openssh bugs - Dec 2014 - [Bug 2322] New: please let the server enable/disable delayed compression on a per user basis

https://bugzilla.mindrot.org/show_bug.cgi?id=2322

            Bug ID: 2322
           Summary: please let the server enable/disable delayed
                    compression on a per user basis
           Product: Portable OpenSSH
           Version: 3.7p1
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: sshd
          Assignee: unassigned-bugs at mindrot.org
          Reporter: philcerf at gmail.com

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

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https://bugzilla.mindrot.org/show_bug.cgi?id=2322

Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |djm at mindrot.org
           Assignee|unassigned-bugs at mindrot.org |djm at mindrot.org

--- Comment #1 from Damien Miller <djm at mindrot.org> ---
Created attachment 2515
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2515&action=edit
Require multiple publickey entries in AuthenticationMethods use
different keys

Thanks for reminding me to do this - I've been planning it for a while.
Here's a patch that implements it for -current.

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https://bugzilla.mindrot.org/show_bug.cgi?id=2322

--- Comment #2 from Damien Miller <djm at mindrot.org> ---
Comment on attachment 2515
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2515
Require multiple publickey entries in AuthenticationMethods use
different keys

oops, wrong bug - please disregard

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #3 from Damien Miller <djm at mindrot.org> ---
Unfortunately this isn't trivially possible - in the SSH protocol,
compression is negotiated before the authentication, and it is only at
the authentication stage that the server learns the name of the user
who is connecting.

It is theoretically possible to force a rekeying operation after
authentication to renegotiate the encryption/compression options but
this is expensive and IMO not worth the complexity needed to implement
it.

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #4 from Damien Miller <djm at mindrot.org> ---
closing resolved bugs as of 8.6p1 release

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