openssh unix dev - Feb 2019 - Can we disable diffie-hellman-group-exchange-sha1 by default?

I ask because the removal of diffie-hellman-group-exchange-sha1 happened
accidently in 7.8 due to a mistake in a change to readconf.c. I noticed
this and filed a bug about it along with a patch to fix readconf.c to use
KEX_CLIENT_* like it used to:

https://github.com/openssh/openssh-portable/commit/1b9dd4aa
https://bugzilla.mindrot.org/show_bug.cgi?id=2967

Its clear the removal was unintentional because myproposal.h still
lists diffie-hellman-group-exchange-sha1 under KEX_CLIENT_KEX:

https://github.com/openssh/openssh-portable/blob/V_7_8_P1/myproposal.h#L102


I was just thinking that if this hasn't been causing a problem for the
last 2 releases then maybe its time to disable this by default in the
client. This algorithm has been disabled by default in the server since
the 6.7 release in October 2014.


-- 
Jon
Doge Wrangler
X(7): A program for managing terminal windows. See also screen(1) and tmux(1).

Hi John,

The short answer is YES.

Jon DeVree <nuxi at vault24.org> writes:
> I ask because the removal of diffie-hellman-group-exchange-sha1 happened
> accidently in 7.8 due to a mistake in a change to readconf.c. I noticed
> this and filed a bug about it along with a patch to fix readconf.c to use
> KEX_CLIENT_* like it used to:
The diffie-hellman-group-exchange-sha1 is an optional key exchange
method provided by RFC4419 and updated by RFC8270.

Support for it is not required and may (and in my opinion should) be
disabled by default without any impact to the SSHv2 protocol.

The only two Mandatory To Implement (MTI) key exchange methods are those
in RFC3253 (diffie-hellman-group1-sha1 and diffie-hellman-group14-sha1).
Even though they are MTIs, that just means you need to be able configure
them, there is no mandatory requirement that a given installation enable
them by default.

	Enjoy!
	-- Mark

Can we disable diffie-hellman-group14-sha1 too?

On Thu, Feb 14, 2019 at 10:23 PM Mark D. Baushke <mdb at juniper.net>
wrote:
>
> Hi John,
>
> The short answer is YES.
>
> Jon DeVree <nuxi at vault24.org> writes:
>
> > I ask because the removal of diffie-hellman-group-exchange-sha1
happened
> > accidently in 7.8 due to a mistake in a change to readconf.c. I
noticed
> > this and filed a bug about it along with a patch to fix readconf.c to
use
> > KEX_CLIENT_* like it used to:
>
> The diffie-hellman-group-exchange-sha1 is an optional key exchange
> method provided by RFC4419 and updated by RFC8270.
>
> Support for it is not required and may (and in my opinion should) be
> disabled by default without any impact to the SSHv2 protocol.
>
> The only two Mandatory To Implement (MTI) key exchange methods are those
> in RFC3253 (diffie-hellman-group1-sha1 and diffie-hellman-group14-sha1).
> Even though they are MTIs, that just means you need to be able configure
> them, there is no mandatory requirement that a given installation enable
> them by default.
>
>         Enjoy!
>         -- Mark
> _______________________________________________
> openssh-unix-dev mailing list
> openssh-unix-dev at mindrot.org
> https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev