Cryptology Cryptanalysis of Cylinder Ciphers

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Steps

(Depending on previous knowledge of the cryptanalyst.)

• Determine the period
  • by KASISKI's or FRIEDMAN's method
    [This is prevented or at least impeded by changing generatrices for each block.]
  • or by capturing a device
  • or by »social engineering« (for example espionage)
• Determine the disks
  • capturing or espionage—otherwise this will be difficult and need a lot of known plaintext.
• Determine the order of the disks (that is the key).

FRIEDMAN developed a systematic approach to cryptanalysis of cylinders in Chapter III of

W. F. FRIEDMAN,
Several Machine Ciphers and Methods for Their Solution.
Riverbank Publ. No. 20, Geneva IL 1918.

Useful Observations

• The permuted alphabets are cyclic.
  In particular a letter is never encrypted by itself.
  This makes negative pattern search for a probable word possible
• Each disk can occur only at one position of the cylinder.
• The possible substitutes of the single plaintext letters are not uniformly distributed
  (except when the table of cycles is a latin square).
• The advantage of independent alphabets is lost as soon as these become known to the enemy.
• To identify the correct generatrix language recognition methods may be helpful.

Example. DE VIARIS breaks the BAZERIES cylinder.