the certificates in the certificate chain SHOULD be issued by one of the listed CAs.
• The certificates MUST be signed using an acceptable hash/ signature algorithm pair, as described in Section 6.3.3.2. Note that this relaxes the constraints on certificate-signing algorithms found in prior versions of TLS.
• If the certificate_extensions list in the certificate request message was non-empty, the end-entity certificate MUST match the extension OIDs recognized by the client, as described in Section 6.3.3.2.

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

6.3.4.1.3. 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 terminate the handshake with a fatal “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 respond with a fatal “handshake_failure” 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 send a fatal alert.

Any endpoint receiving any certificate signed using any signature algorithm using an MD5 hash MUST send a “bad_certificate” alert message and close the connection.

SHA-1 is deprecated and therefore NOT RECOMMENDED. Endpoints that reject certification paths due to use of a deprecated hash MUST send a fatal “bad_certificate” alert message before closing the connection. 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).

6.3.4.2. Certificate Verify

When this message will be sent:

• This message is used to provide explicit proof that an endpoint possesses the private key corresponding to its certificate and also provides integrity for the handshake up to this point. Servers MUST send this message when using a cipher suite which is authenticated 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 {
        digitally-signed struct {
           opaque hashed_data[hash_length];
        };
   } CertificateVerify;

• Where hashed_data is the hash output described in Section 6.3.4, namely Hash(Handshake Context + Certificate) + Hash(resumption_context). For concreteness, this means that the value that is signed is:

       padding + context_string + 00 + hashed_data

• The context string for a server signature is “TLS 1.3, server CertificateVerify” and for a client signature is “TLS 1.3, client CertificateVerify”. A hash of the handshake messages is signed rather than the messages themselves because the digitally-signed format requires padding and context bytes at the beginning of the input. Thus, by signing a digest of the messages, an implementation only needs to maintain a single running hash per hash type for CertificateVerify, Finished and other messages.

• If 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 6.3.2.2). Note that there is a possibility for inconsistencies here. For instance, the client might offer ECDHE_ECDSA key exchange but omit any ECDSA and EdDSA values from its “signature_algorithms” extension. In order to negotiate correctly, the server MUST check any candidate cipher suites against the “signature_algorithms” extension before selecting them. This is somewhat inelegant but is a compromise designed to minimize changes to the original cipher suite design.

• 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 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-PKCS-v1_5 algorithms appear in “signature_algorithms”. SHA-1 MUST NOT be used in any signatures in CertificateVerify. (Note that rsa_pkcs1_sha1 and dsa_sha1, the only defined SHA-1 signature algorithms, are undefined for CertificateVerify signatures.)

Note: When used with non-certificate-based handshakes (e.g., PSK), the client’s signature does not cover the server’s certificate directly, although it does cover the server’s Finished message, which transitively includes the server’s certificate when the PSK derives from a certificate-authenticated handshake. [PSK-FINISHED] describes a concrete attack on this mode if the Finished is omitted from the signature. 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. In order to ensure this, implementations MUST NOT mix certificate-based client authentication with pure PSK modes (i.e., those where the PSK was not derived from a previous non-PSK handshake).

6.3.4.3. Finished

When this message will be sent:

• 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.

Meaning of this message:

• Recipients of Finished messages MUST verify that the contents are correct. 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.

The key used to compute the finished message is computed from the Base key defined in Section 6.3.4 using HKDF (see Section 7.1). Specifically:

client_finished_key =
    HKDF-Expand-Label(BaseKey, "client finished", "", L)

server_finished_key =
    HKDF-Expand-Label(BaseKey, "server finished", "", L)

Structure of this message:

   struct {
       opaque verify_data[verify_data_length];
   } Finished;

The verify_data value is computed as follows:

   verify_data =
       HMAC(finished_key, Hash(
              Handshake Context + Certificate* + CertificateVerify* 
           ) + Hash(resumption_context)
           )

   * Only included if present.

Where 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 the current version of TLS, 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.

6.3.5. 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 application traffic key.

6.3.5.1. 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 pre-shared key (PSK) binding between the ticket value and the following two values derived from the resumption master secret:

   resumption_psk = HKDF-Expand-Label(resumption_secret,
                                      "resumption psk", "", L)

   resumption_context = HKDF-Expand-Label(resumption_secret,
                                          "resumption context", "", L)

The client MAY use this PSK for future handshakes by including the ticket value in the “pre_shared_key” extension in its ClientHello (Section 6.3.2.5) and supplying a suitable PSK cipher suite. Servers may send multiple tickets on a single connection, for instance after post-handshake authentication. For handshakes that do not use a resumption_psk, the resumption_context is a string of L zeroes.

 enum { (65535) } TicketExtensionType;

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

 enum {
   allow_early_data(1)
   allow_dhe_resumption(2),
   allow_psk_resumption(4)
 } TicketFlags;
 
 struct {
     uint32 ticket_lifetime;
     uint32 flags;
     TicketExtension extensions<2..2^16-2>;
     opaque ticket<0..2^16-1>;
 } NewSessionTicket;

flags

A 32-bit value indicating the ways in which this ticket may be used (as an OR of the flags values).

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 more than 604800 seconds (7 days). The value of zero indicates that the ticket should be discarded immediately. Clients MUST NOT cache session tickets for longer than 7 days, regardless of the ticket_lifetime. It 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_extensions

A placeholder for extensions in the ticket. Clients MUST ignore unrecognized extensions.

ticket

The value of the ticket to be used as the PSK identifier. 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.

The meanings of the flags are as follows:

allow_early_data

When resuming with this ticket, the client MAY send data in its first flight (early data) encrypted under a key derived from this PSK.

allow_dhe_resumption

This ticket MAY be used with (EC)DHE-PSK cipher suite

allow_psk_resumption

This ticket MAY be used with a pure PSK cipher suite.

In all cases, the PSK or (EC)DHE-PSK cipher suites that the client offers/uses MUST have the same symmetric parameters (cipher/hash) as the cipher suite negotiated for this connection. If no flags are set that the client recognizes, it MUST ignore the ticket.

6.3.5.2. Post-Handshake Authentication

The server is permitted to request client authentication at any time after the handshake has completed by sending a CertificateRequest message. The client SHOULD respond with the appropriate Authentication messages. 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.

Note: Because client authentication may require 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).

6.3.5.3. Key and IV Update

struct {} KeyUpdate;

The KeyUpdate handshake message is used to indicate that the sender is updating its sending cryptographic keys. This message can be sent by the server after sending its first flight and the client after sending its second flight. Implementations that receive a KeyUpdate message prior to receiving a Finished message as part of the 1-RTT handshake MUST generate a fatal “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 their receiving keys and if they have not already updated their sending state up to or past the then current receiving generation MUST send their own KeyUpdate prior to sending any other messages. This mechanism allows either side to force an update to the entire connection. Note that implementations may receive an arbitrary number of messages between sending a KeyUpdate and receiving the peer’s KeyUpdate because those messages may already be in flight.

