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Editing: PROTOCOL.agent
The SSH agent protocol is described in https://tools.ietf.org/html/draft-miller-ssh-agent This file documents 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 identifier' 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 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 string reserved keyspec[] from_hostkeys string to_username string to_hostname string reserved keyspec[] to_hostkeys string reserved And 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 initial 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 against 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. 3. associated-certs-v00@openssh.com key constraint extension The key constraint extension allows certificates to be associated with private keys as they are loaded from a PKCS#11 token. byte SSH_AGENT_CONSTRAIN_EXTENSION (0xff) string associated-certs-v00@openssh.com bool certs_only string certsblob Where "certsblob" consists of one or more certificates encoded as public key blobs: string[] certificates This extension is only valid for SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests. When an agent receives this extension, it will attempt to match each certificate in the request with a corresponding private key loaded from the requested PKCS#11 token. When a matching key is found, the agent will graft the certificate contents to the token-hosted private key and store the result for subsequent use by regular agent operations. If the "certs_only" flag is set, then this extension will cause ONLY the resultant certificates to be loaded to the agent. The default behaviour is to load the PKCS#11-hosted private key as well as the resultant certificate. A SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED will return SSH_AGENT_SUCCESS if any key (plain private or certificate) was successfully loaded, or SSH_AGENT_FAILURE if no key was loaded. $OpenBSD: PROTOCOL.agent,v 1.23 2024/04/30 05:45:56 djm Exp $
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