[gnutls-help] The signature_algorithms extension and login.live.com

[Mark Doliner]

On Thu, Sep 11, 2014 at 3:02 AM, Nikos Mavrogiannopoulos
<nmav at gnutls.org> wrote:
> On Wed, Sep 10, 2014 at 8:43 PM, Mark Doliner <mark at kingant.net> wrote:
>> Hi! I work on the instant messaging program Pidgin. I'm fiddling with
>> the priority string that we pass to gnutls_priority_init(), trying to
>> find a value that disables weaker ciphers while remaining compatible
>> with the majority of servers.
>
> Well, I'd suggest in using each versions NORMAL:%COMPAT option in that
> case (or gnutls_priority_set_default()). The ciphers are prioritized
> by security margin and remain compatible with the majority if not all
> of the servers (I think your use case is the exact reason for the
> default settings).

Hmm, yeah, using NORMAL is very reasonable. I would certainly prefer
if we could use the default, but it does seem like we can improve
things slightly by using a custom string. I checked in a change to our
development branch for now to use a custom string. Assuming no one has
problems with it I think it's likely we'll release it at some point.
But I don't have strong feelings either way and I could be persuaded
to switch back to NORMAL.

The string I used is:
SECURE192:+SECURE128:-RSA:+RSA:+SIGN-RSA-SHA1:-DHE-DSS:-VERS-SSL3.0

I wrote a really long comment explaining my reasoning, if you're curious:
https://hg.pidgin.im/pidgin/main/rev/76a2a6d75768

>> Related question:
>> RFC5246 section 7.4.1.4.1. Signature Algorithms says "this extension
>> is not meaningful for TLS versions prior to 1.2. Clients MUST NOT
>> offer it if they are offering prior versions." However it looks like
>> GnuTLS sends the signature algorithms extension even when it sends a
>> version of SSL 3.0. Seems like it should only send the extension when
>> version is TLS 1.2 or higher. Is that a bug? Am I misinterpreting the
>> spec?
>
> Does this happen with the gnutls 3.x versions or is it only in 2.12.x?

If I'm interpreting Wireshark correctly, yes. I see it happen with
GnuTLS 2.12.23, 3.2.11, 3.2.17, and 3.3.7.