Cryptology Cylinder Ciphers Introduction

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Principle

Take some disks and write permuted alphabets on the outer edges of them. Then put these disks together as a cylinder in such a way that you can turn them around the cylinder axis independently. This picture shows a famous example, the cipher device M-94, in use by the US Army from 1922 to 1943:

Picture from Wikimedia Commons by Wapcaplet, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported

For encryption partition the plaintext into blocks of the right length. Then for each block turn the disks until the plaintext appears in a row of the cylinder. Then read off the ciphertext from another row. This picture (of a simplified cylinder device) shows the German plaintext KLARTEXT together with the ciphertext DNISNPIR:

Usually the ciphertext row is chosen at random, in the example we could also take SUWUSDUG or EEMMEATE as ciphertext. Therefore the encryption has a probabilistic ingredient. The rows are called »generatrices«, the first generatrix being the row below the plaintext and so on. For our standard 26 letter alphabet we have 25 generatrices.

In analogy the exhaustion method as for a shift cipher is sometimes called »generatrix method«.

For decryption adjust the disk such that the ciphertext appears in one row. Then the plaintext appears in one of the other rows. Usually there is only one row that shows a meaningful plaintext.

For an automatic decryption procedure the language recognition procedures from Chapter 3 may be helpful.

Description by Paper Strips

As with cipher disk and cipher slide also for the cipher cylinder we can give a cryptographically equivalent description by paper strips. For typographic reasons we arrange them horizontally instead of vertically—then we can shift the rows in a simple text editor.

Example:

Disk                                                 
  No.                     |                              
  1   X A M S W U L N O C K V P E G B Z T F H R I J Y Q D
  2   C S Y U K B E N X H L Q J A P G F V O Z M R I D T W
  3   Q W E R T Z U I O P A S D F G H J K L Y X C V B N M
  4   L K J H G F D S A P R O I U Z T E W Q M N B V C X Y
  5   F G H I J K M N W S T U V W X Y Z P A R O L E B C D
  6   A D G J M P S V Y B E H K N Q T W Z C F I L O R U X
  7   D F H J L N P R T V X Z A C E G I K M O Q S U W Y B
  8   K M P Q X Y W E V I T Z L S C O U R A N D B F G H J
                          |                              

Using the first generatrix the plaintext KLARTEXT is transformed to the ciphertext VQSOUHZZ.

To get the full flexibility of the cylinder we have to double the strips in order to see all possible generatrices after shifting the strips.

Here are some pictures from an attempt to build a cipher cylinder from wood. Be warned that you must work very precisely to get a usable device!