Re: libssh-0.4.1 breaks application

[Eugene Starozhilov <estarozhilov@xxxxxxxxx>]

Hi Aris,

When I wrote "Now client and server choose different
encryption algorithms during key negotiation" I meant that
server chose aes256-ctr and client chose aes128-cbc so 
client and server just can't communicate.

According to RFC 4253:
A name-list of acceptable symmetric encryption algorithms (also
         known as ciphers) in order of preference.  The chosen
         encryption algorithm to each direction MUST be the first
         algorithm on the client's name-list that is also on the
         server's name-list.  If there is no such algorithm, both sides
         MUST disconnect.

It seems that server side (libssh-0.4.1) doesn't follow RFC 4253 in choosing encryption algorithm.

Regards,
Eugene



--- On Tue, 3/2/10, Aris Adamantiadis <aris@xxxxxxxxxxxx> wrote:

From: Aris Adamantiadis <aris@xxxxxxxxxxxx>
Subject: Re: libssh-0.4.1 breaks application
To: libssh@xxxxxxxxxx
Date: Tuesday, March 2, 2010, 3:20 AM

Hi Eugene,

This is due to two fixes issued in 0.4.1:
-introduction of aes128-ctr, aes192-ctr and aes256-ctr
-Change in client key selector which gives the client priority on the
algorithms to choose.

I'd say it's a feature and not a bug. aes256-cbc is left for
compatibility but by default, aes256-ctr is choosen. The latter is more
secure and fix a cryptographic bug in the cbc version.

Do these changes really impact the performances or usability of your
application ? In last resort, you can set the preferred keys using
ssh_set_options().

Aris

Eugene Starozhilov a écrit :
> Hi,
> 
> I just moved my application which uses sever libssh API from
> libssh-0.4.0 to libssh-0.4.1. Now client and server choose different
> encryption algorithms during key negotiation. This problem can be
> reproduced using samplesshd as server and ssh as a client.
> 
> CLIENT (ssh)
> 
> debug2: mac_init: found hmac-sha1
> 
> debug1: kex: server->client aes128-cbc hmac-sha1 none
> 
> debug2: mac_init: found hmac-sha1
> 
> debug1: kex: client->server aes128-cbc hmac-sha1 none
> 
> SERVER (samplesshd)
> 
> [3] Type 20
> 
> [3] Set output algorithm aes256-ctr
> 
> [3] Set input algorithm aes256-ctr
> 
> 
> Any suggestions how to fix the problem will be greatly appreciated.
> Thanks,
> Eugene
> 
>