English

Noun

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.