One of the distinguishing features of a good encryption algorithm, is that it is easy to encrypt, and hard to crack. Are there any that are easy enough for average folk to remember, and calculate by hand, and still stand up to brute force attacks on a computer.
Imagine, a prisoner (with pen and paper) sending a message to another inmate, and the guards seize the handwritten message - and put their prison-crypto-cracking department on it.
Currently, I am thinking TEA is the best candidate, but pretty hard to remember I think.
