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Jefferson disk
Manual wheel cipher - wanted item

The Jefferson Disk is a manual cipher system that consists of a set of wheels on an axle. Each wheel has the 26 letters of the Latin alphabet on its circumfere in a pre-determinded scrambled arrangement. Each wheel has a unique number and the order is determinded by a code book.

The image on the right shows a real Jefferson disk that is on public display at the NSA's National Cryptologic Museum [1] . It has 36 alphabet disks (or: wheels) that should be mounted on the axle in the same order by sender and receiver, before messages can be exchanged.

Once this is done, the sender rotates the wheels until the message is spelled out on a particular row (see the example below). The sender then takes any row, other than the plain text, and transmits that to the receiver.
  
Image taken from Wikipedia

The receiver aligns his wheels so that the cipher text is spelled out in full on a particular row. He then scans the other rows for a line of text that makes sense (i.e. a line that is non-random). In some cases, the receiver would read the plain text message from a predetermined distance (offset) from the received cipher 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 by Commandant Etienne Bazeries about a century later [1]. The Jefferson disk was later refined to the M-94 that was used by the United States Army between 1923 and 1942.

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

References
  1. Wikipedia, Jefferson disk
Further information
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Crypto Museum. Last changed: Friday, 04 September 2015 - 10:21 CET.
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