KEM-based Authentication for TLS 1.3

1.1. Organization

After a brief introduction to KEMs, we will introduce the AuthKEM authentication mechanism. For clarity, we discuss unilateral and mutual authentication separately. Next, we introduce the abbreviated AuthKEM handshake, and its opportunistic client authentication mechanism. In the remainder of the draft, we will discuss the necessary implementation mechanics, such as code points, extensions, new protocol messages and the new key schedule.¶

3.1. Key Encapsulation Mechanisms

As this proposal relies heavily on KEMs, which are not originally used by TLS, we will provide a brief overview of this primitive. Other cryptographic operations will be discussed later.¶

A Key Encapsulation Mechanism (KEM) is a cryptographic primitive that defines the methods Encapsulate and Decapsulate. In this draft, we extend these operations with context separation strings:¶

Encapsulate(pkR, context_string): Takes a public key, and produces a shared secret and encapsulation.¶

Decapsulate(enc, skR, context_str): Takes the encapsulation and the private key. Returns the shared secret.¶

We implement these methods through the KEMs defined in [RFC9180] to export shared secrets appropriate for using with the HKDF in TLS 1.3:¶

def Encapsulate(pk, context_string):
  enc, ctx = HPKE.SetupBaseS(pk, "tls13 auth-kem " + context_string)
  ss = ctx.Export("", HKDF.Length)
  return (enc, ss)

def Decapsulate(enc, sk, context_string):
  return HPKE.SetupBaseR(enc,
                         sk,
                         "tls13 auth-kem " + context_string)
             .Export("", HKDF.Length)

Keys are generated and encoded for transmission following the conventions in [RFC9180].¶

4.1. Client authentication

For client authentication, the server sends the CertificateRequest message as in [RFC8446]. This message can not be authenticated in the AuthKEM handshake: we will discuss the implications below.¶

As in [RFC8446], section 4.4.2, if and only if the client receives CertificateRequest, it MUST send a Certificate message. If the client has no suitable certificate, it MUST send a Certificate message containing no certificates. If the server is satisfied with the provided certificate, it MUST send back a KEMEncapsulation message, containing the encapsulation to the client's certificate. The resulting shared secret is mixed into the key schedule. This ensures any messages sent using keys derived from it are covered by the authentication.¶

The AuthKEM handshake with client authentication is given in Figure 2.¶

       Client                                     Server

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

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

        +  Indicates noteworthy extensions sent in the
           previously noted message.
        <> Indicates messages protected using keys
           derived from a [sender]_handshake_traffic_secret.
        {} Indicates messages protected using keys
           derived from a
           [sender]_authenticated_handshake_traffic_secret.
        [] Indicates messages protected using keys
           derived from [sender]_application_traffic_secret_N.

       Figure 2: Message Flow for KEM-Authentication (KEM-Auth)
                 Handshake with client authentication.

If the server is not satisfied with the client's certificates, it MAY, at its discretion, decide to continue or terminate the handshake.¶

Unfortunately, AuthKEM client authentication requires an extra round-trip. Clients that know the server's long-term public KEM key MAY choose to use the abbreviated AuthKEM handshake and opportunistically send the client certificate as a 0-RTT-like message. We will discuss this later.¶

4.2. Relevant handshake messages

After the Key Exchange and Server Parameters phase of TLS 1.3 handshake, the client and server exchange implicitly authenticated messages. KEM-based authentication uses the same set of messages every time that certificate-based authentication is needed. Specifically:¶

• Certificate: The certificate of the endpoint and any per-certificate extensions. This message is omitted by the client if the server did not send a CertificateRequest message (thus indicating that the client should not authenticate with a certificate). For AuthKEM, Certificate MUST include the long-term KEM public key. Certificates MUST be handled in accordance with [RFC8446], section 4.4.2.4.¶

  Certificates MUST be handled in accordance with [RFC8446], section 4.4.2.4.¶

• KEMEncapsulation: A key encapsulation against the certificate's long-term public key, which yields an implicitly authenticated shared secret.¶

