gnutls-cli fails to handshake with Exchange server that uses DES-CBC3-SHA cipher

[Nikos Mavrogiannopoulos]

Thank you. It seems however that the server you're talking to isn't a
valid TLS server. If you check the negotiation in the attached files,
gnutls suggests some ciphersuites and the server decides to use one
outside the suggested set. Moreover the server decided ciphersuite is
an insecure one using DES-56bit. I don't know if it is configuration
issue (i.e. somebody configured the server to intentionally negotiate
a weak ciphersuite), or bug. In both cases I'd suggest to keep away
from this server. If you cannot then just find a priority string that
works for this server and use it. It is broken and there is nothing we
can do in gnutls to fix it.

If openssl negotiates with this server it means that it accepts weak
ciphersuites, something that we don't do unless explicitly instructed.

regards,
Nikos

On Tue, Mar 27, 2012 at 12:13 AM, Thomas Fitzsimmons
<fitzsim at fitzsim.org> wrote:
> Nikos Mavrogiannopoulos <nmav at gnutls.org> writes:
>
>> On 03/24/2012 10:57 PM, Thomas Fitzsimmons wrote:
>>
>>> Hi,
>>> gnutls-cli --verbose --debug 10 --port 993 "<imap_hostname>"
>>> fails to handshake with my Exchange server, whereas
>>> openssl s_client -debug -port 993 -host "<imap_hostname>"
>>> succeeds.  OpenSSL reports that the server is using the DES-CBC3-SHA
>>> cipher.
>>> For background on this issue see:
>>> http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10904#14
>>
>>
>> Hello,
>>  The comment below in the thread is very interesting. Could you send me
>> a capture of a failed handshake?
>>
>>> gnutls.c: [1] Received unexpected handshake message 'CERTIFICATE'
>>> (11). Expected 'SERVER HELLO' (2)
>
> Yes, attached the redacted output of:
>
> ./gnutls-cli --debug 10 --verbose --port 993 <imap_hostname>
>
> from gnutls HEAD.
>
>> Do priority strings like the ones below help?
>> http://www.gnu.org/software/gnutls/manual/html_node/Interoperability.html
>
> Also attached the redacted output after adding:
>
> 1. --priority "NORMAL:%COMPAT"
> 2. --priority "NORMAL:-VERS-TLS-ALL:+VERS-TLS1.0:+VERS-SSL3.0:%COMPAT"
> 3. --priority "NORMAL:-VERS-TLS-ALL:+VERS-TLS1.0:+VERS-SSL3.0:-CIPHER-ALL:+ARCFOUR-128:%COMPAT"
>
> The third priority setting works by using ARCFOUR-128.
>
> Thomas