Cryptography 101 :)
Symmetric Stream Ciphers: http://media.grc.com/sn/SN-031.mp3
Symmetric Block Ciphers: http://media.grc.com/sn/SN-033.mp3
Public Key Cryptographie: http://media.grc.com/sn/SN-034.mp3
Cryptographic Hashes: http://media.grc.com/sn/SN-035.mp3
Crypte Series Wrap Up: http://media.grc.com/sn/SN-037.mp3
They just rock :)
p.s. I already blogged about them