4.3. Overview of key differences with RFC8446 TLS 1.3

• New types of signature_algorithms for KEMs.¶
• Public keys in certificates are KEM algorithms¶
• New handshake message KEMEncapsulation¶
• The key schedule mixes in the shared secrets from the authentication.¶
• The Certificate is sent encrypted with a new handshake encryption key.¶
• The client sends Finished before the server.¶
• The clients sends data before the server has sent Finished.¶

4.4. Implicit and explicit authentication

The data that the client MAY transmit to the server before having received the server's Finished is encrypted using ciphersuites chosen based on the client's and server's advertised preferences in the ClientHello and ServerHello messages. The ServerHello message can however not be authenticated before the Finished message from the server is verified. The full implications of this are discussed in the Security Considerations section.¶

Upon receiving the client's authentication messages, the server responds with its Finished message, which achieves explicit authentication. Upon receiving the server's Finished message, the client achieves explicit authentication. Receiving this message retroactively confirms the server's cryptographic parameter choices.¶

4.5. Authenticating CertificateRequest

The CertificateRequest message can not be authenticated during the AuthKEM handshake; only after the Finished message from the server has been processed, it can be proven as authentic. The security implications of this are discussed later.¶

This is dicussed in Github issue #16. We would welcome feedback there.¶

Clients MAY choose to only accept post-handshake authentication.¶

TODO: Should they indicate this? TLS Flag?¶

5.1. Negotiation

A client that knows a server's long-term KEM public key MAY choose to attempt the abbreviated AuthKEM handshake. If it does so, it MUST include the stored_auth_key extension in the ClientHello message. This message MUST contain the encapsulation against the long-term KEM public key. Details of the extension are described below. The shared secret resulting from the encapsulation is mixed in to the EarlySecret computation.¶

The client MAY additionally choose to send a certificate to the server. It MUST know what ciphersuites the server accepts before it does so. If it chooses to do so, it MUST send the early_auth extension to the server. The Certificate is encrypted with the client_early_handshake_traffic_secret.¶

The server MAY accept the abbreviated AuthKEM handshake. If it does, it MUST reply with a stored_auth_key extension. If it does not accept the abbreviated AuthKEM handshake, for instance because it does not have access to the correct secret key anymore, it MUST NOT reply with a stored_auth_key extension. The server, if it accepts the abbreviated AuthKEM handshake, MAY additionally accept the Certificate message. If it does, it MUST reply with a early_auth extension.¶

If the client, who sent a stored_auth_key extension, receives a ServerHello without stored_auth_key extension, it MUST recompute EarlySecret without the encapsulated shared secret.¶

If the client sent a Certificate message, it MUST drop that message from its transcript. The client MUST then continue with a full AuthKEM handshake.¶

5.2. 0-RTT, forward secrecy and replay protection

The client MAY send 0-RTT data, as in [RFC8446] 0-RTT mode. The Certificate MUST be sent before the 0-RTT data.¶

As the EarlySecret is derived only from a key encapsulated to a long-term secret, it does not have forward secrecy. Clients MUST take this into consideration before transmitting 0-RTT data or opting in to early client auth. Certificates and 0-RTT data may also be replayed.¶

This will be discussed in full under Security Considerations.¶

6.1. Negotiation of AuthKEM

Clients will indicate support for this mode by negotiating it as if it were a signature scheme (part of the signature_algorithms extension). We thus add these new signature scheme values (even though, they are not signature schemes) for the KEMs defined in [RFC9180] Section 7.1. Note that we will be only using their internal KEM's API defined there.¶

enum {
  dhkem_p256_sha256   => TBD,
  dhkem_p384_sha384   => TBD,
  dhkem_p521_sha512   => TBD,
  dhkem_x25519_sha256 => TBD,
  dhkem_x448_sha512   => TBD,
}

When present in the signature_algorithms extension, these values indicate AuthKEM support with the specified key exchange mode. These values MUST NOT appear in signature_algorithms_cert, as this extension specifies the signing algorithms by which certificates are signed.¶

6.2. ClientHello and ServerHello extensions

A number of AuthKEM messages contain tag-length-value encoded extensions structures. We are adding those extensions to the ExtensionType list from TLS 1.3.¶

enum {
  ...
  stored_auth_key (TBD),                 /* RFC TBD */
  early_auth (TBD),                      /* RFC TBD */
  (65535)
} ExtensionType;