Note that if implementations independently send their own KeyUpdates and they cross in flight, this only results in an update of one generation; when each side receives the other side’s update it just updates its receive keys and notes that the generations match and thus no send update is needed.

Note that the side which sends its KeyUpdate first needs to retain the traffic keys (though not the traffic secret) for the previous generation of keys until it receives the KeyUpdate from the other side.

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.

7. Cryptographic Computations

In order to begin connection protection, the TLS Record Protocol requires specification of a suite of algorithms, a master secret, and the client and server random values. The authentication, key exchange, and record protection algorithms are determined by the cipher_suite selected by the server and revealed in the ServerHello message. The random values are exchanged in the hello messages. All that remains is to calculate the key schedule.

7.1. Key Schedule

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 makes use of the following functions, based on HKDF [RFC5869]:

  HKDF-Extract(Salt, IKM) as defined in {{RFC5869}}.
  
  HKDF-Expand-Label(Secret, Label, Messages, Length) =
       HKDF-Expand(Secret, HkdfLabel, Length)

  Where HkdfLabel is specified as:

  struct HkdfLabel {
    uint16 length;
    opaque label<9..255>;
    opaque hash_value<0..255>;
  };

  - HkdfLabel.length is Length
  - HkdfLabel.label is "TLS 1.3, " + Label
  - HkdfLabel.hash_value is HashValue.

  Derive-Secret(Secret, Label, Messages) =
       HKDF-Expand-Label(Secret, Label,
                         Hash(Messages) + Hash(resumption_context), L))

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 0s as long as the size of the Hash that is the basis for the HKDF. Concretely, for the present version of TLS 1.3, secrets are added in the following order:

• PSK
• (EC)DHE shared secret

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

• HKDF-Extract is drawn as taking the Salt argument from the top and the IKM argument from the left.
• Derive-Secret’s Secret argument is indicated by the arrow coming in from the left. For instance, the Early Secret is the Secret for generating the early_traffic-secret.

                 0
                 |
                 v
   PSK ->  HKDF-Extract
                 |
                 v
           Early Secret  --> Derive-Secret(., "early traffic secret",
                 |                         ClientHello)
                 |                         = early_traffic_secret 
                 v
(EC)DHE -> HKDF-Extract
                 |
                 v
              Handshake
               Secret -----> Derive-Secret(., "handshake traffic secret",
                 |                         ClientHello + ServerHello)
                 |                         = handshake_traffic_secret
                 v
      0 -> HKDF-Extract
                 |
                 v
            Master Secret
                 |
                 +---------> Derive-Secret(., "application traffic secret",
                 |                         ClientHello...Server Finished)
                 |                         = traffic_secret_0
                 |                        
                 +---------> Derive-Secret(., "exporter master secret",
                 |                         ClientHello...Client Finished)
                 |                         = exporter_secret       
                 |
                 +---------> Derive-Secret(., "resumption master secret",
                                           ClientHello...Client Finished)
                                           = resumption_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 L zeroes is used.

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 Section 6.3.5.3. The next generation of traffic keys is computed by generating traffic_secret_N+1 from traffic_secret_N as described in this section then re-deriving the traffic keys as described in Section 7.3.

The next-generation traffic_secret is computed as:

traffic_secret_N+1 = HKDF-Expand-Label(traffic_secret_N, “application traffic secret”, “”, L)

Once traffic_secret_N+1 and its associated traffic keys have been computed, implementations SHOULD delete traffic_secret_N. Once the directional keys are no longer needed, they SHOULD be deleted as well.

7.3. Traffic Key Calculation

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

• A secret value
• A phase value indicating the phase of the protocol the keys are being generated for.
• A purpose value indicating the specific value being generated
• The length of the key.

The keying material is computed using:

   key = HKDF-Expand-Label(Secret,
                           phase + ", " + purpose, "",
                           key_length)

The following table describes the inputs to the key calculation for each class of traffic keys:

Record Type	Secret	Phase
0-RTT Handshake	early_traffic_secret	“early handshake key expansion”
0-RTT Application	early_traffic_secret	“early application data key expansion”
Handshake	handshake_traffic_secret	“handshake key expansion”
Application Data	traffic_secret_N	“application data key expansion”

The following table indicates the purpose values for each type of key:

Key Type	Purpose
Client Write Key	“client write key”
Server Write Key	“server write key”
Client Write IV	“client write iv”
Server Write IV	“server write iv”

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).

7.3.1. Diffie-Hellman

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

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

7.3.2. 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:

• The public key to put into the KeyShareEntry.key_exchange structure is the result of applying the ECDH function to the secret key of appropriate length (into scalar input) and the standard public basepoint (into u-coordinate point input).
• The ECDH shared secret is the result of applying ECDH function to the secret key (into scalar input) and the peer’s public key (into u-coordinate point input). The output is used raw, with no processing.

For X25519 and X448, see [RFC7748].

7.3.3. Exporters

[RFC5705] defines keying material exporters for TLS in terms of the TLS PRF. This document replaces the PRF with HKDF, thus requiring a new construction. The exporter interface remains the same, however the value is computed as:

HKDF-Expand-Label(exporter_secret,
                  label, context_value, key_length)

8. Mandatory Algorithms

8.1. MTI Cipher Suites

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

    TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
    TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

These cipher suites MUST support both digital signatures and key exchange with secp256r1 (NIST P-256) and SHOULD support key exchange with X25519 [RFC7748].

A TLS-compliant application SHOULD implement the following cipher suites:

    TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
    TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
    TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
    TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256

8.2. MTI Extensions

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

• Signature Algorithms (“signature_algorithms”; Section 6.3.2.2)
• Negotiated Groups (“supported_groups”; Section 6.3.2.3)
• Key Share (“key_share”; Section 6.3.2.4)
• Pre-Shared Key (“pre_shared_key”; Section 6.3.2.5)
• Server Name Indication (“server_name”; Section 3 of [RFC6066])
• Cookie (“cookie”; Section 6.3.2.1)

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

• “signature_algorithms” is REQUIRED for certificate authenticated cipher suites
• “supported_groups” and “key_share” are REQUIRED for DHE or ECDHE cipher suites
• “pre_shared_key” is REQUIRED for PSK cipher suites
• “cookie” is REQUIRED for all cipher suites.

