This doesn't really explain Vigenere well, since what's left out is where the key comes from: * If key is the same length as plaintext, and truly random, that'

This doesn't really explain Vigenere well, since what's left out is where the key comes from: * If key is the same length as plaintext, and truly random, that's a One-Time Pad. * If the message is appended to the seed key, that's an Autokey Cipher. * If the key is repeated (what the PHP does but not made clear), that's classical Vigenere. It also doesn't describe breaking them: * OTPs are unbreakable. * You can break Autokey by using crib words (e.g. "the") at different locations of the ciphertext, selecting the most phonetically plausible and working backwards to recover the earlier plaintext. * Vigenere can be attacked by trying the same shift for every Nth letter, or using cribs.