This tutorial paper gives a fairly gentle introduction to the basic mathematics required to understand encryption systems. Three main areas are covered: modular arithmetic, prime numbers and probability theory.

http://blossoms.mit.edu/sites/default/files/white-paper-Good-Old-Mathematics.pdf

This page is from the website of Thomas Jefferson's home, Monticello, and discusses the history and mechanics of Jefferson's Wheel Cipher.

http://www.monticello.org/reports/interests/wheel_cipher.html

This page provides an electronic version of Jefferson's Wheel Cipher.

http://www.monticello.org/jefferson/wheelcipher/index2.html

This site provides extensive information on the Enigma Machine, including history, classroom exercises and additional resources on the topic.

http://plus.maths.org/issue34/features/ellis/

Information on cryptography from the Internet Mathematics Library of the Math Forum@Drexel University.

http://mathforum.org/library/topics/cryptography/

A more advanced and comprehensive introduction to cryptography

http://www.ciphersbyritter.com/LEARNING.HTM

Downloadable video clips of famous code breakers taken from history

http://www.secretcodebreaker.com/videos.html

This website, sponsored by the company, Barcodes, Inc., provides articles on a wide variety of topics related to cryptography.

http://www.barcodesinc.com/articles/cryptography2.htm