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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.