The table below indicates the messages where a given extension may appear:¶

+---------------------------------------+-------------+
| Extension                             |    KEM-Auth |
+---------------------------------------+-------------+
| stored_auth_key [RFCTBD]              |      CH, SH |
|                                       |             |
| early_auth  [RFCTBD]                  |      CH, SH |
|                                       |             |
+---------------------------------------+-------------+

struct {
      select (type) {
        case client:
          opaque key_fingerprint<1..255>;
          opaque ciphertext<1..2^16-1>
        case server:
          AcceptedAuthKey '1';
      } body;
} StoredInformation

struct {
} EarlyAuth

6.3. Protocol messages

The handshake protocol is used to negotiate the security parameters of a connection, as in TLS 1.3. It uses the same messages, expect for the addition of a KEMEncapsulation message and does not use the CertificateVerify one.¶

enum {
    ...
    kem_encapsulation(tbd),
    ...
    (255)
  } HandshakeType;

struct {
    HandshakeType msg_type;    /* handshake type */
    uint24 length;             /* remaining bytes in message */
    select (Handshake.msg_type) {
        ...
        case kem_encapsulation:     KEMEncapsulation;
        ...
    };
} Handshake;

Protocol messages MUST be sent in the order defined in Section 4. A peer which receives a handshake message in an unexpected order MUST abort the handshake with an "unexpected_message" alert.¶

The KEMEncapsulation message is defined as follows:¶

struct {
    opaque certificate_request_context<0..2^8-1>
    opaque encapsulation<0..2^16-1>;
} KEMEncapsulation;

The encapsulation field is the result of a Encapsulate function. The Encapsulate() function will also result in a shared secret (ssS or ssC, depending on the peer) which is used to derive the AHS or MS secrets.¶

If the KEMEncapsulation message is sent by a server, the authentication algorithm MUST be one offered in the client's signature_algorithms extension unless no valid certificate chain can be produced without unsupported algorithms.¶

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

In addition, the authentication algorithm MUST be compatible with the key(s) in the sender's end-entity certificate.¶

The receiver of a KEMEncapsulation message MUST perform the Decapsulate(enc, skR) operation by using the sent encapsulation and the private key of the public key advertised in the end-entity certificate sent. The Decapsulate(enc, skR) function will also result on a shared secret (ssS or ssC, depending on the Server or Client executing it respectively) which is used to derive the AHS or MS secrets.¶

certificate_request_context is included to allow the recipient to identify the certificate against which the encapsulation was generated. It MUST be set to the value in the Certificate message to which the encapsulation was computed.¶

6.4. Cryptographic computations

The AuthKEM handshake establishes three input secrets which are combined to create the actual working keying material, as detailed below. The key derivation process incorporates both the input secrets and the handshake transcript. Note that because the handshake transcript includes the random values from the Hello messages, any given handshake will have different traffic secrets, even if the same input secrets are used.¶

            0
            |
            v
    PSK -> HKDF-Extract = Early Secret
            |
            +--> Derive-Secret(., "ext binder" | "res binder", "")
            |                  = binder_key
            |
            +--> Derive-Secret(., "c e traffic", ClientHello)
            |                  = client_early_traffic_secret
            |
            +--> Derive-Secret(., "e exp master", ClientHello)
            |                  = early_exporter_master_secret
            v
            Derive-Secret(., "derived", "")
            |
            v
(EC)DHE -> HKDF-Extract = Handshake Secret
            |
            +--> Derive-Secret(., "c hs traffic",
            |                  ClientHello...ServerHello)
            |                  = client_handshake_traffic_secret
            |
            +--> Derive-Secret(., "s hs traffic",
            |                  ClientHello...ServerHello)
            |                  = server_handshake_traffic_secret
            v
            Derive-Secret(., "derived", "") = dHS
            |
            v
    SSs -> HKDF-Extract = Authenticated Handshake Secret
            |
            +--> Derive-Secret(., "c ahs traffic",
            |                  ClientHello...KEMEncapsulation)
            |         = client_handshake_authenticated_traffic_secret
            |
            +--> Derive-Secret(., "s ahs traffic",
            |                  ClientHello...KEMEncapsulation)
            |         = server_handshake_authenticated_traffic_secret
            v
            Derive-Secret(., "derived", "") = dAHS
            |
            v
SSc||0 * -> HKDF-Extract = Main Secret
            |
            +--> Derive-Secret(., "c ap traffic",
            |                  ClientHello...server Finished)
            |                  = client_application_traffic_secret_0
            |
            +--> Derive-Secret(., "s ap traffic",
            |                  ClientHello...server Finished)
            |                  = server_application_traffic_secret_0
            |
            +--> Derive-Secret(., "exp master",
            |                  ClientHello...server Finished)
            |                  = exporter_master_secret
            |
            +--> Derive-Secret(., "res master",
                               ClientHello...client Finished)
                               = resumption_master_secret