When negotiating use of applicable cipher suites, endpoints MUST abort the connection with a “missing_extension” alert if the required extension was not provided. Any endpoint that receives any invalid combination of cipher suites and extensions MAY abort the connection with a “missing_extension” alert, regardless of negotiated parameters.

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 with a fatal “missing_extension” alert.

Servers MUST NOT send the “signature_algorithms” extension; if a client receives this extension it MUST respond with a fatal “unsupported_extension” alert and close the connection.

9. Application Data Protocol

Application data messages are carried by the record layer and are fragmented and encrypted based on the current connection state. The messages are treated as transparent data to the record layer.

10. Security Considerations

Security issues are discussed throughout this memo, especially in Appendices B, C, and D.

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:

• TLS Cipher Suite Registry: Values with the first byte in the range 0-254 (decimal) are assigned via Specification Required [RFC2434]. Values with the first byte 255 (decimal) are reserved for Private Use [RFC2434]. IANA [SHALL add/has added] a “Recommended” column to the cipher suite registry. All cipher suites listed in Appendix A.4 are marked as “Yes”. All other cipher suites are marked as “No”. IANA [SHALL add/has added] add a note to this column reading:
  • Cipher suites marked as “Yes” are those allocated via Standards Track RFCs. Cipher suites marked as “No” are not; cipher suites marked “No” range from “good” to “bad” from a cryptographic standpoint.
• TLS ContentType Registry: Future values are allocated via Standards Action [RFC2434].
• TLS Alert Registry: Future values are allocated via Standards Action [RFC2434].
• TLS HandshakeType Registry: Future values are allocated via Standards Action [RFC2434].

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

• TLS ExtensionType Registry: Values with the first byte in the range 0-254 (decimal) are assigned via Specification Required [RFC2434]. Values with the first byte 255 (decimal) are reserved for Private Use [RFC2434]. IANA [SHALL update/has updated] this registry to include the “key_share”, “pre_shared_key”, and “early_data” extensions as defined in this document.
  
  IANA [shall update/has updated] this registry to include a “TLS 1.3” column with the following four values: “Client”, indicating that the server shall not send them. “Clear”, indicating that they shall be in the ServerHello. “Encrypted”, indicating that they shall be in the EncryptedExtensions block, “Early”, indicating that they shall be only in the client’s 0-RTT EncryptedExtensions block, and “No” indicating that they are not used in TLS 1.3. This column [shall be/has been] initially populated with the values in this document. IANA [shall update/has updated] this registry to add a “Recommended” column. IANA [shall/has] initially populated this column with the values in the table below. This table has been generated by marking Standards Track RFCs as “Yes” and all others as “No”.

Extension	Recommended	TLS 1.3
server_name [RFC6066]	Yes	Encrypted
max_fragment_length [RFC6066]	Yes	Encrypted
client_certificate_url [RFC6066]	Yes	Encrypted
trusted_ca_keys [RFC6066]	Yes	Encrypted
truncated_hmac [RFC6066]	Yes	No
status_request [RFC6066]	Yes	No
user_mapping [RFC4681]	Yes	Encrypted
client_authz [RFC5878]	No	Encrypted
server_authz [RFC5878]	No	Encrypted
cert_type [RFC6091]	Yes	Encrypted
supported_groups [RFC-ietf-tls-negotiated-ff-dhe]	Yes	Encrypted
ec_point_formats [RFC4492]	Yes	No
srp [RFC5054]	No	No
signature_algorithms [RFC5246]	Yes	Client
use_srtp [RFC5764]	Yes	Encrypted
heartbeat [RFC6520]	Yes	Encrypted
application_layer_protocol_negotiation [RFC7301]	Yes	Encrypted
status_request_v2 [RFC6961]	Yes	Encrypted
signed_certificate_timestamp [RFC6962]	No	Encrypted
client_certificate_type [RFC7250]	Yes	Encrypted
server_certificate_type [RFC7250]	Yes	Encrypted
padding [RFC7685]	Yes	Client
encrypt_then_mac [RFC7366]	Yes	No
extended_master_secret [RFC7627]	Yes	No
SessionTicket TLS [RFC4507]	Yes	No
renegotiation_info [RFC5746]	Yes	No
key_share [[this document]]	Yes	Clear
pre_shared_key [[this document]]	Yes	Clear
early_data [[this document]]	Yes	Clear
ticket_age [[this document]]	Yes	Early
cookie [[this document]]	Yes	Encrypted/HelloRetryRequest

In addition, this document defines two new registries to be maintained by IANA

• TLS SignatureScheme Registry: Values with the first byte in the range 0-254 (decimal) are assigned via Specification Required [RFC2434]. Values with the first byte 255 (decimal) are reserved for Private Use [RFC2434]. This registry SHALL have a “Recommended” column. The registry [shall be/ has been] initially populated with the values described in Section 6.3.2.2. The following values SHALL be marked as “Recommended”: ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, rsa_pss_sha256, rsa_pss_sha384, rsa_pss_sha512, ed25519.

12. References

12.1. Normative References

[AES]	National Institute of Standards and Technology, "Specification for the Advanced Encryption Standard (AES)", NIST FIPS 197, November 2001.
[DH]	Diffie, W. and M. Hellman, "New Directions in Cryptography", IEEE Transactions on Information Theory, V.IT-22 n.6 , June 1977.
[I-D.ietf-tls-chacha20-poly1305]	Langley, A., Chang, W., Mavrogiannopoulos, N., Strombergson, J. and S. Josefsson, "ChaCha20-Poly1305 Cipher Suites for Transport Layer Security (TLS)", Internet-Draft draft-ietf-tls-chacha20-poly1305-04, December 2015.
[I-D.irtf-cfrg-eddsa]	Josefsson, S. and I. Liusvaara, "Edwards-curve Digital Signature Algorithm (EdDSA)", Internet-Draft draft-irtf-cfrg-eddsa-05, March 2016.
[I-D.mattsson-tls-ecdhe-psk-aead]	Mattsson, J. and D. Migault, "ECDHE_PSK with AES-GCM and AES-CCM Cipher Suites for Transport Layer Security (TLS)", Internet-Draft draft-mattsson-tls-ecdhe-psk-aead-05, April 2016.
[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.
[RFC2434]	Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, DOI 10.17487/RFC2434, October 1998.
[RFC3447]	Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 2003.
[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.
[RFC5288]	Salowey, J., Choudhury, A. and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, DOI 10.17487/RFC5288, August 2008.
[RFC5289]	Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289, DOI 10.17487/RFC5289, August 2008.
[RFC5487]	Badra, M., "Pre-Shared Key Cipher Suites for TLS with SHA-256/384 and AES Galois Counter Mode", RFC 5487, DOI 10.17487/RFC5487, March 2009.
[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.
[RFC6209]	Kim, W., Lee, J., Park, J. and D. Kwon, "Addition of the ARIA Cipher Suites to Transport Layer Security (TLS)", RFC 6209, DOI 10.17487/RFC6209, April 2011.
[RFC6367]	Kanno, S. and M. Kanda, "Addition of the Camellia Cipher Suites to Transport Layer Security (TLS)", RFC 6367, DOI 10.17487/RFC6367, September 2011.
[RFC6655]	McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for Transport Layer Security (TLS)", RFC 6655, DOI 10.17487/RFC6655, July 2012.
[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.
[RFC7251]	McGrew, D., Bailey, D., Campagna, M. and R. Dugal, "AES-CCM Elliptic Curve Cryptography (ECC) Cipher Suites for TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014.
[RFC7748]	Langley, A., Hamburg, M. and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016.
[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

