The fundamental principle of SSL is this: a client connects to an SSL-enabled server, and the server returns enough information to a) encrypt the communication channel, and b) authenticate itself enough that you can prove that they’re the intended system. (b) is performed by the server providing both its certificate information as well as the CA (certificate authority) that produced it. This latter item is the area in which you might occasionally encounter problems when you have a client that complains about not being able to verify a hostname. This is where some people recommend just passing a flag to skip the check, thus completely compromising the integrity of SSL.

Depending on how reputable your CA is, they might provide additional CA authorities (referred to as IAs, or “intermediate authorities”), such that these authorities form a “certificate chain” that sufficiently proves that all of the authorities that lend their credibility to your certificate can be traced back to one very well-known authority (the “root CA”) that any client (browser, tool, or OS) would know about.

In special or proprietary situations, you might have to physically go into the configuration for your browser/tool/OS, and add a new root CA that the client did not previously know about. Otherwise, the client might forbid access to your website on that system. Unless your dealing with some hightened security situation regarding the intranet at your place of business, this is rarely necessary.

Sometimes, it’s necessary to physically inspect what CAs are being reported by the server, for as simple a reason as just verifying that you’ve configured it correctly. Until recently, and even, arguably, at the present time, Python has been unable to provide this information, as it comes prepackaged with a natively-compiled SSL module and the underlying mechanics simply don’t expose these calls. If you want this information, you’d be forced to just invoke OpenSSL’s “s_client” subcommand on the command-line.

Just recently, a patch was released that exposes this functionality. As a warning, since this hasn’t yet been introduced into the source-tree, its implementation might change by the time it has.

This is some cut-up and reassembled code, to show it in action:

import socket

from ssl import wrap_socket, CERT_NONE, PROTOCOL_SSLv23
from ssl import SSLContext  # Modern SSL?
from ssl import HAS_SNI  # Has SNI?

from pprint import pprint

def ssl_wrap_socket(sock, keyfile=None, certfile=None, cert_reqs=None,
                    ca_certs=None, server_hostname=None,
                    ssl_version=None):
    context = SSLContext(ssl_version)
    context.verify_mode = cert_reqs

    if ca_certs:
        try:
            context.load_verify_locations(ca_certs)
        # Py32 raises IOError
        # Py33 raises FileNotFoundError
        except Exception as e:  # Reraise as SSLError
            raise SSLError(e)

    if certfile:
        # FIXME: This block needs a test.
        context.load_cert_chain(certfile, keyfile)

    if HAS_SNI:  # Platform-specific: OpenSSL with enabled SNI
        return (context, context.wrap_socket(sock, server_hostname=server_hostname))

    return (context, context.wrap_socket(sock))

hostname = 'www.google.com'

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((hostname, 443))

(context, ssl_socket) = ssl_wrap_socket(s,
                                       ssl_version=2, 
                                       cert_reqs=2, 
                                       ca_certs='/usr/local/lib/python3.3/dist-packages/requests/cacert.pem', 
                                       server_hostname=hostname)

pprint(ssl_socket.getpeercertchain())

s.close()

The output is a tuple of dictionaries:

({'issuer': ((('countryName', 'US'),),
             (('organizationName', 'Google Inc'),),
             (('commonName', 'Google Internet Authority G2'),)),
  'notAfter': 'Sep 11 11:04:38 2014 GMT',
  'notBefore': 'Sep 11 11:04:38 2013 GMT',
  'serialNumber': '50C71E48BCC50676',
  'subject': ((('countryName', 'US'),),
              (('stateOrProvinceName', 'California'),),
              (('localityName', 'Mountain View'),),
              (('organizationName', 'Google Inc'),),
              (('commonName', 'www.google.com'),)),
  'subjectAltName': (('DNS', 'www.google.com'),),
  'version': 3},
 {'issuer': ((('countryName', 'US'),),
             (('organizationName', 'GeoTrust Inc.'),),
             (('commonName', 'GeoTrust Global CA'),)),
  'notAfter': 'Apr  4 15:15:55 2015 GMT',
  'notBefore': 'Apr  5 15:15:55 2013 GMT',
  'serialNumber': '023A69',
  'subject': ((('countryName', 'US'),),
              (('organizationName', 'Google Inc'),),
              (('commonName', 'Google Internet Authority G2'),)),
  'version': 3},
 {'issuer': ((('countryName', 'US'),),
             (('organizationName', 'Equifax'),),
             (('organizationalUnitName',
               'Equifax Secure Certificate Authority'),)),
  'notAfter': 'Aug 21 04:00:00 2018 GMT',
  'notBefore': 'May 21 04:00:00 2002 GMT',
  'serialNumber': '12BBE6',
  'subject': ((('countryName', 'US'),),
              (('organizationName', 'GeoTrust Inc.'),),
              (('commonName', 'GeoTrust Global CA'),)),
  'version': 3},
 {'issuer': ((('countryName', 'US'),),
             (('organizationName', 'Equifax'),),
             (('organizationalUnitName',
               'Equifax Secure Certificate Authority'),)),
  'notAfter': 'Aug 22 16:41:51 2018 GMT',
  'notBefore': 'Aug 22 16:41:51 1998 GMT',
  'serialNumber': '35DEF4CF',
  'subject': ((('countryName', 'US'),),
              (('organizationName', 'Equifax'),),
              (('organizationalUnitName',
                'Equifax Secure Certificate Authority'),)),
  'version': 3})

The topmost item is the most specific and describes the certificate for the domain itself, whereas the bottommost one is the least specific, and describes the highest, most well known authority involved in the operation (in this case, Equifax).