Autokey (Keyletter) Cipher
The Keyword (sometimes Autokey) Cipher is a constantly rotating cipher using the Vigenère tableau, but unlike a Vigenère cipher, which uses a word as the key and encrypts the text just once, the autokey cipher uses a single letter as the key. The entire original text is first rotated by the key letter chosen. The first letter of the result becomes the first letter of the final encrypted text and the key for the next rotation. The new text is rotated again using this new key. The second letter of the newly encrypted text becomes the next key, and the second letter of the final encrypted text. This process continues using the third, fourth, etc. letter until all characters of the text have been encoded.