An introduction to number theory covering basics, congruences, primes, the Chinese Remainder Theorem, Wilson's Theorem, Fermat's Little Theorem, Euler's Phi function, etc.
Covers some connections to cryptology, from basic ideas and terminology through RSA, Diffie-Hellman, and ElGamal.
Suitable for a one-semester undergraduate course.
See http://www.poritz.net/jonathan/share/yaintt/ for much more information.