Clock method

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The clock method ( Polish metoda zegara ; English clock method ) was a cryptanalytic technique that was invented around 1934 by the Polish code breaker Jerzy Różycki and was used in the Biuro Szyfrów (“cipher office” of the Polish General Staff ) to form part of the key to the German Enigma -Machine , namely which of the three Enigma rollers was located on the far right in the roller set and was operated as a "fast" roller.

Background and history

Ten years after Arthur Scherbius invented the Enigma in 1918 , the German Reichswehr decided to use the Scherbius machine on a trial basis. The most modern commercial version at the time, the Enigma D , was supplemented with a secret additional device, the plug board , exclusively for military use . This strengthened the cryptographic security of the Enigma (see also: cryptographic strengths of the Enigma ). The Reichswehr machine, known as Enigma I (read: "Enigma Eins"), embodied one of the most modern and secure encryption methods in the world at the time. While it failed French and the British, in the encryption break and they classified the Enigma as "unbreakable", succeeded Polish cryptanalysts in charge of Germany Unit BS4 of Biuro Szyfrów already in 1932, the first slump (see also: Deciphering the Enigma ) .

Saying key duplication

Jerzy Różycki used together with his colleagues Marian Rejewski and Henryk Zygalski a serious procedural errors which underwent the Germans: To ensure reliable transmission, the initial position of the rollers was placed twice in succession at the beginning of a message and the message key is encrypted ( " Slogan key duplication " ). To encrypt the initial position, it was mandatory at that time to turn the three reels to a "basic position", which was prescribed as part of the key for all participants. Thus the first and fourth, the second and fifth as well as the third and sixth ciphertext letters were each assigned to the same plain text letter. This (incorrect) regulation was only changed by the Germans on September 15, 1938, when a new indicator method was introduced with a freely selectable basic position for the key encryption.

method

The Poles usually had a hundred or more Enigma radio messages, so that they were usually able to find a few dozen in which the first two letters of the encrypted message key were identical and the third different. From this they could conclude that the corresponding ciphertexts had been encrypted with identical starting positions of the left and middle rollers. By comparing the different ciphertexts, knowing that the roller transfers on rollers I, II and III are made with different letters (QR, EF or VW) and using the coincidence index , they were able to determine which of the three rollers is on the right-hand side was in the set of rollers. Because of the statistically known unequal frequency of letters in German-language texts, there was a clear increase in the coincidence index above that of random or incorrectly arranged ciphertexts from 3.8% (≈ 1/26) to a noticeable 5 only when the two ciphertexts were arranged “in the correct phase” % until 6 %. The correct phase position established in this way and the known transfer letters of the three possible rollers revealed the right roller that was used.

The British Codebreakers in Bletchley Park (BP), England, learned the clock method, as well as other cryptanalytic methodologies, from the Poles at the secret Pyry meeting in July 1939 , and expanded them to Banburism .

literature

  • IJ Good : Enigma and fish . In Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, pp. 149-166. ISBN 0-19-280132-5 .
  • Rudolf Kippenhahn : Encrypted messages, secret writing, Enigma and chip card . Rowohlt, Reinbek near Hamburg 1999. ISBN 3-499-60807-3
  • Władysław Kozaczuk : Secret Operation Wicher . Bernard et al. Graefe, Koblenz 1989, Karl Müller, Erlangen 1999. ISBN 3-7637-5868-2 , ISBN 3-86070-803-1
  • Władysław Kozaczuk: Under the spell of Enigma . Military publishing house of the German Democratic Republic , 1987, ISBN 3-327-00423-4
  • Władysław Kozaczuk & Jerzy Straszak: Enigma - How the Poles Broke the Nazi Code . Hyppocrene Books, New York 2004. ISBN 0-7818-0941-X
  • Marian Rejewski: An Application of the Theory of Permutations in Breaking the Enigma Cipher . Applicationes Mathematicae, 16 (4), 1980, pp. 543-559.
  • Marian Rejewski: How Polish Mathematicians Deciphered the Enigma . Annals of the History of Computing, 3 (3), July 1981, pp. 213-234.
  • Gordon Welchman : From Polish Bomba to British Bombe: The Birth of Ultra . Intelligence and National Security, 1986.

Web links

Individual evidence

  1. ^ Marian Rejewski: How Polish Mathematicians Deciphered the Enigma . Annals of the History of Computing, 3 (3), July 1981, p. 223.
  2. ^ Rudolf Kippenhahn: Encrypted messages, secret writing, Enigma and chip card . Rowohlt, Reinbek bei Hamburg 1999, p. 211. ISBN 3-499-60807-3
  3. ↑ Simon Singh: Secret Messages . Carl Hanser Verlag, Munich 2000, p. 178. ISBN 3-446-19873-3
  4. ↑ Simon Singh: Secret Messages . Carl Hanser Verlag, Munich 2000, p. 199. ISBN 3-446-19873-3
  5. ^ Marian Rejewski: An Application of the Theory of Permutations in Breaking the Enigma Cipher . Applicationes Mathematicae, 16 (4), 1980, pp. 543–559, cryptocellar.org (PDF; 1.6 MB), accessed on May 27, 2019.
  6. ^ Gordon Welchman: The Hut Six Story - Breaking the Enigma Codes . Allen Lane, London 1982; Cleobury Mortimer M&M, Baldwin Shropshire 2000, p. 11. ISBN 0-947712-34-8
  7. ↑ Jack Good: Enigma and fish . In Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, p. 155. ISBN 0-19-280132-5 .