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85 lines
3.0 KiB
Plaintext
85 lines
3.0 KiB
Plaintext
The SSH agent protocol is described in
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https://tools.ietf.org/html/draft-miller-ssh-agent-04
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This file document's 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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identfier' 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 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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keyspec[] from_hostkeys
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string to_username
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string to_hostname
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keyspec[] to_hostkeys
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An 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 inital 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 agains 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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$OpenBSD: PROTOCOL.agent,v 1.15 2021/12/19 22:15:21 djm Exp $
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