Beaufort cipher

[1] Its most famous application was in a rotor-based cipher machine, the Hagelin M-209.

[2] The Beaufort cipher is based on the Beaufort square which is essentially the same as a Vigenère square but in reverse order starting with the letter "Z" in the first row,[3] where the first row and the last column serve the same purpose.

[4] To encrypt, first choose the plaintext character from the top row of the tableau; call this column P. Secondly, travel down column P to the corresponding key letter K. Finally, move directly left from the key letter to the left edge of the tableau, the ciphertext encryption of plaintext P with key K will be there.

For example if encrypting plain text character "d" with key "m" the steps would be: To decrypt, the process is reversed.

This obviously reduces errors in handling the table which makes it useful for encrypting larger volumes of messages by hand, for example in the manual DIANA crypto system, used by U.S. Special Forces during the Vietnam War (compare DIANA-table in the image).

In the above example in the column with "m" on top one would find in the reciprocal "d" row the ciphertext "K".

This results in "trigram" combinations where two parts suffice to identify the third.

After eliminating the identical trigrams only 126 of the initial 676 combinations remain (see below) and could be memorized in any order (e.g. AMN can be memorized as "man" and CIP as "pic") to speed up encoding and decoding.

A format of one-time pad used by the U.S. National Security Agency , code named DIANA. The table on the right is an aid for converting between plaintext and ciphertext using the characters at left as the key.