pyOpenSSL

do_handshake resets cipher list (server side)

Asked by Israel Nir

Hi folks, you are my last hope!
I'm trying to force my server to use 'RC4-SHA' for debugging reasons.
I first do
ctx.set_options(OP_CIPHER_SERVER_PREFERENCE)
ctx.set_cipher_list('RC4-SHA')

and everything looks great, that is,

connection = SSL.Connection(ctx, connection)
connection.set_accept_state()
print "before", connection.get_cipher_list()

prints "before ['RC4-SHA']"

However, next I call do_handshake:

connection.do_handshake()
print "after", connection.get_cipher_list()

and it prints the default cipher list. Moreover, in Wireshark I can see that my server's reply chose a cipher other than 'RC4-SHA', even though the client handshake declared that it supports it.

Am I doing something wrong, or is this a bug?

I'm using OS.X 10.6.8 with Python 2.7.5, OpenSSL 1.0.1e and pyOpenSSL 0.13

Any help will be greatly appreciated.

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Israel Nir
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Jean-Paul Calderone (exarkun) said : 		#1

Consider including an <http://sscce.org/> rather than just a few fragments of a program.

Also, I haven't noticed a post on the mailing list about this topic yet, so you have at least one more forum to which to resort (ie, Launchpad bugs needn't be your last hope).

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Israel Nir (israeln) said : 		#2

Thanks Jean-Paul,

The code is actually part of mitmproxy's netlib (https://github.com/mitmproxy/netlib).
Here's the full method, but I think my few fragments are more "sscce" in this case :)

    def convert_to_ssl(self, cert, key, method=SSLv23_METHOD, options=None, handle_sni=None, request_client_cert=False, cipher_list=None):
        ctx = SSL.Context(method)
        if not options is None:
            ctx.set_options(options)
        ctx.set_options(OP_CIPHER_SERVER_PREFERENCE)
        if cipher_list:
            ctx.set_cipher_list(cipher_list)
        if handle_sni:
            # SNI callback happens during do_handshake()
            ctx.set_tlsext_servername_callback(handle_sni)
        ctx.use_privatekey_file(key)
        ctx.use_certificate(cert.x509)
        if request_client_cert:
            def ver(*args):
                self.clientcert = certutils.SSLCert(args[1])
            ctx.set_verify(SSL.VERIFY_PEER, ver)
        self.connection = SSL.Connection(ctx, self.connection)
        self.ssl_established = True
        self.connection.set_accept_state()
        try:
            print "before", self.connection.getpeername(), self.connection.get_cipher_list()
            self.connection.do_handshake()
            print "after", self.connection.getpeername(), self.connection.get_cipher_list()
        except SSL.Error, v:
            raise NetLibError("SSL handshake error: %s"%str(v))
        self.rfile.set_descriptor(self.connection)
        self.wfile.set_descriptor(self.connection)

Revision history for this message
Jean-Paul Calderone (exarkun) said : 		#3

I might run an SSCCE in a few environments to try to reproduce the problem and see if I can understand why it happens. I'm not going to try to write a program that reproduces the problem, though.

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Israel Nir (israeln) said : 		#4

You are absolutely right.
I had to overcome my fear of openssl, and make some demo code. While doing so, I've discovered the problem.

In the callback defined by

ctx.set_tlsext_servername_callback(handle_sni)

No cipher list was defined. I guess that once this callback is called during the handshake process, previous configuration is reset.