, "

[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.
[FI06]	Finney, H., "Bleichenbacher's RSA signature forgery based on implementation error", August 2006.
[GCM]	Dworkin, M., "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", NIST Special Publication 800-38D, November 2007.
[I-D.ietf-tls-cached-info]	Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", Internet-Draft draft-ietf-tls-cached-info-23, May 2016.
[I-D.ietf-tls-negotiated-ff-dhe]	Gillmor, D., "Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for TLS", Internet-Draft draft-ietf-tls-negotiated-ff-dhe-10, June 2015.
[IEEE1363]	IEEE, "Standard Specifications for Public Key Cryptography", IEEE 1363 , 2000.
[PKCS6]	RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax Standard, version 1.5", November 1993.
[PKCS7]	RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax Standard, version 1.5", November 1993.
[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.
[RFC0793]	Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981.
[RFC1948]	Bellovin, S., "Defending Against Sequence Number Attacks", RFC 1948, DOI 10.17487/RFC1948, May 1996.
[RFC4086]	Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC4279]	Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", RFC 4279, DOI 10.17487/RFC4279, December 2005.
[RFC4302]	Kent, S., "IP Authentication Header", RFC 4302, DOI 10.17487/RFC4302, December 2005.
[RFC4303]	Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 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.
[RFC4506]	Eisler, M., "XDR: External Data Representation Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May 2006.
[RFC4507]	Salowey, J., Zhou, H., Eronen, P. and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 4507, DOI 10.17487/RFC4507, May 2006.
[RFC4681]	Santesson, S., Medvinsky, A. and J. Ball, "TLS User Mapping Extension", RFC 4681, DOI 10.17487/RFC4681, October 2006.
[RFC5054]	Taylor, D., Wu, T., Mavrogiannopoulos, N. and T. Perrin, "Using the Secure Remote Password (SRP) Protocol for TLS Authentication", RFC 5054, DOI 10.17487/RFC5054, November 2007.
[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.
[RFC5081]	Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport Layer Security (TLS) Authentication", RFC 5081, DOI 10.17487/RFC5081, November 2007.
[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.
[RFC5746]	Rescorla, E., Ray, M., Dispensa, S. and N. Oskov, "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010.
[RFC5763]	Fischl, J., Tschofenig, H. and E. Rescorla, "Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May 2010.
[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.
[RFC5878]	Brown, M. and R. Housley, "Transport Layer Security (TLS) Authorization Extensions", RFC 5878, DOI 10.17487/RFC5878, 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.
[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.
[RFC7366]	Gutmann, P., "Encrypt-then-MAC for Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 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.
[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.
[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. 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.

A.1. Record Layer

   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

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

   struct {
       ContentType type;
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       opaque fragment[TLSPlaintext.length];
   } TLSPlaintext;

   struct {
       ContentType opaque_type = application_data(23); /* see fragment.type */
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       aead-ciphered struct {
          opaque content[TLSPlaintext.length];
          ContentType type;
          uint8 zeros[length_of_padding];
       } fragment;
   } TLSCiphertext;

A.2. Alert Messages

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

   enum {
       close_notify(0),
       end_of_early_data(1),
       unexpected_message(10),               /* fatal */
       bad_record_mac(20),                   /* fatal */
       decryption_failed_RESERVED(21),       /* fatal */
       record_overflow(22),                  /* fatal */
       decompression_failure_RESERVED(30),   /* fatal */
       handshake_failure(40),                /* fatal */
       no_certificate_RESERVED(41),          /* fatal */
       bad_certificate(42),
       unsupported_certificate(43),
       certificate_revoked(44),
       certificate_expired(45),
       certificate_unknown(46),
       illegal_parameter(47),                /* fatal */
       unknown_ca(48),                       /* fatal */
       access_denied(49),                    /* fatal */
       decode_error(50),                     /* fatal */
       decrypt_error(51),                    /* fatal */
       export_restriction_RESERVED(60),      /* fatal */
       protocol_version(70),                 /* fatal */
       insufficient_security(71),            /* fatal */
       internal_error(80),                   /* fatal */
       inappropriate_fallback(86),           /* fatal */
       user_canceled(90),
       no_renegotiation_RESERVED(100),       /* fatal */
       missing_extension(109),               /* fatal */
       unsupported_extension(110),           /* fatal */
       certificate_unobtainable(111),
       unrecognized_name(112),
       bad_certificate_status_response(113), /* fatal */
       bad_certificate_hash_value(114),      /* fatal */
       unknown_psk_identity(115),
       (255)
   } AlertDescription;

   struct {
       AlertLevel level;
       AlertDescription description;
   } Alert;

A.3. Handshake Protocol

   enum {
       hello_request_RESERVED(0),
       client_hello(1),
       server_hello(2),
       session_ticket(4),
       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),
       (255)
   } HandshakeType;

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

A.3.1. Key Exchange Messages

   struct {
       opaque random_bytes[32];
   } Random;

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

   struct {
       ProtocolVersion client_version = { 3, 4 };    /* TLS v1.3 */
       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<0..2^16-1>;
   } ClientHello;

   struct {
       ProtocolVersion server_version;
       Random random;
       CipherSuite cipher_suite;
       Extension extensions<0..2^16-1>;
   } ServerHello;

   struct {
       ProtocolVersion server_version;
       CipherSuite cipher_suite;
       NamedGroup selected_group;
       Extension extensions<0..2^16-1>;
   } HelloRetryRequest;

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

   enum {
       supported_groups(10),
       signature_algorithms(13),
       key_share(40),
       pre_shared_key(41),
       early_data(42),
       ticket_age(43),
       cookie (44),
       (65535)
   } ExtensionType;

