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Uncertain; possibly a pun on silly and the letters CIL used as a cryptographic key.


cilly (plural cillies)

  1. (cryptography, dated) Any predictable facet of the Enigma Machine cipher due to operator error or laziness, whose existence helped the Allies to break the code.
    • 2000, Simon Singh, The Code Book:
      Another type of cilly was the repeated use of the same message key, perhaps the initials of the operator's girlfriend []
    • 2002, Ronald L Krutz; Russell Dean Vines, The CISSP Prep Guide: Gold Edition:
      Answer c is a reference to a cilly, which was a three-character message key used in the German Enigma machine.
    • 2005, Brian J Winkel; Cipher A Deavours; David Kahn; Louis Kruh, The German Enigma cipher machine:
      Taunt describes the work involved in breaking Enigma messages, cribs, cillies, the Herivel tip, help from Bombes and more.
    • 2006, Rebecca Ann Ratcliff, Delusions of intelligence: Enigma, Ultra and the end of secure ciphers:
      By 1944, cillies had already become rare; hence, Bletchley Park had already found other breaks into the Enigma nets.
    • 2007, Friedrich Ludwig Bauer, Decrypted secrets: methods and maxims of cryptology:
      Cillies resulted from a combination of two different mistakes in a multi-part message by some Enigma operators.