[mod_gnutls-devel] mod-gnutls: Ciphers selection problem ?

[Daniel Kahn Gillmor]

On Fri 2017-12-08 17:13:44 +0100, Thomas Klute wrote:
> Am 08.12.2017 um 12:44 schrieb osg at free.fr:
>> I have configured this servername with cipher SECURE, and using curl I have this negotiation: TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384
>> ServerName      ssl-secure.tst.osgpcq.net
>> GnuTLSPriorities        SECURE
>> curl -k -v https://ssl-secure.tst.osgpcq.net
>> * SSL connection using TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384
>> 
>> 
>> If I'm trying to configure an another servername with only one cypher: TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384
>> ServerName      ssl-mono.tst.osgpcq.net
>> GnuTLSPriorities        NONE:+VERS-TLS-ALL:+ECDHE-RSA:+AES-256-GCM:+SHA384:+COMP-NULL
>> curl -k -v https://ssl-mono.tst.osgpcq.net
>> * error:14077410:SSL routines:SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure                                                                                                                                                                  
>> Curl is not able to negotiate...
>> 
>> 
>> Could you indicate me if I haven't configured correctly the GnuTLSPriorities or if the problem is elsewhere ?
>
> If I run the 00_basic test of the mod_gnutls test suite with the
> GnuTLSPriorities changed to your value, the error log reports this:
>
> [Fri Dec 08 16:50:57.773880 2017] [gnutls:info] [pid 27056:tid
> 139957247129344] [client ::1:35478] GnuTLS: Handshake Failed (-326) 'No
> or insufficient priorities were set.'
>
> There are two problems here:
>
> 1) The only MAC algorithm you have allowed is SHA384, but you are using
> an AEAD cipher. You have to allow the AEAD MAC algorithm ("+AEAD").
>
> 2) Your GnuTLSPriorities do not include any signature algorithm and
> group configuration, both of which are required for the handshake. The
> GnuTLSPriorities string is passed transparently to the GnuTLS library,
> so please check the GnuTLS priority documentation [1] for possible
> values. The catch-all solution would be to add "+SIGN-ALL:+GROUP-ALL".

In general, you can also test these sorts of things taking mod_gnutls
out of the loop by experimenting with gnutls-serv (this binary ships in
the gnutls-bin package in debian).  If you can't get the priority string
to work with gnutls-serv, you probably won't be able to get it to work
with mod_gnutls.  otoh, if you *can* get it to work with gnutls-serv,
but it doesn't work with mod_gnutls, then that might indicate a bug in
mod_gnutls.

hth,

        --dkg