Some SSH-1 servers cannot deal with RSA authentication messages at all. If Pageant is running and contains any SSH-1 keys, PuTTY will normally automatically try RSA authentication before falling back to passwords, so these servers will crash when they see the RSA attempt.
If this bug is detected, PuTTY will go straight to password authentication. If this bug is enabled when talking to a correct server, the session will succeed, but of course RSA authentication will be impossible.
This is an SSH-1-specific bug.
4.26.4 ‘Chokes on SSH-2 ignore messages’
An ignore message (SSH_MSG_IGNORE) is a message in the SSH protocol which can be sent from the client to the server, or from the server to the client, at any time. Either side is required to ignore the message whenever it receives it. PuTTY uses ignore messages in SSH-2 to confuse the encrypted data stream and make it harder to cryptanalyse. It also uses ignore messages for connection keepalives (see
section 4.13.1
).
If it believes the server to have this bug, PuTTY will stop using ignore messages. If this bug is enabled when talking to a correct server, the session will succeed, but keepalives will not work and the session might be less cryptographically secure than it could be.
4.26.5 ‘Chokes on PuTTY's SSH-2 ‘winadj’ requests’
PuTTY sometimes sends a special request to SSH servers in the middle of channel data, with the name winadj@putty.projects.tartarus.org (see
section F.1
). The purpose of this request is to measure the round-trip time to the
server, which PuTTY uses to tune its flow control. The server does not actually have to understand the message; it is expected to send back a SSH_MSG_CHANNEL_FAILURE message indicating that it didn't understand it. (All PuTTY needs for its timing calculations is some kind of response.)
It has been known for some SSH servers to get confused by this message in one way or another – because it has a long name, or because they can't cope with unrecognised request names even to the extent of sending back the correct failure response, or because they handle it sensibly but fill up the server's log file with pointless spam, or whatever. PuTTY therefore supports this bug-compatibility flag: if it believes the server has this bug, it will never send its ‘winadj@putty.projects.tartarus.org’ request, and will make do without its timing data.
4.26.6 ‘Miscomputes SSH-2 HMAC keys’
Versions 2.3.0 and below of the SSH server software from ssh.com compute the keys for their HMAC message authentication codes incorrectly. A typical symptom of this problem is that PuTTY dies unexpectedly at the beginning of the session, saying ‘Incorrect MAC received on packet’.
If this bug is detected, PuTTY will compute its HMAC keys in the same way as the buggy server, so that communication will still be possible. If this bug is enabled when talking to a correct server, communication will fail.
This is an SSH-2-specific bug.
4.26.7 ‘Miscomputes SSH-2 encryption keys’
Versions below 2.0.11 of the SSH server software from ssh.com compute the keys for the session encryption incorrectly. This problem can cause various error messages, such as ‘Incoming packet was garbled on decryption’, or possibly even ‘Out of memory’.
If this bug is detected, PuTTY will compute its encryption keys in the same way as the buggy server, so that communication will still be possible. If this bug is enabled when talking to a correct server, communication will fail.
This is an SSH-2-specific bug.
4.26.8 ‘Requires padding on SSH-2 RSA signatures’