You might want to try Bruce Schneier's Solitaire cryptosystem - you'll find a link to perl sourcecode near the top of the page. It's not totally secure as you'll read on his site but it does do fairly good encryption and doesn't add appreciably to the length of the encrypted string. On a quick test run, plain text was 439 characters long and encrypted text was 457 characters long. A shorter 16 character plain text was encrypted into a 19 character string. The program does spit out the encrypted text in 5 character groups but a simple join statement will fix that.

BTW, you'll also find reference to Neal Stephenson's novel Cryptonomicon. If you haven't read it (and the prequel System of the World series), do yourself an favor and get them. You won't be sorry.

Hope this helps,

Jack

UPDATE Since Solitaire is emulating a standard deck of 52 cards, this gives you double the 26 letter English alphabet. Because of this, the program as written doesn't handle numbers, punctuation or special characters. Those characters are dropped/ignored.