'Get a context factory with which to negotiate TLS.
@rtype: L{None} or
L{ClientContextFactory <twisted.internet.ssl.ClientContextFactory>}
@return: A context factory or L{None} if TLS is not supported on the
client.'

def _getContextFactory(self):

    try:
        from twisted.internet import ssl
    except ImportError:
        return None
    else:
        context = ssl.ClientContextFactory()
        context.method = ssl.SSL.TLSv1_METHOD
        return context
