I took an intro course last semester on computer security (the encryption part of it). We covered:
RSA (based on not being able to factor large (200 digit) numbers into two primes)
Public-key Cryptography
DES (leads to 3-DES and AES; two modern techniques)
I can't remember what else we went over. Some of it is mathematically intensive. For RSA, I'd suggest reading on Galois Fields.
The links are introductory material on the algorithms but work through them. Let me know if you have any trouble.