17 Dec
2003
17 Dec
'03
11:17 p.m.
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.