Birthday Paradox and Hash Collisions ( pdf )

Put 23 people into a room and there is a 50% chance that two of them will share a birthday. This has implications for the security of stored credentials and sets a fundamental limit on our ability to ensure the integrity of data.
The Monographic Substitution Cipher: from Julius Caesar to the KGB ( pdf )

A tour of monographic encryption schemes from simple shift ciphers to the one-time-pad.
Perfect Secrecy and the Unbreakable Cipher ( pdf )

In the 1940's Claude Shannon proved the existence of a cryptosystem that afforded perfect secrecy -- a cipher with no mathematical weaknesses. The recipe? A random key equal in length to the message that can be used only once. Leave it to the Soviets to re-use keys, a fatal mistake that did not go unnoticed by American cryptanalysts during the Cold War.
A word on ethics, hacking, and ethical hacking

Disclaimer and ground rules.
Eavesdropping on the LAN, Part 1: Active sniffing attacks and defenses

If an intruder gains access to your network, these are some of the methods they will employ to eavesdrop on and hijack communications. After demonstrating several examples, we discuss recommended defenses.

Eavesdropping on the LAN, Part 2: Applications to Cleartext and Encrypted Protocols

Upon establishing a man-in-the-middle on the LAN, the intruder attacks a variety of cleartext and encrypted communications. We demonstrate what this looks like from the intruder's perspective, and describe several techniques for getting around the encrypted protocols in common use today.