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

   struct {
       select (role) {
           case client:
               KeyShareEntry client_shares<0..2^16-1>;

           case server:
               KeyShareEntry server_share;
       }
   } KeyShare;

   opaque psk_identity<0..2^16-1>;

   struct {
       select (Role) {
           case client:
               psk_identity identities<2..2^16-1>;

           case server:
               uint16 selected_identity;
       }
   } PreSharedKeyExtension;

   struct {
       select (Role) {
           case client:
               opaque context<0..255>;

           case server:
              struct {};
       }
   } EarlyDataIndication;

   struct {
       uint32 ticket_age;
   } TicketAge;

A.3.1.1. Cookie Extension

   struct {
       opaque cookie<0..255>;
   } Cookie;

A.3.1.2. Signature Algorithm Extension

   enum {
       /* RSASSA-PKCS-v1_5 algorithms */
       rsa_pkcs1_sha1 (0x0201),
       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 (0x0700),
       rsa_pss_sha384 (0x0701),
       rsa_pss_sha512 (0x0702),

       /* EdDSA algorithms */
       ed25519 (0x0703),
       ed448 (0x0704),

       /* Reserved Code Points */
       dsa_sha1_RESERVED (0x0202),
       dsa_sha256_RESERVED (0x0402),
       dsa_sha384_RESERVED (0x0502),
       dsa_sha512_RESERVED (0x0602),
       obsolete_RESERVED (0x0000..0x0200),
       obsolete_RESERVED (0x0203..0x0400),
       obsolete_RESERVED (0x0404..0x0500),
       obsolete_RESERVED (0x0504..0x0600),
       obsolete_RESERVED (0x0604..0x06FF),
       private_use (0xFE00..0xFFFF),
       (0xFFFF)
   } SignatureScheme;

   SignatureScheme supported_signature_algorithms<2..2^16-2>;

A.3.1.3. Named Group Extension

   enum {
       /* Elliptic Curve Groups (ECDHE) */
       obsolete_RESERVED (1..22),
       secp256r1 (23), secp384r1 (24), secp521r1 (25),
       obsolete_RESERVED (26..28),
       x25519 (29), x448 (30),

       /* Finite Field Groups (DHE) */
       ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
       ffdhe6144 (259), ffdhe8192 (260),

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

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

Values within “obsolete_RESERVED” ranges were 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.

A.3.1.4. Deprecated Extensions

The following extensions are no longer applicable to TLS 1.3, although TLS 1.3 clients MAY send them if they are willing to negotiate them with prior versions of TLS. TLS 1.3 servers MUST ignore these extensions if they are negotiating TLS 1.3: truncated_hmac [RFC6066], srp [RFC5054], encrypt_then_mac [RFC7366], extended_master_secret [RFC7627], SessionTicket [RFC5077], and renegotiation_info [RFC5746].

A.3.2. Server Parameters Messages

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

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

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

   struct {
       opaque certificate_request_context<0..2^8-1>;
       SignatureScheme
         supported_signature_algorithms<2..2^16-2>;
       DistinguishedName certificate_authorities<0..2^16-1>;
       CertificateExtension certificate_extensions<0..2^16-1>;
   } CertificateRequest;

A.3.3. Authentication Messages

   opaque ASN1Cert<1..2^24-1>;

   struct {
       opaque certificate_request_context<0..2^8-1>;
       ASN1Cert certificate_list<0..2^24-1>;
   } Certificate;

   struct {
        digitally-signed struct {
           opaque hashed_data[hash_length];
        };
   } CertificateVerify;

   struct {
       opaque verify_data[verify_data_length];
   } Finished;

A.3.4. Ticket Establishment

 enum { (65535) } TicketExtensionType;

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

 enum {
   allow_early_data(1)
   allow_dhe_resumption(2),
   allow_psk_resumption(4)
 } TicketFlags;
 
 struct {
     uint32 ticket_lifetime;
     uint32 flags;
     TicketExtension extensions<2..2^16-2>;
     opaque ticket<0..2^16-1>;
 } NewSessionTicket;

A.4. Cipher Suites

A cipher suite defines a cipher specification supported in TLS and negotiated via hello messages in the TLS handshake. Cipher suite names follow a general naming convention composed of a series of component algorithm names separated by underscores:

   CipherSuite TLS_KEA_AUTH_WITH_CIPHER_HASH = VALUE;

Component	Contents
TLS	The string “TLS”
KEA	The key exchange algorithm (e.g. ECDHE, DHE)
AUTH	The authentication algorithm (e.g. certificates, PSK)
WITH	The string “WITH”
CIPHER	The symmetric cipher used for record protection
HASH	The hash algorithm used with HKDF
VALUE	The two byte ID assigned for this cipher suite

The “CIPHER” component commonly has sub-components used to designate the cipher name, bits, and mode, if applicable. For example, “AES_256_GCM” represents 256-bit AES in the GCM mode of operation. Cipher suite names that lack a “HASH” value that are defined for use with TLS 1.2 or later use the SHA-256 hash algorithm by default.

The primary key exchange algorithm used in TLS is Ephemeral Diffie-Hellman [DH]. The finite field based version is denoted “DHE” and the elliptic curve based version is denoted “ECDHE”. Prior versions of TLS supported non-ephemeral key exchanges, however these are not supported by TLS 1.3.

See the definitions of each cipher suite in its specification document for the full details of each combination of algorithms that is specified.

The following is a list of standards track server-authenticated (and optionally client-authenticated) cipher suites which are currently available in TLS 1.3:

Cipher Suite Name	Value	Specification
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256	{0x00,0x9E}	[RFC5288]
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384	{0x00,0x9F}	[RFC5288]
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256	{0xC0,0x2B}	[RFC5289]
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384	{0xC0,0x2C}	[RFC5289]
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256	{0xC0,0x2F}	[RFC5289]
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384	{0xC0,0x30}	[RFC5289]
TLS_DHE_RSA_WITH_AES_128_CCM	{0xC0,0x9E}	[RFC6655]
TLS_DHE_RSA_WITH_AES_256_CCM	{0xC0,0x9F}	[RFC6655]
TLS_DHE_RSA_WITH_AES_128_CCM_8	{0xC0,0xA2}	[RFC6655]
TLS_DHE_RSA_WITH_AES_256_CCM_8	{0xC0,0xA3}	[RFC6655]
TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256	{0xCC,0xA8}	[I-D.ietf-tls-chacha20-poly1305]
TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256	{0xCC,0xA9}	[I-D.ietf-tls-chacha20-poly1305]
TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256	{0xCC,0xAA}	[I-D.ietf-tls-chacha20-poly1305]

Note: The values listed for ChaCha/Poly are preliminary but are being or will be used for interop testing and therefore are likely to be assigned.

