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HOWTO: Programming SSL in Python with M2Crypto

Ng Pheng Siong (ngps@netmemetic.com) and Heikki Toivonen (heikki@osafoundation.org)


M2Crypto is a Python interface to OpenSSL. It makes available to the Python programmer SSL functionality to implement clients and servers, S/MIME v2, RSA, DSA, DH, symmetric ciphers, message digests and HMACs.

This document demonstrates programming HTTPS with M2Crypto.

A bit of history

M2Crypto was created during the time of Python 1.5, which features a module httplib providing client-side HTTP functionality. M2Crypto sports a httpslib based on httplib.

Beginning with version 2.0, Python's socket module provided (rudimentary) SSL support. Also in the same version, httplib was enhanced with class HTTPConnection, which is more sophisticated than the old class HTTP, and HTTPSConnection, which does HTTPS.

Subsequently, M2Crypto.httpslib grew a compatible (but not identical) class HTTPSConnection.

The primary interface difference between the two HTTPSConnection classes is that M2Crypto's version accepts an M2Crypto.SSL.Context instance as a parameter, whereas Python 2.x's SSL support does not permit Pythonic control of the SSL context.

Within the implementations, Python's HTTPSConnection employs a FakeSocket object, which collects all input from the SSL connection before returning it to the application as a StringIO buffer, whereas M2Crypto's HTTPSConnection uses a buffering M2Crypto.BIO.IOBuffer object that works over the underlying M2Crypto.SSL.Connection directly.

Since then M2Crypto has gained a Twisted wrapper that allows securing Twisted SSL connections with M2Crypto.

Secure SSL

It is recommended that you read the book Network Security with OpenSSL by John Viega, Matt Messier and Pravir Chandra, ISBN 059600270X.

Using M2Crypto does not automatically make an SSL connection secure. There are various steps that need to be made before we can make that claim. Let's see how a simple client can establish a secure connection:

ctx = SSL.Context()
ctx.set_verify(SSL.verify_peer | SSL.verify_fail_if_no_peer_cert, depth=9)
if ctx.load_verify_locations('ca.pem') != 1: raise Exception('No CA certs')
s = SSL.Connection(ctx)
# Normal protocol (for example HTTP) commands follow

The first line creates an SSL context. The defaults allow any SSL version (except SSL version 2 which has known weaknesses) and sets the allowed ciphers to secure ones.

The second line tells M2Crypto to perform certificate validation. The flags shown above are typical for clients, and requires the server to send a certificate. The depth parameter tells how long certificate chains are allowed - 9 is pretty common default, although probably too long in practice.

The third line loads the allowed root (certificate authority) certificates.

The fourth line creates an SSL connection object with the secure context.

The fifth line connects to the server. During this time we perform the last security step: just after connection, but before exchanging any data, we compare the commonName (or subjectAltName DNS field) field in the certificate the server returned to the server address we tried to connect to. This happens automatically with SSL.Connection and the Twisted wrapper class, and anything that uses those. In all other cases you must do the check manually. It is recommended you call the SSL.Checker to do the actual check.

SSL servers are different in that they typically do not require the client to send a certificate, so there is usually no certificate checking. Also, it is typically useless to perform host name checking.

Code Samples

The best samples of how to use the various SSL objects are in the tests directory, and the test_ssl.py file specifically. There are additional samples in the demo directory, but they are not quaranteed to be up to date.

NOTE: The tests and demos may not be secure as is. Use the information above on how to make them secure.


ssldump "is an SSLv3/TLS network protocol analyser. It identifies TCP connections on the chosen network interface and attempts to interpret them as SSLv3/TLS traffic. When it identifies SSLv3/TLS traffic, it decodes the records and displays them in a textual form to stdout. If provided with the appropriate keying material, it will also decrypt the connections and display the application data traffic.

If linked with OpenSSL, ssldump can display certificates in decoded form and decrypt traffic (provided that it has the appropriate keying material)."

ssldump is written by Eric Rescorla.