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3. Key Exchange Method: sntrup761x25519-sha512

This section preserves the RFC text for the SSH hybrid key exchange method, including sntrup761, X25519, SHA-512, SSH message names, public value lengths, IANA registration, security considerations, and test vectors.

3.  Key Exchange Method: sntrup761x25519-sha512

   The key agreement is done by the X25519 Diffie-Hellman protocol as
   described in Section 3 ("Key Exchange Methods") of [RFC8731] and by
   the key encapsulation method described in [NTRUPrimePQCS].

   The key exchange procedure reuses the ECDH key exchange defined in
   Sections 4 ("ECDH Key Exchange") and 7.1 ("ECDH Message Numbers") of
   [RFC5656].

   The protocol flow and the SSH_MSG_KEX_ECDH_INIT and
   SSH_MSG_KEX_ECDH_REPLY messages are identical, except that we use
   different ephemeral public values Q_C and Q_S and shared secret K as
   described below.

   Implementations MAY use the name SSH_MSG_KEX_HYBRID_INIT where
   SSH_MSG_KEX_ECDH_INIT is used and the name SSH_MSG_KEX_HYBRID_REPLY
   where SSH_MSG_KEX_ECDH_REPLY is used, as long as the encoding on the
   wire is identical.  These symbolic names do not appear on the wire;
   they are merely used in specifications to refer to particular byte
   values.  For consistency with [RFC5656], which defines the packet
   syntax, we use those names in the rest of this document.

   The SSH_MSG_KEX_ECDH_INIT's value Q_C that holds the client's
   ephemeral public key MUST be constructed by concatenating the
   1158-byte public key output from the key generator of sntrup761 with
   the 32-byte K_A = X25519(a, 9) as described in [NTRUPrimePQCS] and
   [RFC8731].  The Q_C value is thus 1190 bytes.

   The SSH_MSG_KEX_ECDH_REPLY's value Q_S that holds the server's
   ephemeral public key MUST be constructed by concatenating the
   1039-byte ciphertext output from the key encapsulation mechanism of
   sntrup761 with the 32-byte K_B = X25519(b, 9) as described in
   [NTRUPrimePQCS] and [RFC8731].  The Q_S value is thus 1071 bytes.

   Clients and servers MUST abort if the length of the received public
   keys Q_C or Q_S are not the expected lengths.  An abort for these
   purposes is defined as a disconnect (SSH_MSG_DISCONNECT) of the
   session and SHOULD use the SSH_DISCONNECT_KEY_EXCHANGE_FAILED reason
   for the message; see Section 11.1 ("Disconnection Message") of
   [RFC4253].  No further validation is required beyond what is
   described in [RFC7748], [RFC8731], and [NTRUPrimePQCS].

   The SSH_MSG_KEX_ECDH_REPLY's signature value is computed as described
   in ECC for SSH [RFC5656] with the following changes.  Instead of
   encoding the shared secret K as "mpint", it MUST be encoded as
   "string".  The shared secret K value MUST be the 64-byte output octet
   string of the SHA-512 hash computed with the input as the 32-byte
   octet string key output from the key encapsulation mechanism of
   sntrup761 concatenated with the 32-byte octet string of X25519(a,
   X25519(b, 9)) = X25519(b, X25519(a, 9)).

   Some earlier implementations may implement this protocol only through
   the name [email protected]; therefore, it is
   RECOMMENDED to announce and accept that name as an alias of this
   protocol to increase chances for successfully negotiating the
   protocol.
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