1The SSH agent protocol is described in 2https://tools.ietf.org/html/draft-miller-ssh-agent-04 3 4This file documents OpenSSH's extensions to the agent protocol. 5 61. session-bind@openssh.com extension 7 8This extension allows a ssh client to bind an agent connection to a 9particular SSH session identifier as derived from the initial key 10exchange (as per RFC4253 section 7.2) and the host key used for that 11exchange. This binding is verifiable at the agent by including the 12initial KEX signature made by the host key. 13 14The 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 23Where 'hostkey' is the encoded server host public key, 'session 24identifier' is the exchange hash derived from the initial key 25exchange, 'signature' is the server's signature of the session 26identifier using the private hostkey, as sent in the final 27SSH2_MSG_KEXDH_REPLY/SSH2_MSG_KEXECDH_REPLY message of the initial key 28exchange. 'is_forwarding' is a flag indicating whether this connection 29should be bound for user authentication or forwarding. 30 31When an agent received this message, it will verify the signature and 32check the consistency of its contents, including refusing to accept 33a duplicate session identifier, or any attempt to bind a connection 34previously bound for authentication. It will then record the 35binding for the life of the connection for use later in testing per-key 36destination constraints. 37 382. restrict-destination-v00@openssh.com key constraint extension 39 40The key constraint extension supports destination- and forwarding path- 41restricted keys. It may be attached as a constraint when keys or 42smartcard 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 48Where 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 57And a keyspec consists of: 58 59 string keyblob 60 bool is_ca 61 62When receiving this message, the agent will ensure that the 63'from_username' field is empty, and that 'to_hostname' and 'to_hostkeys' 64have been supplied (empty 'from_hostname' and 'from_hostkeys' are valid 65and signify the initial hop from the host running ssh-agent). The agent 66will then record the constraint against the key. 67 68Subsequent operations on this key including add/remove/request 69identities and, in particular, signature requests will check the key 70constraints against the session-bind@openssh.com bindings recorded for 71the agent connection over which they were received. 72 733. SSH_AGENT_CONSTRAIN_MAXSIGN key constraint 74 75This key constraint allows communication to an agent of the maximum 76number of signatures that may be made with an XMSS key. The format of 77the constraint is: 78 79 byte SSH_AGENT_CONSTRAIN_MAXSIGN (0x03) 80 uint32 max_signatures 81 82This option is only valid for XMSS keys. 83 84$OpenBSD: PROTOCOL.agent,v 1.19 2023/04/12 08:53:54 jsg Exp $ 85