Depth (cryptology)

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As depth (from English literally "depth"; plural: depths ; German technical term : "Klartext-Klartext-Kompromittierung" , "Klartext-Klartext-Kompromiss" or "in phase" for short) are two or more ciphertexts in cryptanalysis , that have been encrypted with the same key .

history

This expression was used in particular during the Second World War by British code breakers in Bletchley Park (BP), England, when the German Enigma and Lorenz key machines were successfully broken , which the German Wehrmacht used to encrypt their secret communications.

method

In practice, it was not uncommon for two texts intercepted by the British Y service to be encrypted not with the same starting position of the cipher machines , i.e. already in depth , but with different starting positions (for example the key wheels) of the machines, but otherwise same key. The texts are then not (yet) in depth , but can be adjusted to match one another by clever shifting and thus brought into depth . Clicks are observed and used as an indicator, i.e. identical characters or groups of characters that appear repeatedly in the ciphertext.

The “in-phase alignment” of two or more texts is referred to in English as setting in depth . One then speaks of to be in depth when this is successful. Texts that are in depth are referred to in German as "in phase" .

example

In the history of cryptography there have been several cases in which ciphertexts that were in depth led to their decipherment or even to the exposure of the encryption process . A famous example is the telex message sent from Athens to Vienna by a German intelligence soldier on August 30, 1941. It was a message encoded with the Lorenz key suffix SZ 40 , which the recipient in Vienna could not read correctly. Therefore, he asked the remote station in Athens in a short plain text message to repeat the broadcast. This happened what the British, who listened carefully, were able to follow closely. Forbidden, the radio operator in Athens used the "used" key again and, presumably for convenience, the second time he did not enter the plaintext in the same way as the first time in the telex, but slightly shortened it. Thus the two now different ciphertexts were slightly out of phase, but otherwise almost in depth . This allowed the British code breaker John Tiltman (1894–1982) to make the decisive first break-in into the SZ 40 and subsequently the British to decipher German telex traffic.

See also

literature

Individual evidence

  1. ↑ 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 .
  2. ^ 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
  3. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 378.
  4. ↑ Tony Sale: The Bletchley Park 1944 Cryptographic Dictionary . Publication, Bletchley Park, 2001, p. 27 (English). Accessed: January 2, 2017. PDF; 0.4 MB
  5. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 388.