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119 lines
4.3 KiB
Plaintext
119 lines
4.3 KiB
Plaintext
The SSH agent protocol is described in
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https://datatracker.ietf.org/doc/draft-ietf-sshm-ssh-agent/
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This file documents OpenSSH's extensions to the agent protocol.
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1. session-bind@openssh.com extension
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This extension allows a ssh client to bind an agent connection to a
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particular SSH session identifier as derived from the initial key
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exchange (as per RFC4253 section 7.2) and the host key used for that
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exchange. This binding is verifiable at the agent by including the
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initial KEX signature made by the host key.
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The message format is:
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byte SSH_AGENTC_EXTENSION (0x1b)
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string session-bind@openssh.com
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string hostkey
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string session identifier
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string signature
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bool is_forwarding
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Where 'hostkey' is the encoded server host public key, 'session
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identifier' is the exchange hash derived from the initial key
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exchange, 'signature' is the server's signature of the session
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identifier using the private hostkey, as sent in the final
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SSH2_MSG_KEXDH_REPLY/SSH2_MSG_KEXECDH_REPLY message of the initial key
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exchange. 'is_forwarding' is a flag indicating whether this connection
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should be bound for user authentication or forwarding.
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When an agent received this message, it will verify the signature and
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check the consistency of its contents, including refusing to accept
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a duplicate session identifier, or any attempt to bind a connection
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previously bound for authentication. It will then record the
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binding for the life of the connection for use later in testing per-key
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destination constraints.
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2. restrict-destination-v00@openssh.com key constraint extension
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The key constraint extension supports destination- and forwarding path-
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restricted keys. It may be attached as a constraint when keys or
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smartcard keys are added to an agent.
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byte SSH_AGENT_CONSTRAIN_EXTENSION (0xff)
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string restrict-destination-v00@openssh.com
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constraint[] constraints
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Where a constraint consists of:
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string from_username (must be empty)
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string from_hostname
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string reserved
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keyspec[] from_hostkeys
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string to_username
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string to_hostname
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string reserved
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keyspec[] to_hostkeys
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string reserved
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And a keyspec consists of:
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string keyblob
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bool is_ca
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When receiving this message, the agent will ensure that the
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'from_username' field is empty, and that 'to_hostname' and 'to_hostkeys'
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have been supplied (empty 'from_hostname' and 'from_hostkeys' are valid
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and signify the initial hop from the host running ssh-agent). The agent
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will then record the constraint against the key.
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Subsequent operations on this key including add/remove/request
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identities and, in particular, signature requests will check the key
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constraints against the session-bind@openssh.com bindings recorded for
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the agent connection over which they were received.
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3. SSH_AGENT_CONSTRAIN_MAXSIGN key constraint
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This key constraint allows communication to an agent of the maximum
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number of signatures that may be made with an XMSS key. The format of
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the constraint is:
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byte SSH_AGENT_CONSTRAIN_MAXSIGN (0x03)
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uint32 max_signatures
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This option is only valid for XMSS keys.
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3. associated-certs-v00@openssh.com key constraint extension
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The key constraint extension allows certificates to be associated
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with private keys as they are loaded from a PKCS#11 token.
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byte SSH_AGENT_CONSTRAIN_EXTENSION (0xff)
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string associated-certs-v00@openssh.com
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bool certs_only
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string certsblob
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Where "certsblob" consists of one or more certificates encoded as public
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key blobs:
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string[] certificates
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This extension is only valid for SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED
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requests. When an agent receives this extension, it will attempt to match
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each certificate in the request with a corresponding private key loaded
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from the requested PKCS#11 token. When a matching key is found, the
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agent will graft the certificate contents to the token-hosted private key
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and store the result for subsequent use by regular agent operations.
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If the "certs_only" flag is set, then this extension will cause ONLY
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the resultant certificates to be loaded to the agent. The default
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behaviour is to load the PKCS#11-hosted private key as well as the
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resultant certificate.
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A SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED will return SSH_AGENT_SUCCESS
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if any key (plain private or certificate) was successfully loaded, or
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SSH_AGENT_FAILURE if no key was loaded.
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$OpenBSD: PROTOCOL.agent,v 1.24 2024/11/27 13:27:34 djm Exp $
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