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Swiss Caesar
Chiffrierschieber Cäsar

Chiffrierschieber Cäsar was a manual transposition cipher system, or shift cipher, in the shape of a slide rule, used from 1949 onwards, by the EVU-section 1 of the Swiss Army, mainly for training purposes. Although a normal Caesar Cipher uses a fixed ROT13 shift, this one is more flexible.

The device measures 30 x 3 cm and is made of eloxed aluminium. A lane is milled-out at the centre over the full length of the device, in which a tight fitting slider is placed. The fixed part has the Latin alphabet engraved twice along the lower edge, in the normal A-Z order. The slider has a single alphabet engraved in reverse (Z-A).

This effectively gives us a so-called Reverse Caesar Cipher — much like the Reverse Caesar cipher wheel shown elsewhere on this website — which has the advantage of being fully reciproke (reversible), even when the alphabet is shifted.
  
Close-up of the Swiss Caesar slide rule (Chiffrierschieber Csar)

The system is named after Julius Caesar (55 BC) who used it for his private correspondence. Although Julius Caesar used a fixed offset between the two alphabets — he shifted each letter of the plaintext 3 positions down in the alphabet — the name Caesar Cipher is commonly used for all ciphers which feature linear alphabet transposition. In practice, many variations are possible.

A special variant of the Caesar Cipher is ROT13, which shifts each letter by 13 positions (exactly half the available 26 letters of the Latin alphabet). It is often used in simple software, and has the advantage to be reciproke: applying the ROT13 function to the ciphertext, reveals the plaintext.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Z Y X W V U T S R Q P O N M L K J I H G F E D C B A
The Reverse Caesar Cipher used by the slide rule shown here, is yet another variant, in which the sliding alphabet is ordered in reverse direction. It has the advantage of being reciproke at any position of the slider. This means that a single procedure can be used for coding and decoding. To make the cipher stronger, and hence more difficult to break, a complex procedure involving key words and some calculations was used. The exact procedure is described in the manual [A].

  1. (EVU) = Eidgenössiger Vor Unterricht (Federal For Teaching).

Front Shifting the center part Close-up of the Swiss Caesar slide rule (Chiffrierschieber Csar) Detail Knob Side view Rear side Serial number engraved at the rear
A
×
A
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Front
A
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Shifting the center part
A
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Close-up of the Swiss Caesar slide rule (Chiffrierschieber Csar)
A
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Detail
A
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Knob
A
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Side view
A
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Rear side
A
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Serial number engraved at the rear

Swiss Caesar Rule with storage box


Documentation
  1. Anleitung zum Chiffrieren mit dem Chiffrierschieber 'Cäsar'
    Operating instructions for the Caesar cipher slide rule (German).
    Bern (Switzerland), 18 March 1949. 1
  1. Document kindly provided by Walter Schmid.

References
  1. Günter Hütter, Chiffrierschieber Cäsar
    Photographed with kind permission. September 2017.

  2. Walter Schmid, Personal correspondence
    December 2017.
Further information
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Crypto Museum. Created: Wednesday 10 January 2018. Last changed: Thursday, 16 August 2018 - 15:56 CET.
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