This might be a good time to get the RC4 algorithm wedged into your
brain.  It's a far cry from PGP (it relies on a secret key), but it's a
nice programming exercise: http://ciphersaber.gurus.com/

from the page: 
> CipherSaber-1 uses Ron Rivest's RC4 algorithm as published in the second edition of Bruce Schneier's Applied Cryptography. RC4 is widely respected and used in a
> number of products, including SSL, the tool Web browsers use to secure credit card forms. With a long enough key RC4 is considered strong by most experts. RC4 is also
> extraordinarily easy to explain and to reproduce. As Schneier says, "The algorithm is so simple that most programmers can quickly code it from memory." Implementations
> of RC4 are widely available on the Internet but it is actually easier to write your own version.