Just bringing you another API, it's the RC4 encryption algorithm re implemented for ComputerCraft, RC4 is a fast stream algorithm and is used in protocols like SSL and WEP.
Download Link: http://pastebin.com/rDUe5wGe
Each time you encrypt something, the internal state changes, so you have to decrypt the same amount of data as you encrypt for the sessions to stay in sync, Meaning that if you encrypt two messages, you need to decrypt both messages otherwise the decryption stream wont have the same key state as the encryption.
I'm not very good with documentation. So please just leave any comments here and I'll help the best I can. Thanks