Previous | Contents | Index | Next

Appendix G: SSH-2 names specified for PuTTY

There are various parts of the SSH-2 protocol where things are specified using a textual name. Names ending in are reserved for allocation by the PuTTY team. Allocated names are documented here.

G.1 Connection protocol channel request names

These names can be sent in a SSH_MSG_CHANNEL_REQUEST message.
This is sent by a client to announce that it will not have more than one channel open at a time in the current connection (that one being the one the request is sent on). The intention is that the server, knowing this, can set the window on that one channel to something very large, and leave flow control to TCP. There is no message-specific data.
PuTTY sends this request along with some SSH_MSG_CHANNEL_WINDOW_ADJUST messages as part of its window-size tuning. It can be sent on any type of channel. There is no message-specific data. Servers MUST treat it as an unrecognised request and respond with SSH_MSG_CHANNEL_FAILURE.

(Some SSH servers get confused by this message, so there is a bug-compatibility mode for disabling it. See section 4.26.3.)

G.2 Key exchange method names
These appeared in various drafts of what eventually became RFC 4432. They have been superseded by rsa1024-sha1 and rsa2048-sha256.

G.3 Encryption algorithm names
These were used in drafts of what eventually became RFC 4345. They have been superseded by arcfour128 and arcfour256.

G.4 Agent extension request names

The SSH agent protocol, which is only specified in an Internet-Draft at the time of writing (draft-miller-ssh-agent), defines an extension mechanism. These names can be sent in an SSH_AGENTC_EXTENSION message.
The payload is a single SSH-2 string containing a keypair in the PPK format defined in appendix C. Compared to the standard SSH_AGENTC_ADD_IDENTITY, this extension allows adding keys in encrypted form, with the agent requesting a decryption passphrase from the user on demand, and able to revert the key to encrypted form.
The payload is a single SSH-2 string specifying a public key blob, as in SSH_AGENTC_REMOVE_IDENTITY. Requests that the agent forget any cleartext form of a specific key.

Returns SSH_AGENT_SUCCESS if the agent ended up holding the key only in encrypted form (even if it was already encrypted); returns SSH_AGENT_EXTENSION_FAILURE if not (if it wasn't held by the agent at all, or only in cleartext form).
No payload. Requests that the agent forget the cleartext form of any keys for which it holds an encrypted form.

If the agent holds any keys with an encrypted form (or no keys at all), returns SSH_AGENT_SUCCESS to indicate that no such keys are now held in cleartext form, followed by a uint32 specifying how many keys remain in cleartext form (because the agent didn't hold an encrypted form for them). If the agent holds nothing but keys in cleartext form, returns SSH_AGENT_EXTENSION_FAILURE.
No payload. Returns SSH_AGENT_SUCCESS followed by a list of identities similar to SSH_AGENT_IDENTITIES_ANSWER, except that each key has an extra SSH-2 string at the end. Currently that string contains a single uint32 flags word, with the following bits defined:
Bit 0
If set, key is held with an encrypted form (so that the reencrypt extension can do something useful with it).
Bit 1
If set, key's cleartext form is not currently held (so the user will have to supply a passphrase before the key can be used).

If you want to provide feedback on this manual or on the PuTTY tools themselves, see the Feedback page.

[PuTTY development snapshot 2021-04-12.a21056a]