Autokey cipher

The autokey cipher, also called the autoclave cipher, is a classical polyalphabetic cipher that folds the message itself into the key. The sender and receiver agree on a short primer, usually a keyword, and the key is that primer followed by the plaintext, so the key stretches to the exact length of the message and never repeats within it. Encryption looks up each plaintext letter against the matching key letter in a tabula recta, a square of 26 shifted alphabets. A key-autokey variant draws later key elements from earlier keystream members, while a text-autokey variant draws them from earlier message text. The scheme is credited to Blaise de Vigenere in 1586. Because the key does not repeat, Kasiski examination and the index of coincidence do not work against it, but the plaintext leaks into the key, so an attacker can guess likely words and test them at every key position. A single error in encryption also spoils the rest of the message for the reader.