CY method

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The CY procedure (also: CY method ; English CY procedure ) was a procedure used in the end phase of the Second World War for encryption using the Enigma rotor key machine .

Procedure

Here the "slow" left roller shows the letter R and could now be manually changed to E, for example, using the CY process.

It was introduced to the German army on September 15, 1944. The basic rule for the Air Force and the Army was that radio messages encrypted with the Enigma were not allowed to exceed a maximum length of 250 letters. Longer messages had to be split into correspondingly short parts, encrypted with different Enigma basic positions and sent separately. This served as security against unauthorized decipherment .

As a further measure to strengthen cryptographic security, it has now been prescribed to insert the letters CY into the plain text to be encrypted for longer phrases with more than 150 letters, approximately in the middle of the text, i.e. after 70 to 130 characters . Then any letter of the alphabet had to be added, but it should deviate significantly (at least five) from the one currently displayed for the left cylinder. In the picture you can see the letter R for the left cylinder. A letter far away from this in the alphabet would be E. After CY, E was entered and then the letter immediately following in the alphabet, here F, totaling “CYEF ". This last measure served as an aid to the detection of transmission errors. Then the encryptor adjusted the left roller. In the example he turned it from R to E using the knurl of the hand. Immediately afterwards he entered the remaining plaintext to be encrypted into his Enigma.

The receiver initially decrypted the received radio message as usual, until he suddenly noticed the unusual letter combination CY, known as the “Weisergruppe”, in plain text, followed by the letters EF in this example. He saw that E and F directly follow one another in the alphabet and thus recognized the correctness of the transmission and decryption. The letter E indicated that he now had to manually turn the left cylinder to E before he could continue deciphering the rest of the ciphertext .

safety

The CY procedure was introduced by the German authorities as a method to strengthen the Enigma encryption, as they increasingly doubted its security towards the end of the war. In fact, however, it was a cryptographically weak measure that did not justify the additional effort for the operator. Half the length of the message was enough for the British Codebreakers in Bletchley Park , England, to use their tried and tested methods to "crack" the Enigma radio messages as usual .

literature

  • Michael Pröse: Encryption machines and deciphering devices in World War II - history of technology and aspects of IT history . Dissertation, 2004. PDF; 7.5 MB . Retrieved May 9, 2016.
  • Geoff Sullivan and Frode Weierud : Breaking German Army Ciphers . Cryptologia , 29 (3), July 2005, pp. 193-232. PDF; 1.4 MB . Retrieved May 9, 2016.
  • Heinz Ulbricht: The Enigma cipher machine - deceptive security. A contribution to the history of the intelligence services. Dissertation Braunschweig 2005. PDF; 4.7 MB . Retrieved May 18, 2016.

Web links

  • Enigma security measures on Christos military and intelligence corner with description of the CY procedure (English). Retrieved May 18, 2016.

Individual evidence

  1. ↑ Michael Pröse: Encryption machines and deciphering devices in the Second World War - the history of technology and aspects of the history of IT . Dissertation, 2004, p. 143. PDF; 7.5 MB . Retrieved May 9, 2016.
  2. ^ Hugh Sebag-Montefiore: Enigma - The battle for the code . Cassell Military Paperbacks, London 2004, p. 404. ISBN 0-304-36662-5 .
  3. ↑ Heinz Ulbricht: The Enigma encryption machine - Deceptive security. A contribution to the history of the intelligence services. Dissertation Braunschweig 2005, p. 22. PDF; 4.7 MB . Retrieved May 18, 2016.
  4. ↑ a b Geoff Sullivan, Frode Weierud: Breaking German Army Ciphers . Cryptologia , 29 (3), July 2005, p. 213.
  5. ↑ Fish Notes Report 103  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. in the WWII Archives (English). Retrieved May 18, 2016.@1@ 2Template: Dead Link / www.wwiiarchives.net  
  6. ↑ Michael Pröse: Encryption machines and deciphering devices in the Second World War - the history of technology and aspects of the history of IT . Dissertation, 2004, p. 144. PDF; 7.5 MB . Retrieved May 9, 2016.