Note: ECDHE AES GCM was not yet standards track prior to the publication of this specification. This document promotes the above-listed ciphers to standards track.

The following is a list of standards track ephemeral pre-shared key cipher suites which are currently available in TLS 1.3:

Cipher Suite Name	Value	Specification
TLS_DHE_PSK_WITH_AES_128_GCM_SHA256	{0x00,0xAA}	[RFC5487]
TLS_DHE_PSK_WITH_AES_256_GCM_SHA384	{0x00,0xAB}	[RFC5487]
TLS_DHE_PSK_WITH_AES_128_CCM	{0xC0,0xA6}	[RFC6655]
TLS_DHE_PSK_WITH_AES_256_CCM	{0xC0,0xA7}	[RFC6655]
TLS_PSK_DHE_WITH_AES_128_CCM_8	{0xC0,0xAA}	[RFC6655]
TLS_PSK_DHE_WITH_AES_256_CCM_8	{0xC0,0xAB}	[RFC6655]
TLS_ECDHE_PSK_WITH_AES_128_GCM_SHA256	{0xD0,0x01}	[I-D.mattsson-tls-ecdhe-psk-aead]
TLS_ECDHE_PSK_WITH_AES_256_GCM_SHA384	{0xD0,0x02}	[I-D.mattsson-tls-ecdhe-psk-aead]
TLS_ECDHE_PSK_WITH_AES_128_CCM_8_SHA256	{0xD0,0x03}	[I-D.mattsson-tls-ecdhe-psk-aead]
TLS_ECDHE_PSK_WITH_AES_128_CCM_SHA256	{0xD0,0x04}	[I-D.mattsson-tls-ecdhe-psk-aead]
TLS_ECDHE_PSK_WITH_AES_256_CCM_SHA384	{0xD0,0x05}	[I-D.mattsson-tls-ecdhe-psk-aead]
TLS_ECDHE_PSK_WITH_CHACHA20_POLY1305_SHA256	{0xCC,0xAC}	[I-D.ietf-tls-chacha20-poly1305]
TLS_DHE_PSK_WITH_CHACHA20_POLY1305_SHA256	{0xCC,0xAD}	[I-D.ietf-tls-chacha20-poly1305]

Note: The values listed for ECDHE and ChaCha/Poly are preliminary but are being or will be used for interop testing and therefore are likely to be assigned.

Note: [RFC6655] is inconsistent with respect to the ordering of components within PSK AES CCM cipher suite names. The names above are as defined.

All cipher suites in this section are specified for use with both TLS 1.2 and TLS 1.3, as well as the corresponding versions of DTLS. (see Appendix C)

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

A.4.1. Unauthenticated Operation

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

• Raw public keys [RFC7250].
• Using a public key contained in a certificate but without validation of the certificate chain or any of its contents.

Either technique used alone is are 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 channel bindings [RFC5929]. [[NOTE: TLS 1.3 needs a new channel binding definition that has not yet been defined.]] 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.

A.5. The Security Parameters

These security parameters are determined by the TLS Handshake Protocol and provided as parameters to the TLS record layer in order to initialize a connection state. SecurityParameters includes:

   enum { server, client } ConnectionEnd;

   enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;

   enum { aes_gcm } RecordProtAlgorithm;

   /* The algorithms specified in KDFAlgorithm and
      RecordProtAlgorithm may be added to. */

   struct {
       ConnectionEnd          entity;
       KDFAlgorithm           kdf_algorithm;
       RecordProtAlgorithm    record_prot_algorithm;
       uint8                  enc_key_length;
       uint8                  iv_length;
       opaque                 hs_master_secret[48];
       opaque                 master_secret[48];
       opaque                 client_random[32];
       opaque                 server_random[32];
   } SecurityParameters;

A.6. Changes to RFC 4492

RFC 4492 [RFC4492] adds Elliptic Curve cipher suites to TLS. This document changes some of the structures used in that document. This section details the required changes for implementors of both RFC 4492 and TLS 1.2. Implementors of TLS 1.2 who are not implementing RFC 4492 do not need to read this section.

This document adds an “algorithm” field to the digitally-signed element in order to identify the signature and digest algorithms used to create a signature. This change applies to digital signatures formed using ECDSA as well, thus allowing ECDSA signatures to be used with digest algorithms other than SHA-1, provided such use is compatible with the certificate and any restrictions imposed by future revisions of [RFC5280].

As described in Section 6.3.4.1.1, the restrictions on the signature algorithms used to sign certificates are no longer tied to the cipher suite. Thus, the restrictions on the algorithm used to sign certificates specified in Sections 2 and 3 of RFC 4492 are also relaxed. As in this document, the restrictions on the keys in the end-entity certificate remain.

Appendix B. Implementation Notes

The TLS protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementors.

B.1. Random Number Generation and Seeding

TLS requires a cryptographically secure pseudorandom number generator (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash operations, most notably SHA-256, are acceptable, but cannot provide more security than the size of the random number generator state.

To estimate the amount of seed material being produced, add the number of bits of unpredictable information in each seed byte. For example, keystroke timing values taken from a PC compatible 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of the counter value is 16 bits or more. Seeding a 128-bit PRNG would thus require approximately 100 such timer values.

[RFC4086] provides guidance on the generation of random values.

B.2. Certificates and Authentication

Implementations are responsible for verifying the integrity of certificates and should generally support certificate revocation messages. Certificates should always be verified to ensure proper signing by a trusted Certificate Authority (CA). The selection and addition of trusted CAs should be done very carefully. Users should be able to view information about the certificate and root CA.

B.3. Cipher Suite Support

TLS supports a range of key sizes and security levels, including some that provide no or minimal security. A proper implementation will probably not support many cipher suites. 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. See also Appendix C.4.

B.4. 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:

• Do you correctly handle handshake messages that are fragmented to multiple TLS records (see Section 5.2.1)? Including corner cases like a ClientHello that is split to several small fragments? Do you fragment handshake messages that exceed the maximum fragment size? In particular, the certificate and certificate request handshake messages can be large enough to require fragmentation.
• Do you ignore the TLS record layer version number in all TLS records? (see Appendix C)
• Have you ensured that all support for SSL, RC4, EXPORT ciphers, and MD5 (via the “signature_algorithm” extension) is completely removed from all possible configurations that support TLS 1.3 or later, and that attempts to use these obsolete capabilities fail correctly? (see Appendix C)
• Do you handle TLS extensions in ClientHello correctly, including unknown extensions or omitting the extensions field completely?
• When the server has requested a client certificate, but no suitable certificate is available, do you correctly send an empty Certificate message, instead of omitting the whole message (see Section 6.3.4.1.2)?
• When processing the plaintext fragment produced by AEAD-Decrypt and scanning from the end for the ContentType, do you avoid scanning past the start of the cleartext in the event that the peer has sent a malformed plaintext of all-zeros?
• When processing a ClientHello containing a version of { 3, 5 } or higher, do you respond with the highest common version of TLS rather than requiring an exact match?
• Do you ignore unrecognized cipher suites (see Section 6.3.1.1), named groups (see Section 6.3.2.3), and signature algorithms (see Section 6.3.2.2)?

