The SSH agent protocol is described in https://tools.ietf.org/html/draft-miller-ssh-agent-04 This file document's OpenSSH's extensions to the agent protocol. 1. session-bind@openssh.com extension This extension allows a ssh client to bind an agent connection to a particular SSH session identifier as derived from the initial key exchange (as per RFC4253 section 7.2) and the host key used for that exchange. This binding is verifiable at the agent by including the initial KEX signature made by the host key. The message format is: byte SSH_AGENTC_EXTENSION (0x1b) string session-bind@openssh.com string hostkey string session identifier string signature bool is_forwarding Where 'hostkey' is the encoded server host public key, 'session identfier' is the exchange hash derived from the initial key exchange, 'signature' is the server's signature of the session identifier using the private hostkey, as sent in the final SSH2_MSG_KEXDH_REPLY/SSH2_MSG_KEXECDH_REPLY message of the initial key exchange. 'is_forwarding' is a flag indicating whether this connection should be bound for user authentication or forwarding. When an agent received this message, it will verify the signature and check the consistency of its contents, including refusing to accept a duplicate session identifier, or any attempt to bind a connection previously bound for authentication. It will then then record the binding for the life of the connection for use later in testing per-key destination constraints. 2. restrict-destination-v00@openssh.com key constraint extension The key constraint extension supports destination- and forwarding path- restricted keys. It may be attached as a constraint when keys or smartcard keys are added to an agent. byte SSH_AGENT_CONSTRAIN_EXTENSION (0xff) string restrict-destination-v00@openssh.com constraint[] constraints Where a constraint consists of: string from_username (must be empty) string from_hostname keyspec[] from_hostkeys string to_username string to_hostname keyspec[] to_hostkeys An a keyspec consists of: string keyblob bool is_ca When receiving this message, the agent will ensure that the 'from_username' field is empty, and that 'to_hostname' and 'to_hostkeys' have been supplied (empty 'from_hostname' and 'from_hostkeys' are valid and signify the inital hop from the host running ssh-agent). The agent will then record the constraint against the key. Subsequent operations on this key including add/remove/request identities and, in particular, signature requests will check the key constraints agains the session-bind@openssh.com bindings recorded for the agent connection over which they were received. 3. SSH_AGENT_CONSTRAIN_MAXSIGN key constraint This key constraint allows communication to an agent of the maximum number of signatures that may be made with an XMSS key. The format of the constraint is: byte SSH_AGENT_CONSTRAIN_MAXSIGN (0x03) uint32 max_signatures This option is only valid for XMSS keys. $OpenBSD: PROTOCOL.agent,v 1.15 2021/12/19 22:15:21 djm Exp $