== Using Python to find out what cipher a SSL server is using

I have a [[new-found interest ../tech/SSLForwardSecrecy]] in finding out
what ciphers various SSL servers around here are using in some easy and
convenient way. Doing this in Perl is easy (there's an example [[here
http://seclists.org/basics/2003/Jun/0702.html]]), but I prefer Python.
I'd normally use [[pyOpenSSL http://pyopenssl.sourceforge.net/]],
[[my favorite Python OpenSSL module PyOpenSSLComments]], but
unfortunately it doesn't (currently) have an interface to the
necessary 'get the connection's cipher' OpenSSL routine, and
there's no visible substitute for it.

(While the _[[.get_cipher_list()|]]_ method of _Connection_ objects
looks tempting, I discovered through some experimentation that it
doesn't return anything like the list of ciphers that are common between
the server and the client.)

However, all is not lost; it turns out that
the commonly available [[M2Crypto module
http://chandlerproject.org/bin/view/Projects/MeTooCrypto]] does have
enough functionality to do this. In fact, it's pretty easy, and so here
is sample code to do it (omitting error checking, as usual):

 from M2Crypto import SSL
 
 def printcipher(host, port):
     ctx = SSL.Context('tlsv1')
     s = SSL.Connection(ctx)
 
     # No, really, all I care is that you successfully
     # negotiated SSL, especially as some of your checks
     # are broken.
     s.postConnectionCheck = None
     s.connect((host, port))
 
     # The space at the end is [sic]
     if s.get_state() == "SSLOK ":
         c = s.get_cipher()
         cp = c.name()
         print "Cipher %s, %d bits" % (cp, len(c))
         if cp.startswith("DHE-") or cp.startswith("EDH-"):
             print "forward secrecy: yes"
         else:
             print "forward secrecy: probably not"
     s.close()

This code works for protocols that start SSL/TLS immediately on
connection, such as https or imaps. Extension to protocols that have a
plaintext conversation and then start TLS (such as ESTMP with STARTTLS)
is left as an exercise for the sufficiently interested.

I believe but am not sure that there is no point in asking for anything
except TLS v1; the OpenSSL _ciphers(1)_ manpage suggests that SSLv3 and
TLSv1 have the same set of ciphers. (Presumably this is TLS v1.0.)

(Note that M2Crypto is pretty under-documented; reading the source is
not so much recommended as required. Fortunately it comes with a large
set of examples.)