PAKE Usage in TLS 1.3

3.1.1. PAKE Algorithms

When sent by the client, the "pake_algorithms" extension indicates PAKE algorithms which the client supports for authenticated key exchange, ordered from most preferred to least preferred.¶

The "extension_data" field of this extension contains a "PAKEAlgorithmList" value:¶

　enum {
    /* Balanced PAKE algorithms */
    0x0000~0x3FFF

    /* Augmented PAKE algorithms */
    0x4000~0x7FFF

    /* Post-Quantum Balanced PAKE algorithms */
    0x8000~0xBFFF

    /* Post-Quantum Augmented PAKE algorithms */
    0xC000~0xFFFF
    (0xFFFF)
　} PAKEAlgorithm;

  struct {
    　PAKEAlgorithm supported_pake_algorithms<2..2^16-1>;
  } PAKEAlgorithmList;

Here, each PAKE algorithm suite consists of two part, the first part is represented by the first byte and specifies different PAKE algorithms, and the second part is represented by the second byte and specifies different ciphersuites for the fixed PAKE algorithm. For example, the first bytes "0x00" and "0x01" represent CPace [I-D.draft-irtf-cfrg-cpace-12] and SPAKE2 [RFC9382], respectively; and the first bytes "0x40" and "0x41" represent OPAQUE [I-D.draft-irtf-cfrg-opaque-17] and SPAKE2+ [RFC9383], respectively. For the SPAKE2+ algorithm with the first byte "0x41", the second bytes "0x00~0xFF" can be used to represent its different ciphersuites (see Section 4 of [RFC9383]).¶

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

3.1.2. PAKE Share

The "pake_share" extension contains the endpoint’s PAKE parameters. The "PAKEShareEntry" value is defined as follows:¶

  struct {
      PAKEAlgorithm pake_algorithm;
      opaque pake_message<1..2^16-1>;
  } PAKEShareEntry;

The "pake_algorithm" field indicates the PAKE algorithm used. The "pake_message" field indicates the PAKE key exchange information, its contents are determined by the specified PAKE algorithm:¶

  struct {
      select (pake_algorithm) {
      case CPace or SPAKE2 or SPAKE2+:
          opaque dh_share;
      case OPAQUE:
          opaque dh_share;
          opaque oprf_message;
      };
  } PAKEMessage;

Here, the "oprf_message" field indicates the "blinded_message" value or the "evaluated_message" value, which are defined in Section 6.3.1 of [I-D.draft-irtf-cfrg-opaque-17]; the "dh_share" field indicates a (EC)DH public element that is determined by the underlying group.¶

(1) In the ClientHello message, the "extension_data" field of this extension contains a "PAKEShareClientHello" value:¶

  struct {
      opaque client_identity<0..2^16-1>;
      PAKEShareEntry client_shares<0..2^16-1>;
      PAKEExtension pake_extensions<0..2^16-1>;
  } PAKEShareClientHello;

The "client_identity" field indicates a client's identity used in the PAKE algorithm. The "client_shares" field contains a list of offered PAKEShareEntry values in descending order of client preference. The "pake_extensions" field indicates a list of PAKE extensions allowed in the structure of PAKEShareClientHello, clients can use this field to send data to servers and request extended functionality from servers.¶

The contents of "client_shares" field MAY be empty if the client is requesting a HelloRetryRequest. Each PAKEShareEntry value MUST correspond to a PAKE algorithm offered in the "pake_algorithms" extension and MUST appear in the same order. However, the values MAY be a non-contiguous subset of the "pake_algorithms" extension and MAY omit the most preferred PAKE algorithms. Such a situation could arise if the most preferred PAKE algorithms are new and unlikely to be supported in enough places to make pregenerating PAKE shares for them efficient.¶

Clients can offer as many PAKEShareEntry values as the number of supported PAKE algorithms it is offering, each representing a single set of PAKE key exchange parameters. For instance, a client might offer shares for several PAKE algorithms. The "pake_message" values for each PAKEShareEntry MUST be generated independently. Clients MUST NOT offer multiple PAKEShareEntry values for the same PAKE algorithm. Clients MUST NOT offer any PAKEShareEntry values for PAKE algorithms not listed in the client’s "pake_algorithms" extension. Servers MAY check for violations of these rules and abort the handshake with an "illegal_parameter" alert if one is violated. In addition, servers MUST verify that (1) the list of client shares to see if there is one with the provided "client_identity" value, and (2) if the "pake_message" contents in the "client_shares" field are valid (e.g., valid group elements). If either of these checks fails, then servers MUST abort the handshake with an "illegal_parameter" alert.¶

(2) In a HelloRetryRequest message, the "extension_data" field of this extension contains a "PAKEShareHelloRetryRequest" value:¶

  struct {
      PAKEAlgorithm selected_pake_algorithm;
  } PAKEShareHelloRetryRequest;

