[Ach] SSLyze / GnuTLS

Adi Kriegisch adi at kriegisch.at
Fri Nov 22 08:32:12 CET 2013


> So here's some documentation that isn't very helpful, I found:
> http://www.exim.org/exim-html-current/doc/html/spec_html/ch-encrypted_smtp_connections_using_tlsssl.html 
> But it points to http://www.gnutls.org/manual/html_node/Priority-Strings.html
Great! This documentation is for GnuTLS3 whereas Debian/stable comes with
GnuTLS2 (2.12.20).
> So here's one that I came up with that exim actually starts with:
> tls_require_ciphers =
For which kind of communication? 

> One can test what that would result in with
> $ gnutls-cli --priority "$prioritylist" -l
Hmmm... yeah -- that works fine on GnuTLS3 but doesn't for older versions.
AFAIK there is no way to expand a GnuTLS ciper string in older GnuTLS
versions. Although there are some library functions that might be used to
get exactly that...
> That might be broken on wheezy (can somebody confirm?) but on Fedora it sort of
> works. The results for the list above are very minimal:
Compared to the cipher string you gave to it, it looks correct; compare it
(as the string starts with NONE, I just count the '+' parts):

> TLS_RSA_AES_256_CBC_SHA1                            0x00, 0x35  SSL3.0
> TLS_RSA_AES_256_CBC_SHA256                          0x00, 0x3d  TLS1.2
> TLS_RSA_AES_128_CBC_SHA1                            0x00, 0x2f  SSL3.0
> TLS_RSA_AES_128_CBC_SHA256                          0x00, 0x3c  TLS1.2

> Certificate types: none
> Protocols: VERS-TLS1.2, VERS-TLS1.1, VERS-TLS1.0, VERS-SSL3.0, VERS-DTLS1.0

> Compression: COMP-NULL

> Elliptic curves: none
> PK-signatures: SIGN-RSA-SHA512, SIGN-RSA-SHA384, SIGN-RSA-SHA256,

> But my output on wheezy looks different from that, listing MACs and key
> exchange algorithms … Hrmrmrm. Very sceptical.
GnuTLS on Wheezy does not filter because it does not support a version
string. So it shows all it supports.
On Redhat/Fedora elliptic curves were disabled for the past years due to an
unclear legal situation. Only very recently they started enabling ECC in
> Funky bits like %SERVER_PRECEDENCE don't seem to work with exim either. It
Hmm... This is only available in GnuTLS3, I guess. Same issue with apache's

> would really help if another tool could enumerate whatever it actually offers.
> I remember some shell script that uses openssl in a really inefficient way to
> list ciphers - given that nothing else worked that might be an option? Pointers
> welcome …
Hehe... openssl is to blame here: IANA lists the official cipher suite
names which gnutls (mostly) uses. Only openssl has its own names. eg.

IMHO the only reasonable resolution is via the IANA list and the hex codes:
This table is also available as CSV and XML... So pretty coder friendly.

> > In case you want to help out, would you want to join our meeting next
> > monday?
> I'm not in Vienna at the moment. I'll be back in about a month from now.
Probably we can continue that work over the mailing list, then?! ;-)

-- Adi 

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