This is a very nice intro to the most important concepts. I'm doing a (short) series on cryptography in Python. Mine is more targeted at the hobbyist programmer. Here's Part 1: <a href="http://bityard.blogspot.com/2010/01/symmetric-encryption-with-pycrypto-part.html" rel="nofollow">http://bityard.blogspot.com/2010/01/symmetric-encryption-wit...</a>