Diffie Hellman key exchange algorithms

Balaraman, Srinath srinath_balaraman at mentor.com
Tue Sep 25 02:46:51 EST 2007

Hello All,

To add to this question, I also am interested in knowing if the size of
the DH key to be negotiated both from the Server and the Client can be
configured? If yes, how?


-----Original Message-----
From: openssh-unix-dev-bounces+srinath_balaraman=mentor.com at mindrot.org
[mailto:openssh-unix-dev-bounces+srinath_balaraman=mentor.com at mindrot.or
g] On Behalf Of Damien Miller
Sent: Friday, September 21, 2007 10:06 PM
To: Vikram Mhetre
Cc: openssh-unix-dev at mindrot.org
Subject: Re: Diffie Hellman key exchange algorithms

On Thu, 20 Sep 2007, Vikram Mhetre wrote:

> A few questions regarding the OpenSSH support for the Diffie Hellman
key exchange algorithms:
> (1) Are the diffie-hellman-group-exchange-sha256",
> "diffie-hellman-group-exchange-sha1" , "diffie-hellman-group14-sha1"
> "diffie-hellman-group1-sha1" (as defined in RFCs 4253 and RFC 4419)
> the complete list of key exchange algorithms supported by OpenSSH?


> (2) Is there a way to configure the DH key exchange algorithms to be
> supported?


> (3) The /etc/primes file is used for the
> "diffie-hellman-group-exchange-sha256",
> "diffie-hellman-group-exchange-sha1" algorithms. However if the primes
> file does not exist does the SSH connection simply fail if one of
> these two algorithms is chosen during the initial part of the setup.
> That is, based on RFC 4419, since the server cannot choose a suitable
> prime group (p,g) from the primes file (since it does not exist) will
> it simply reject the SSH connection?

That is incorrect. The group exchange methods will fall back to a fixed
modulus (the same used in diffie-hellman-group14-sha1) if they cannot
open /etc/moduli.

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