Jefferson disk

The Jefferson disk, or wheel cypher as Thomas Jefferson named it, also known as the Bazeries Cylinder, is a cipher system using a set of wheels or disks, each with the 26 letters of the alphabet arranged around their edge. The order of the letters is different for each disk and is usually scrambled in some random way. Each disk is marked with a unique number. A hole in the centre of the disks allows them to be stacked on an axle. The disks are removable and can be mounted on the axle in any order desired. The order of the disks is the cipher key, and both sender and receiver must arrange the disks in the same predefined order. Jefferson's device had 36 disks. [Kahn, p. 194]

Once the disks have been placed on the axle in the agreed order, the sender rotates each disk up and down until a desired message is spelled out in one row. Then the sender can copy down any row of text on the disks other than the one that contains the plaintext message. The recipient simply has to arrange the disks in the agreed-upon order, rotate the disks so they spell out the encrypted message on one row, and then look around the rows until he sees the plaintext message, i.e. the row that's not complete gibberish. There is an extremely small chance that there would be two readable messages, but that can be checked quickly by the person coding.

First invented by Thomas Jefferson in 1795, this cipher did not become well known and was independently invented by Commandant Etienne Bazeries, the conqueror of the Great Cipher, a century later. The system was used by the United States Army from 1923 until 1942 as the M-94.

This system is not considered secure against modern codebreaking if it is used to encrypt more than one row of text with the same ordering of disks (i.e. using the same key). See #Cryptanalysis.

To encrypt a message, Alice rotates the disks to produce the plaintext message along one "row" of the stack of disks, and then selects another row as the ciphertext. To decrypt the message, Bob rotates the disks on his cylinder to produce the ciphertext along a row. It is handy if both Alice and Bob know the offset of the row, but not really necessary since Bob can simply look around the cylinder to find a row that makes sense.

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