If a client receives a second HelloRetryRequest in the same connection (i.e., where the ClientHello was itself in response to a HelloRetryRequest), it MUST abort the handshake with an "unexpected_message" alert. Upon receipt of this extension in a HelloRetryRequest, the client MUST verify that (1) the "selected_pake_algorithm" field corresponds to a PAKE algorithm which was provided in the "pake_algorithms" extension in the original ClientHello, and (2) the "selected_pake_algorithm" field does not correspond to a PAKE algorithm which was provided in the "pake_share" extension in the original ClientHello. If either of these checks fails, then the client MUST abort the handshake with an "illegal_parameter" alert. Otherwise, when sending the new ClientHello, the client MUST replace the original "pake_share" extension with one containing only a new PAKEShareEntry for the PAKE algorithm indicated in the "selected_pake_algorithm" field of the triggering HelloRetryRequest.¶

(3) In a ServerHello message, the "extension_data" field of this extension contains a PAKEShareServerHello value:¶

  struct {
      opaque server_identity<0..2^16-1>;
      PAKEShareEntry server_share;
      PAKEExtension pake_extensions<0..2^16-1>;
  } PAKEShareServerHello;

The "server_identity" field indicates a server's identity used in the PAKE algorithm. The "server_share" field contains a single PAKEShareEntry value that is in the same PAKE algorithm as one of the client’s shares. The "pake_extensions" field indicates a list of PAKE extensions allowed in the structure of PAKEShareServerHello, servers can use this field to send data to clients and provide extended functionality to clients.¶

If using PAKE key establishment, servers offer exactly one PAKEShareEntry in the ServerHello. This value MUST be in the same PAKE algorithm as the PAKEShareEntry value offered by the client that the server has selected for the negotiated PAKE key exchange. Servers MUST NOT send a PAKEShareEntry for any PAKE algorithm not indicated in the client’s "pake_algorithms" extension. If using PAKE key establishment and a HelloRetryRequest containing a "pake_share" extension was received by the client, the client MUST verify that the selected PAKEAlgorithm in the ServerHello is the same as that in the HelloRetryRequest. If this check fails, the client MUST abort the handshake with an "illegal_parameter" alert. In addition, the client MUST verify that (1) the "server_identity" value to see if it is the target server's name, and (2) if the "pake_message" content in the "server_share" field is valid (e.g., a valid group element). If either of these checks fails, then the client MUST abort the handshake with an "illegal_parameter" alert.¶

(4) In this document, we define a PAKEExtension structure and three PAKE extension types that SHOULD be used in some PAKE algorithms.¶

  enum {
      session_identifier (0),
      nonce (1),
      credential_response (2),
      (255)
  } PAKEExtensionType;

  struct {
      PAKEExtensionType pake_extension_type;
      opaque pake_extension_data<0..2^16-1>;
  } PAKEExtension;

The "session_identifier" extension indicates a unique session identifier of the CPace algorithm (see Section 3.1 of [I-D.draft-irtf-cfrg-cpace-12]). This extension SHOULD only be presented in the structure of "PAKEShareClientHello" when the CPace algorithm is selected, and MUST NOT be used with other PAKE algorithms that don't need it.¶

The "nonce" extension indicates the "client_nonce" value or "server_nonce" value in the OPAQUE algorithm (see Section 6.1 of [I-D.draft-irtf-cfrg-opaque-17]). This extension SHOULD only be presented in the structure of "PAKEShareClientHello" or "PAKEShareServerHello" when the OPAQUE algorithm is selected, and MUST NOT be used with other PAKE algorithms that don't need it.¶

The "pake_extension_data" field of the "credential_response" extension contains a "CredentialResponse" structure without the "evaluated_message" field in the OPAQUE algorithm (see Section 6.3.1 of [I-D.draft-irtf-cfrg-opaque-17]). This extension SHOULD only be presented in the structure of "PAKEShareServerHello" when the OPAQUE algorithm is negotiated, and MUST NOT be used with other PAKE algorithms that don't need it.¶

3.1.3. PAKE Authentication Modes

In order to use PAKE, clients MUST also send a "pake_auth_modes" extension. The semantics of this extension are that the client wants to identify which authentication modes will be used, and its content MAY be multiple authentication modes. The "extension_data" field of this extension contains a "PAKEAuthModes" value:¶

  enum {
      pake_auth (0),
      pake_certificate_auth (1),
      (255)
  } PAKEAuthMode;

  struct {
      PAKEAuthMode auth_modes<1..255>;
  } PAKEAuthModes;

The "pake_auth" mode indicates PAKE-only authentication. In this mode, the server MUST NOT send the Certificate, CertificateVerify and CertificateRequest messages. The "pake_certificate_auth" mode indicates PAKE authentication combined with certificate authentication. In this mode, the server MUST send the Certificate and CertificateVerify messages to conduct certificate-based server authentication, and optionally sends the CertificateRequest message if certificate-based client authentication is desired.¶

A client MUST provide a "pake_auth_modes" extension if it offers "pake_algorithms" and "pake_share" extensions. If clients offer "pake_algorithms" and "pake_share" without a "pake_auth_modes" extension, servers MUST abort the handshake with an "illegal_parameter" alert. Servers MUST provide only one authentication mode in the "pake_auth_modes" extension to unambiguously identify which mode was selected, but MUST NOT select an authentication mode that is not listed by the client. If servers offer "pake_share" without a "pake_auth_modes" extension, clients MUST abort the handshake with an "illegal_parameter" alert.¶