Crib

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As Crib (German: Likely word , French: probable Mot ; literally, " mnemonic " or " cheat sheet ") is in cryptology , a plaintext phrase referred to by the code breaker knows suspected or guesses that they are in a ciphertext in encrypted form occurs. In the German technical term, this is also referred to as "clear text-ciphertext compromise" (or somewhat shorter as "clear text ciphertext compromise").

If this is actually the case and if the location (position) of the crib in the ciphertext can be identified, for example by means of a pattern search , then the encryption has been breached, which is often sufficient to decipher the entire text.

Cribs served the British code breakers at Bletchley Park as an aid to the German Enigma encryption during the Second World War to break . For this they used a specially constructed electromechanical deciphering machine, called the Turing bomb, as the most important aid . In the early days of the war, when this machine was not yet ready for use, they made do with the Eins catalog . (Because the Enigma's character set only contained letters, numbers had to be written out in digits. The code breakers had prepared an alphabetically sorted list of all possible encodings and associated keys for the frequent digit " one ".)

A popular mistake of the (German) opposing side, which the British " greeted with a kiss ", called Kiss ( German  "Kuss" ), and which they used to obtain cribs , were two or more intercepted secret texts from which they could assume that they came from the same plaintext . The German technical term for this is " ciphertext-ciphertext compromise ". An example of this are less important messages such as weather reports, which were weakly encrypted for small units and sent to submarines encrypted with the same name as the Enigma. The British were able to break the weak encryption and use the clear text obtained as a crib .

Another way to win Cribs is "plaintext-plaintext compromises" , referred to by the British as Depth . This is the case when two or more ciphertexts have been (incorrectly) encrypted with the same key .

literature

  • Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin a. a. 2000, ISBN 3-540-67931-6 .
  • Tony Sale : The Bletchley Park 1944 Cryptographic Dictionary . Publication, Bletchley Park, 2001, p. 22. Accessed: March 26, 2008. PDF; 0.4 MB

Individual evidence

  1. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 378.
  2. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 399ff.
  3. ↑ James A. Reeds, Whitfield Diffie , JV Field: Breaking Teleprinter Ciphers at Bletchley Park: An edition of IJ Good , D. Michie and G. Timms: General Report on Tunny with Emphasis on Statistical Methods (1945). Wiley - IEEE Press, 2015, p. 396 (English). ISBN 978-0-470-46589-9 .