*: if client authentication was requested, the `SSc` value should
   be used. Otherwise, the `0` value is used.

SSs, encapsulation <- Encapsulate(public_key_server,
                                  "server authentication")
               SSc <- Decapsulate(encapsulation, private_key_client,
                                  "client authentication")

               SSs <- Decapsulate(encapsulation, private_key_server
                                  "server authentication")
SSc, encapsulation <- Encapsulate(public_key_client,
                                  "client authentication")

server/client_finished_key =
  HKDF-Expand-Label(MainSecret,
                    server/client_label,
                    "", Hash.length)
server_label = "tls13 server finished"
client_label = "tls13 client finished"

server/client_verify_data =
      HMAC(server/client_finished_key,
           Transcript-Hash(Handshake Context,
                           Certificate*,
                           KEMEncapsulation*,
                           Finished**)

*  Only included if present.
** The party who last sends the finished message in terms of flights
   includes the other party's Finished message.

7.1. Implicit authentication

Because preserving a 1/1.5RTT handshake in KEM-Auth requires the client to send its request in the same flight when the ServerHello message is received, it can not yet have explicitly authenticated the server. However, through the inclusion of the key encapsulated to the server's long-term secret, only an authentic server should be able to decrypt these messages.¶

However, the client can not have received confirmation that the server's choices for symmetric encryption, as specified in the ServerHello message, were authentic. These are not authenticated until the Finished message from the server arrived. This may allow an adversary to downgrade the symmetric algorithms, but only to what the client is willing to accept. If such an attack occurs, the handshake will also never successfully complete and no data can be sent back.¶

If the client trusts the symmetric algorithms advertised in its ClientHello message, this should not be a concern. A client MUST NOT accept any cryptographic parameters it does not include in its own ClientHello message.¶

If client authentication is used, explicit authentication is reached before any application data, on either client or server side, is transmitted.¶

Application Data MUST NOT be sent prior to sending the Finished message, except as specified in Section 2.3 of [RFC8446]. Note that while the client MAY send Application Data prior to receiving the server's last explicit Authentication message, any data sent at that point is, being sent to an implicitly authenticated peer.¶

7.2. Authentication of Certificate Request

Due to the implicit authentication of the server's messages during the full AuthKEM handshake, the CertificateRequest message can not be authenticated before the client received Finished.¶

The key schedule guarantees that the server can not read the client's certificate message (as discussed above). An active adversary that maliciously inserts a CertificateRequest message will also result in a mismatch in transcript hashes, which will cause the handshake to fail.¶

However, there may be side effects. The adversary might learn that the client has a certificate by observing the length of the messages sent. There may also be side-effects, especially in situations where the client is prompted to e.g. approve use or unlock a certificate stored encrypted or on a smart card.¶

B.1. Authentication concerns for client authentication requests.

Tracked by Issue #16.¶

The certificate request message from the server can not be authenticated by the AuthKEM mechanism. This is already somewhat discussed above and under security considerations. We might want to allow clients to refuse client auth for scenarios where this is a concern.¶

B.2. Interaction with signing certificates

Tracked by Issue #20.¶

In the current state of the draft, we have not yet discussed combining traditional signature-based authentication with KEM-based authentication. One might imagine that the Client has a sigining certificate and the server has a KEM public key.¶

In the current draft, clients MUST use a KEM certificate algorithm if the server negotiated AuthKEM.¶