Cryptographic details:

• What countermeasures do you use to prevent timing attacks against RSA signing operations [TIMING]?
• When verifying RSA signatures, do you accept both NULL and missing parameters (see Section 4.8)? Do you verify that the RSA padding doesn’t have additional data after the hash value? [FI06]
• When using Diffie-Hellman key exchange, do you correctly preserve leading zero bytes in the negotiated key (see Section 7.3.1)?
• Does your TLS client check that the Diffie-Hellman parameters sent by the server are acceptable (see Appendix D.1.1.1)?
• Do you use a strong and, most importantly, properly seeded random number generator (see Appendix B.1) Diffie-Hellman private values, the ECDSA “k” parameter, and other security-critical values?
• Do you zero-pad Diffie-Hellman public key values to the group size (see Section 6.3.2.4.1)?

B.5. Client Tracking Prevention

Clients SHOULD NOT reuse a session ticket for multiple connections. Reuse of a session ticket allows passive observers to correlate different connections. Servers that issue session tickets SHOULD offer at least as many session 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 session tickets with every connection. This ensures that clients are always able to use a new session ticket when creating a new connection.

Appendix C. 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.record_version & TLSCiphertext.record_version) As of TLS 1.3, this field is deprecated and its value MUST be ignored by all implementations. Version negotiation is performed using only the handshake versions. (ClientHello.client_version & ServerHello.server_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 6.3.4.1.1)

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.

C.1. Negotiating with an older server

A TLS 1.3 client who wishes to negotiate with such older servers will send a normal TLS 1.3 ClientHello containing { 3, 4 } (TLS 1.3) in ClientHello.client_version. If the server does not support this version 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 resuming a session SHOULD initiate the connection using the version that was previously negotiated.

Note that 0-RTT data is not compatible with older servers. See Appendix C.3.

If the version chosen by the server is not supported by the client (or not acceptable), the client MUST send a “protocol_version” alert message and close the connection.

If a TLS server receives a ClientHello containing a version number greater than the highest version supported by the server, it MUST reply according to the highest version supported by the server.

Some legacy server implementations are known to not implement the TLS specification properly and might abort connections upon encountering TLS extensions or versions which it is 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.

C.2. Negotiating with an older client

A TLS server can also receive a ClientHello containing a version number smaller than the highest supported version. If the server wishes to negotiate with old clients, it will proceed as appropriate for the highest version supported by the server that is not greater than ClientHello.client_version. For example, if the server supports TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the server will proceed with a TLS 1.0 ServerHello. If the server only supports versions greater than client_version, it MUST send a “protocol_version” alert message and close the connection.

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

C.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 fail to complete the handshake. This can cause issues when a client offers 0-RTT, particularly against multi-server deployments. For example, a deployment may deploy TLS 1.3 gradually with some servers implementing TLS 1.3 and some implementing TLS 1.2, or a TLS 1.3 deployment may be downgraded to TLS 1.2.

If a client accepts older versions of TLS and receives an older ServerHello after sending a ClientHello with 0-RTT data, it MAY retry the connection without 0-RTT. It is NOT RECOMMENDED to retry the connection in response to a more generic error or advertise lower versions of TLS.

Multi-server deployments are RECOMMENDED to ensure a stable deployment of TLS 1.3 without 0-RTT prior to enabling 0-RTT.

C.4. Backwards Compatibility Security Restrictions

If an implementation negotiates use of TLS 1.2, then negotiation of cipher suites also supported by TLS 1.3 SHOULD be preferred, if 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 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 or accept any records with a version less than { 3, 0 }.

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

Implementations MUST NOT send a ClientHello.client_version or ServerHello.server_version set to { 3, 0 } or less. Any endpoint receiving a Hello message with ClientHello.client_version or ServerHello.server_version set to { 3, 0 } MUST respond with a “protocol_version” alert message and close the connection.

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

Appendix D. Security Analysis

[[TODO: The entire security analysis needs a rewrite.]]

The TLS protocol is designed to establish a secure connection between a client and a server communicating over an insecure channel. This document makes several traditional assumptions, including that attackers have substantial computational resources and cannot obtain secret information from sources outside the protocol. Attackers are assumed to have the ability to capture, modify, delete, replay, and otherwise tamper with messages sent over the communication channel. This appendix outlines how TLS has been designed to resist a variety of attacks.

D.1. Handshake Protocol

The TLS Handshake Protocol is responsible for selecting a cipher spec and generating a master secret, which together comprise the primary cryptographic parameters associated with a secure session. The TLS Handshake Protocol can also optionally authenticate parties who have certificates signed by a trusted certificate authority.

D.1.1. Authentication and Key Exchange

TLS supports three authentication modes: authentication of both parties, server authentication with an unauthenticated client, and total anonymity. Whenever the server is authenticated, the channel is secure against man-in-the-middle attacks, but completely anonymous sessions are inherently vulnerable to such attacks. Anonymous servers cannot authenticate clients. If the server is authenticated, its certificate message must provide a valid certificate chain leading to an acceptable certificate authority. Similarly, authenticated clients must supply an acceptable certificate to the server. Each party is responsible for verifying that the other’s certificate is valid and has not expired or been revoked.

[[TODO: Rewrite this because the master_secret is not used this way any more after Hugo’s changes.]] The general goal of the key exchange process is to create a master_secret known to the communicating parties and not to attackers (see Section 7.1). The master_secret is required to generate the Finished messages and record protection keys (see Section 6.3.4.3 and Section 7.3). By sending a correct Finished message, parties thus prove that they know the correct master_secret.

D.1.1.1. Diffie-Hellman Key Exchange with Authentication

When Diffie-Hellman key exchange is used, the client and server use the “key_share” extension to send temporary Diffie-Hellman parameters. The signature in the certificate verify message (if present) covers the entire handshake up to that point and thus attests the certificate holder’s desire to use the ephemeral DHE keys.

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

Additionally, using a fresh key for each handshake provides Perfect Forward Secrecy. Implementations SHOULD generate a new X for each handshake when using DHE cipher suites.

