Autokey cipher

An autokey cipher (also known as the autoclave cipher) is a cipher which incorporates the message (the plaintext) into the key. There are two forms of autokey cipher: key autokey and text autokey ciphers. A key-autokey cipher uses previous members of the keystream to determine the next element in the keystream. A text-autokey uses the previous message text to determine the next element in the keystream.

In modern cryptography, self-synchronizing stream ciphers are autokey ciphers.

History

The first autokey cipher was invented by Girolamo Cardano, and contained a fatal defect. Like many autokey ciphers it used the plaintext to encrypt itself; however, since there was no additional key, it is no easier for the intended recipient to read the message than anyone else who knows that the cipher is being used. A number of attempts were made by other cryptographers to produce a system that was neither trivial to break nor too difficult for the intended recipient to decipher. Eventually one was invented in 1564 by Giovan Battista Bellaso using a "reciprocal table" with five alphabets of his invention and another form was described in 1586 by Blaise de Vigenère with a similar reciprocal table of ten alphabets.