x/crypto/ssh: Unmarshal for SSH_AGENTC_EXTENSION

[lpcalisi]

OpenSSH 8.9 will include the ability to control how and where keys in ssh-agent may be used, both locally and when forwarded (subject to some limitations).

This new feature implements a new agent extension an it doesn't compatible with Golang SSH Agent library. This kind of messages could not be parsed by Unmarshall method.

The message format is:

byte            SSH_AGENTC_EXTENSION (0x1b)
string          session-bind@openssh.com
string          hostkey
string          session identifier
string          signature
bool            is_forwarding

The error is ssh: parse error in message type 27, when 27 is the SSH_AGENTC_EXTENSION message type (referenced in SSH Agent Protocol)

[seankhliao]

cc @FiloSottile

[lpcalisi]

hi team! any news about it? thanks you! @seankhliao @FiloSottile

[gopherbot]

Change https://go.dev/cl/412154 mentions this issue: ssh/agent: fix extensionAgentMsg