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Annotation of src/usr.bin/ssh/PROTOCOL.agent, Revision 1.19

1.15      djm         1: The SSH agent protocol is described in
1.14      dtucker     2: https://tools.ietf.org/html/draft-miller-ssh-agent-04
1.13      djm         3:
1.17      dtucker     4: This file documents OpenSSH's extensions to the agent protocol.
1.15      djm         5:
                      6: 1. session-bind@openssh.com extension
                      7:
                      8: This extension allows a ssh client to bind an agent connection to a
                      9: particular SSH session identifier as derived from the initial key
                     10: exchange (as per RFC4253 section 7.2) and the host key used for that
                     11: exchange. This binding is verifiable at the agent by including the
                     12: initial KEX signature made by the host key.
                     13:
                     14: The message format is:
                     15:
                     16:        byte            SSH_AGENTC_EXTENSION (0x1b)
                     17:        string          session-bind@openssh.com
                     18:        string          hostkey
                     19:        string          session identifier
                     20:        string          signature
                     21:        bool            is_forwarding
                     22:
                     23: Where 'hostkey' is the encoded server host public key, 'session
1.16      jsg        24: identifier' is the exchange hash derived from the initial key
1.15      djm        25: exchange, 'signature' is the server's signature of the session
                     26: identifier using the private hostkey, as sent in the final
                     27: SSH2_MSG_KEXDH_REPLY/SSH2_MSG_KEXECDH_REPLY message of the initial key
                     28: exchange. 'is_forwarding' is a flag indicating whether this connection
                     29: should be bound for user authentication or forwarding.
                     30:
                     31: When an agent received this message, it will verify the signature and
                     32: check the consistency of its contents, including refusing to accept
                     33: a duplicate session identifier, or any attempt to bind a connection
1.19    ! jsg        34: previously bound for authentication. It will then record the
1.15      djm        35: binding for the life of the connection for use later in testing per-key
                     36: destination constraints.
                     37:
                     38: 2. restrict-destination-v00@openssh.com key constraint extension
                     39:
                     40: The key constraint extension supports destination- and forwarding path-
                     41: restricted keys. It may be attached as a constraint when keys or
                     42: smartcard keys are added to an agent.
                     43:
                     44:        byte            SSH_AGENT_CONSTRAIN_EXTENSION (0xff)
                     45:        string          restrict-destination-v00@openssh.com
                     46:        constraint[]    constraints
                     47:
                     48: Where a constraint consists of:
                     49:
                     50:        string          from_username (must be empty)
                     51:        string          from_hostname
                     52:        keyspec[]       from_hostkeys
                     53:        string          to_username
                     54:        string          to_hostname
                     55:        keyspec[]       to_hostkeys
                     56:
1.18      dtucker    57: And a keyspec consists of:
1.15      djm        58:
                     59:        string          keyblob
                     60:        bool            is_ca
                     61:
                     62: When receiving this message, the agent will ensure that the
                     63: 'from_username' field is empty, and that 'to_hostname' and 'to_hostkeys'
                     64: have been supplied (empty 'from_hostname' and 'from_hostkeys' are valid
1.16      jsg        65: and signify the initial hop from the host running ssh-agent). The agent
1.15      djm        66: will then record the constraint against the key.
                     67:
                     68: Subsequent operations on this key including add/remove/request
                     69: identities and, in particular, signature requests will check the key
1.16      jsg        70: constraints against the session-bind@openssh.com bindings recorded for
1.15      djm        71: the agent connection over which they were received.
                     72:
                     73: 3. SSH_AGENT_CONSTRAIN_MAXSIGN key constraint
                     74:
                     75: This key constraint allows communication to an agent of the maximum
                     76: number of signatures that may be made with an XMSS key. The format of
                     77: the constraint is:
                     78:
                     79:        byte            SSH_AGENT_CONSTRAIN_MAXSIGN (0x03)
                     80:        uint32          max_signatures
                     81:
                     82: This option is only valid for XMSS keys.
                     83:
1.19    ! jsg        84: $OpenBSD: PROTOCOL.agent,v 1.18 2022/09/21 22:26:50 dtucker Exp $