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Manual wheel cipher
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.
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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.
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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 by Thomas Jefferson in 1795, 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.
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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.
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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.
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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]
.
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© Copyright 2009-2011, Paul Reuvers & Marc Simons. Last changed: Tue,27 Dec 2011.17:47:03
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