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The cyclometer was a cryptologic device designed in the end of 1936 by Marian Rejewski, of the Polish Cipher Bureau's German section (BS-4), to facilitate decryption of German Enigma ciphertext.

Cyclometer (1934), devised by Rejewski to catalogue the cycle structure of Enigma permutations.

The cyclometer was used to prepare a catalog of the length and number of cycles in the "characteristics" for all 17,576 positions of the rotors for a given sequence of rotors. Since there were six such possible sequences, the resulting "catalog of characteristics," or "card catalog,", when it was ready in 1937, comprised a total of (6) (17,576) = 105,456 entries. Preparation of the catalog, writes Rejewski, "was laborious and took over a year, but when it was ready... daily keys [could be obtained] within about fifteen minutes."

On November 1, 1937, however, the Germans changed the "reversing drum," or "reflector." This forced the Cipher Bureau to start over again and produce a new card catalog, "a task," writes Rejewski, "which consumed, on account of our greater experience, probably somewhat less than a year's time."

But then, on September 15, 1938, the Germans changed entirely the procedure for enciphering message keys, and as a result the card-catalog method became completely useless. This spurred the invention of Rejewski's cryptologic bomba and Zygalski's perforated sheets.

• Bomba - Cryptologic bomb: a machine designed about October 1938 by Marian Rejewski to facilitate the retrieval of Enigma keys.
• Bombe: a machine, inspired by Rejewski's "Bomba", that was used by British and American cryptologists during World War II.
• Cryptanalysis of the Enigma and Enigma machine.
• Zygalski sheets: invented about October 1938 by Henryk Zygalski and called "perforated sheets" by the Poles, they made possible the recovery of the Enigma's entire cipher key.

• Władysław Kozaczuk, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two, edited and translated by Christopher Kasparek, Frederick, MD, University Publications of America, 1984, especially pp. 242 and 284-87 on the cyclometer.

• "Polish Enigma Double"
• About the Enigma (National Security Agency)
• "The Enigma Code Breach" by Jan Bury
• The „Enigma” and the Intelligence
• www.enigmahistory.org
• "Codebreaking and Secret Weapons in World War II" By Bill Momsen
• A Brief History of Computing Technology, 1930 to 1939

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