[Bug 769] dh-group-exchange should be configurable off in client and server

[bugzilla-daemon at mindrot.org]

http://bugzilla.mindrot.org/show_bug.cgi?id=769

           Summary: dh-group-exchange should be configurable off in client
                    and server
           Product: Portable OpenSSH
           Version: 3.7p1
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: ssh
        AssignedTo: openssh-bugs at mindrot.org
        ReportedBy: jacobn+mindrot at chiark.greenend.org.uk


I have observed the extra overhead of dh-group-exchange to cause people to 
switch from using SSH-2 back to SSH-1, in several contexts. This is 
unfortunate, as SSH-2 with the default group is still presumably several miles 
more secure than SSH-1.

The OpenSSH client and server should both allow dh-group-exchange to be turned 
off (and in general, allow configuration of kex method preferences).

Supporting data:
On slow machines (e.g., 486-class, old SPARCs), dh-gex takes an unreasonably 
long time (e.g., 14 seconds for gex followed by kex on a SS20 server). 
Disabling gex (at the client end - there's a config option in PuTTY) gives a 
much more sensible delay (2 seconds).

Also, from draft-ietf-secsh-architecture-15.txt:

   The following policy issues SHOULD be addressed in the configuration
   mechanisms of each implementation:
      [...]
   o  Public key algorithms and key exchange method to be used for host
      authentication.

(No patch though, sorry. Also, apologies if this has been addressed recently; I 
was going off the man pages on openssh.com.)



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