About a month ago, I had decided to make programs to encrypt/decrypt messages on-calc. Basically, you store codes into L1 and run prgmDECRYPT to see the message, or you run prgmENCRYPT, enter a phrase, and copy the codes out of L1 to share. It's nothing more than a simple character --> number code, but it works. Feel free to use them however you would like and/or add more code to support more characters.

http://sc.cemetech.net/?xpi=08a298e7eb00d50c89c94bdcd8a44fc8
http://sc.cemetech.net/?xpi=84013841ed0ad04429a570eeb3f0f577

Very tight code, I'm quite impressed! My only two suggestions would be to combine the encoder and decoder, to avoid having to have two separate programs and instances of the key string, and to store the output to a list other than L1, but they're minor complaints.

Suggestion, why not add in a number to encrypt by? Such as instead of a linear security thing. The user can type in a value and the program will encrypt based on that number. If another number is used to decrypt, the message would still be unreadable.

Not like the lists would ever make it into the wrong hands on a daily basis, but it'd be possible to transfer them across the web with a sense of security.

Of course, you'd want a large space of numbers, because a simple brute force / frequency-analysis algorithm could crack the encrypted data.