encryption program

[Ed Falk]

> what do you mean, polyalphabetic substitution?

Someone can correct me if I'm wrong, but I believe it refers to a cipher
where each character is encrypted individually, using a different
monoalphabetic cipher for each one.  The ciphers repeat after
a certain period, usually the key length.

Vigenere is the simplest polyalphabetic cipher of them all, with
the individual ciphers simply being ROT-n.