D.1.2. Version Rollback Attacks

Because TLS includes substantial improvements over SSL Version 2.0, attackers may try to make TLS-capable clients and servers fall back to Version 2.0. This attack can occur if (and only if) two TLS-capable parties use an SSL 2.0 handshake. (See also Appendix C.4.)

Although the solution using non-random PKCS #1 block type 2 message padding is inelegant, it provides a reasonably secure way for Version 3.0 servers to detect the attack. This solution is not secure against attackers who can brute-force the key and substitute a new ENCRYPTED-KEY-DATA message containing the same key (but with normal padding) before the application-specified wait threshold has expired. Altering the padding of the least-significant 8 bytes of the PKCS padding does not impact security for the size of the signed hashes and RSA key lengths used in the protocol, since this is essentially equivalent to increasing the input block size by 8 bytes.

D.1.3. Detecting Attacks Against the Handshake Protocol

An attacker might try to influence the handshake exchange to make the parties select different encryption algorithms than they would normally choose.

For this attack, an attacker must actively change one or more handshake messages. If this occurs, the client and server will compute different values for the handshake message hashes. As a result, the parties will not accept each others’ Finished messages. Without the static secret, the attacker cannot repair the Finished messages, so the attack will be discovered.

D.2. Protecting Application Data

The shared secrets are hashed with the handshake transcript to produce unique record protection secrets for each connection.

Outgoing data is protected using an AEAD algorithm before transmission. The authentication data includes the sequence number, message type, message length, and the message contents. The message type field is necessary to ensure that messages intended for one TLS record layer client are not redirected to another. The sequence number ensures that attempts to delete or reorder messages will be detected. Since sequence numbers are 64 bits long, they should never overflow. Messages from one party cannot be inserted into the other’s output, since they use independent keys.

D.3. Denial of Service

TLS is susceptible to a number of denial-of-service (DoS) attacks. In particular, an attacker who initiates a large number of TCP connections can cause a server to consume large amounts of CPU doing asymmetric crypto operations. However, because TLS is generally used over TCP, it is difficult for the attacker to hide their point of origin if proper TCP SYN randomization is used [RFC1948] by the TCP stack.

Because TLS runs over TCP, it is also susceptible to a number of DoS attacks on individual connections. In particular, attackers can forge RSTs, thereby terminating connections, or forge partial TLS records, thereby causing the connection to stall. These attacks cannot in general be defended against by a TCP-using protocol. Implementors or users who are concerned with this class of attack should use IPsec AH [RFC4302] or ESP [RFC4303].

D.4. Final Notes

For TLS to be able to provide a secure connection, both the client and server systems, keys, and applications must be secure. In addition, the implementation must be free of security errors.

The system is only as strong as the weakest key exchange and authentication algorithm supported, and only trustworthy cryptographic functions should be used. Short public keys and anonymous servers should be used with great caution. Implementations and users must be careful when deciding which certificates and certificate authorities are acceptable; a dishonest certificate authority can do tremendous damage.

Appendix E. 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://www1.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 F. Contributors

• Martin Abadi
  University of California, Santa Cruz
  abadi@cs.ucsc.edu
• Christopher Allen (co-editor of TLS 1.0)
  Alacrity Ventures
  ChristopherA@AlacrityManagement.com
• Steven M. Bellovin
  Columbia University
  smb@cs.columbia.edu
• David Benjamin
  Google
  davidben@google.com
• Benjamin Beurdouche
• Karthikeyan Bhargavan (co-author of [RFC7627])
  INRIA
  karthikeyan.bhargavan@inria.fr
• Simon Blake-Wilson (co-author of [RFC4492])
  BCI
  sblakewilson@bcisse.com
• Nelson Bolyard (co-author of [RFC4492])
  Sun Microsystems, Inc.
  nelson@bolyard.com
• Ran Canetti
  IBM
  canetti@watson.ibm.com
• Pete Chown
  Skygate Technology Ltd
  pc@skygate.co.uk
• Antoine Delignat-Lavaud (co-author of [RFC7627])
  INRIA
  antoine.delignat-lavaud@inria.fr
• Tim Dierks (co-editor of TLS 1.0, 1.1, and 1.2)
  Independent
  tim@dierks.org
• Taher Elgamal
  Securify
  taher@securify.com
• Pasi Eronen
  Nokia
  pasi.eronen@nokia.com
• Cedric Fournet
  Microsoft
  fournet@microsoft.com
• Anil Gangolli
  anil@busybuddha.org
• David M. Garrett
• Vipul Gupta (co-author of [RFC4492])
  Sun Microsystems Laboratories
  vipul.gupta@sun.com
• Chris Hawk (co-author of [RFC4492])
  Corriente Networks LLC
  chris@corriente.net
• Kipp Hickman
• Alfred Hoenes
• David Hopwood
  Independent Consultant
  david.hopwood@blueyonder.co.uk
• Subodh Iyengar
  Facebook
  subodh@fb.com
• Daniel Kahn Gillmor
  ACLU
  dkg@fifthhorseman.net
• Phil Karlton (co-author of SSL 3.0)
• Paul Kocher (co-author of SSL 3.0)
  Cryptography Research
  paul@cryptography.com
• Hugo Krawczyk
  IBM
  hugo@ee.technion.ac.il
• Adam Langley (co-author of [RFC7627])
  Google
  agl@google.com
• Ilari Liusvaara
  Independent
  ilariliusvaara@welho.com
• Jan Mikkelsen
  Transactionware
  janm@transactionware.com
• Bodo Moeller (co-author of [RFC4492])
  Google
  bodo@openssl.org
• Erik Nygren
  Akamai Technologies
  erik+ietf@nygren.org
• Magnus Nystrom
  RSA Security
  magnus@rsasecurity.com
• Alfredo Pironti (co-author of [RFC7627])
  INRIA
  alfredo.pironti@inria.fr
• Andrei Popov
  Microsoft
  andrei.popov@microsoft.com
• Marsh Ray (co-author of [RFC7627])
  Microsoft
  maray@microsoft.com
• Robert Relyea
  Netscape Communications
  relyea@netscape.com
• Jim Roskind
  Netscape Communications
  jar@netscape.com
• Michael Sabin
• Dan Simon
  Microsoft, Inc.
  dansimon@microsoft.com
• Nick Sullivan
  CloudFlare Inc.
  nick@cloudflare.com
• Bjoern Tackmann
  University of California, San Diego
  btackmann@eng.ucsd.edu
• Martin Thomson
  Mozilla
  mt@mozilla.com
• Tom Weinstein
• Hoeteck Wee
  Ecole Normale Superieure, Paris
  hoeteck@alum.mit.edu
• Tim Wright
  Vodafone
  timothy.wright@vodafone.com

Author's Address