Autokey
History
Invented by the Frenchman Blaise de Vigenère (1523-1596), the Autokey cipher was described in his book Traicté des Chiffres in 1585. This cipher is more secure than the Vigenère, named after him but not invented by him, as it used the message itself as its key. The Autokey cipher can be used with the Vigenère, Beaufort, Variant Beaufort or Porta tables.
Description
In each table the key letter is found in the left column of the table to determine the row, and plaintext letter in the top row to determine the column. The intersection of these reveals the cipher letter. So, using the Vigenère type, if the key is T and the plaintext letter is T the cipher letter is M.
Example
Encipher the following Mark Twain quote "The best way to cheer yourself is to try to cheer someone else up.” using the keyword TWAIN gives:
Vigenère Autokey Cipher
The best way to cheer yourself is to try to cheer someone else up.
MDE JRLA ABC LH YHCXF AVYVJCZZ ZK XZ YZQ MC VYCXF UVQIFFS QPGR YT
Beaufort Autokey Cipher
The best way to cheer yourself is to try to cheer someone else up.
APW HJBO IBG ZF UTUPX ETKNZUDP JA LX MRU AA RKUPX KTSADFK ITWJ KP
Variant Autokey Cipher
The best way to cheer yourself is to try to cheer someone else up.
ALE TRZM SZU BV GHGLD WHQNBGXL RA PD OJG AA JQGLD QHIAXVQ SHER QL
Porta Autokey Cipher
The best way to cheer yourself is to try to cheer someone else up.
KSR SXJD HNJ KF NUQNK KLFCKQSP QJ EJ EAC KH YPQNK ELOTGEY XNLX FA
Solving
Solving methods: Brute force and Dictionary keyword search.
After selecting the Autokey cipher, the type should be selected from the Setup drop down menu on the toolbar - Vigenère, Beaufort, Variant or Porta.
Vigenère Table
Beaufort Table
Variant Beaufort Table
Porta Table