Bug 2322

Hi.

This is basically from https://lists.mindrot.org/pipermail/openssh-unix-dev/2014-November/033176.html .


1) In encryption, compression may generally be abused as an oracle for side-channel attack, when the attacker can inject chosen-plaintext (e.g. as in CRIME and BREACH attacks).

While this is far less likely for SSH as in CRIME and BREACH it's not really impossible to happen (see the above mail thread for example scenarios like log files that are transferred via SSH).

The SSH client side can protect against this, simply by disabling encryption.
The SSH server side may want to only allow certain users to use SSH-protocol-level compression. E.g. users which are only used for automated systems where it's known that no chosen plain text can be injected,... or human users which have been educated about that principal problem.
Of course, even when SSH-level compression is disallowed, users could still do any other manual compression, which would be vulnerable to the same problem - but better protect against something than nothing.


2) Even though gzip is a rather fast compression algorithm (at least compared to some others), it may be desirable for a sysadmin to allow only certain users to enable encryption (especially since the user could choose high compression levels) in order to prevent overloading the system.



For these two motivations, it would be nice if one could control on the SSH server side, whether a user is allowed to use compression or not, ideally via the Match block, e.g.

#global setting:
Compression no

#per user settings:
Match User foo bar
     Compression delayed

As Damien suggested in the thread above - a clean solution would be, that if such configuration is given, the server enforces a key-renegotiation immediately after the handshake with the "new" compression algos.


Regards,
Philippe