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Caesar Wheel
Substitution and Transposition Cipher

The Caesar Wheel was a training tool for cryptographers and cryptanalysts at the Dutch Army Intelligence Unit. It was developed and built for instructional purposes by the Centrale Werkplaats Instructiemiddelen (CWI) in the early 1970s. It consisted of three independently movable concentric wheels, each with a full Latin alphabet on one side and a Cryllic one on the other side.

The image on the right shows the Latin side of the Caesar Wheel. It has a diameter of approx. 14cm, which is about the size of a hand. The outer disc is solid and contains the alphabet written as letters (A-Z) and numbers (0-25). High-frequency letters (i.e. letters that appear often in common language) are printed in red.

At the center is the inner ring, which consists of a a transparent plexiglass disc with the letters of the alphabet printed on it. The disc is held in place by a bolt at the center. The center disc can be moved independently from the outer ring.
  
Latin side of the disc

In between the outer ring and the inner disc is a red ring with letters on a black background. The letters are removable and can be mixed at will, allowing the wheel to be used as a transposition cipher [1] but also as a subsitution cipher [2] simply by configuring it. In order to configure the alphabet, the inner disc has to be removed first. This is done by releasing the bolt at the center. The middle ring can be rotated independently from the other two rings, by holding the device in one hand, whilst moving the notch at the letter G with a finger of the other hand.

As the Caesar Wheel was developed during the early 1970s, at the height of the Cold War, the other side of the disc had a full cyrillic alphabet.

As the cyrillic alphabet is larger than the latin alphabet, there are 31 letters on each ring (А-Я), numbered 0-30. Like with the Latin alphabet, the high frequency letters (АВДЕИ...) are marked in red on all three rings. The letters on the middle ring can be removed and mixed at will.

The middle ring can be rotated by pushing the notch at the letter З (the Russian letter 'Z').
  
Close-up of the notch on the middle cyrillic alphabet ring

Cryptanalysis has become an extremely complex expert-task over the years, and tools like the Caesar Wheel shown here, offer a good introduction into the matter. Various other instruction tools were developed, like the Caesar Box and several slide-rulers. Please note that Caesar Wheel is just a nickname for this cipher disc. It should not be confused with the Caesar Cipher.

Caesar wheel (latin side) Latin side of the disc Moving the middle alphabet ring Center disc removed Removing the letters of the middle ring Ordering the letters of the middle ring Close-up of the center disc after removing it from the wheel Rotating the center disc
Caesar wheel (cyrillic side) Cyrillic (Russian) side of the disc Close-up of the three cyrillic alphabets Close-up of the notch on the middle cyrillic alphabet ring Moving the middle cyrillic alphabet
A
×
A
1 / 13
Caesar wheel (latin side)
A
2 / 13
Latin side of the disc
A
3 / 13
Moving the middle alphabet ring
A
4 / 13
Center disc removed
A
5 / 13
Removing the letters of the middle ring
A
6 / 13
Ordering the letters of the middle ring
A
7 / 13
Close-up of the center disc after removing it from the wheel
A
8 / 13
Rotating the center disc
A
9 / 13
Caesar wheel (cyrillic side)
A
10 / 13
Cyrillic (Russian) side of the disc
A
11 / 13
Close-up of the three cyrillic alphabets
A
12 / 13
Close-up of the notch on the middle cyrillic alphabet ring
A
13 / 13
Moving the middle cyrillic alphabet

References
  1. Wikipedia, Transposition cipher
    Retrieved July 2012.

  2. Wikipedia, Substitution cipher
    Retrieved July 2012.
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Crypto Museum. Created: Saturday 21 July 2012. Last changed: Saturday, 08 July 2017 - 08:44 CET.
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