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Jefferson disk
Polyalphabetic substitution cipher - wanted item

The Jefferson Disk is a manual polyalphabetic substitution cipher system, invented in 1795 by Thomas Jefferson, the 3rd President of the United States. The device was independently invented by Étienne Bazeries about one centry later, and is therefore also known as the Bazeries Cylinder. It later evolved into the M-94 cipher unit that was used by the US Army from 1922 to 1945 [1].

The device consists of an axle with 36 wooden discs, each of which holds the letters of the Latin alphabet on its circumference in a (differently) scrambled order. Each disc is identified by a unique number. The order in which the discs are arranged on the axle, is known as the secret KEY and had to be pre-arranged between the parties.

Original Jefferson disks are extremely rare, as only very few were made and even less have survived. The image on the right shows an original one that is on public display at the NSA's National Cryptologic Museum at Fort Meade [1].
  
Photograph via Wikipedia [1]

Sending a message is pretty simple. The sender sets up the secret KEY (i.e. the order of the discs) and then rotates each of the wheels until the first 25 letters of the message are visible in a single row. He then rotates the set of discs by an arbitrary number of steps (other than zero) and reads of the resulting ciphertext. The ciphertext is then sent to the receipient by letter or telegram.

The recipient sets up the same secret KEY and aligns his wheels so that the ciphertext is readable on a particular row. He then rotates the axle by the same number of steps (offset) as the sender (in reverse direction), to read-off the plaintext. If the offset was unknown, he would simply check all rows to find a line that made sense (i.e. a line that produced readable text).

The Jefferson disk is also known as the Jefferson Wheel Cipher or the Bazeries Cylinder. It was invented in 1795 by the third US president, Thomas Jefferson, but became widely known after it was re-invented independently about a century later by Commandant Etienne Bazeries, whilst working for the Cipher Bureau of the French Ministry of Foreign Afairs [1]. The Jefferson disk was later refined to the M-94 that was used by the United States Army between 1923 and 1945 [2].


10-wheel replica
The Jefferson disk is best demonstrated by using a toy version of it. In 2009 and 2010, resonably priced plastic replicas of a 10-wheel Jefferson disk were sold on Ebay. The example below was created on such a replica. It has only 10 disks rather than 36, but is good enough for a demonstration. Detailed images of this toy are available at the bottom of this page.

Let's assume we want to transmit the message RETREATNOW. We would arrange the wheels so that this message is visible on one of the rows (see image #2).

We would then use the text from, say, the second row down, as the cipher text. It reads: WVCTSOKTDN. This cipher text is then transmitted to the receiver.

The receiver arranges the wheels so that the cipher text is readable on one of the rows, and then reads the plain text from the 2nd row up.
  
Typical view of the 'Jefferson' cipher wheel with the ruler on top

This system is, of course, not very safe if more than one line of text is encoded with the same order of the wheels, which is nearly always the case. Due to the repetitive nature of the key (i.e. the number and order of the wheels), it can easily be broken with hand methods. Nevertheless it was considered relatively strong at the time it was first used [1] .

Typical view of the 'Jefferson' cipher wheel with the ruler on top Encoding a line of text Close-up of the wheels and the ruler Axle detail Holding the cipher wheel in a hand Front view with the ruler down Typical view with the ruler down Changing the position of a cipher wheel
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Typical view of the 'Jefferson' cipher wheel with the ruler on top
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Encoding a line of text
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Close-up of the wheels and the ruler
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Axle detail
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Holding the cipher wheel in a hand
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Front view with the ruler down
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Typical view with the ruler down
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Changing the position of a cipher wheel

References
  1. Wikipedia, Jefferson disk
    Retrieved August 2010.

  2. Wikipedia, M-94
    Retrieved September 2017.

  3. David Kahn, The Codebreakers
    1967. pp. 192-195.
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Crypto Museum. Created: Monday 16 August 2010. Last changed: Sunday, 03 September 2017 - 09:37 CET.
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