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.
One popular form of autokey starts with a tabula recta, a square with 26 copies of the alphabet, the first line starting with 'A', the next line starting with 'B', etc., like the one above. In order to encrypt a plaintext, one locates the row with the first letter to be encrypted, and the column with the first letter of the key. The letter where the line and column cross is the ciphertext letter.
Giovan Battista Bellaso used the first letter of each word as a primer to start his text autokey. Blaise de Vigenère used as a primer an agreed-upon single letter of the alphabet.
The autokey cipher as used by the members of the American Cryptogram Association is in the way the key is generated. It starts with a relatively short keyword, and appends the message to it. So if the keyword is "QUEENLY", and the message is "ATTACK AT DAWN", the key would be "QUEENLYATTACKATDAWN"
Plaintext: ATTACK AT DAWN... Key: QUEENL YA TTACK AT DAWN.... Ciphertext: QNXEPV YT WTWP...The ciphertext message would therefore be "QNXEPVYTWTWP".
Read more about this topic: Autokey Cipher
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