History and Modern Cryptanalysis of Enigma’s Pluggable Reflector Olaf Ostwald and Frode Weierud ABSTRACT: The development history of Umkehrwalze Dora (UKWD), Enigma's pluggable reflector, is presented from the first ideas in the mid-1920s to the last development plans and its actual usage in 1945. An Enigma message in three parts, enciphered with UKWD and intercepted by the British on 11 March 1945, is shown. The successful recovery of the key of this message is described. Modern computer-based cryptanalysis is used to recover the wiring of the unknown “Uncle Dick,” which the British called this field-rewirable reflector. The attack is based on the known ciphertext and plaintext pair from the first part of the intercept. After recovery of the unknown reflector wiring and the daily key the plaintext of the second part of the message is revealed. KEYWORDS: Enigma, cryptanalysis, Uncle Dick, Umkehrwalze Dora, UKWD, unsolved ciphers Address correspondence to Frode Weierud, Bjerkealleen 17, 1385 Asker, Norway. Email:
[email protected] 1. Introduction Uncle Dick,1 as it was called by the codebreakers of Bletchley Park (BP), or Umkehrwalze Dora (UKWD), as designated by the Germans, was the nickname of a special pluggable reflector,2 used as the leftmost wheel within the scrambler 3 of the Enigma. The electro-mechanical cipher machine Enigma (from Greek αίνιγµα for “riddle”) was the backbone of the German Wehrmacht during World War II. Arthur Scherbius, a German promoted electrical engineer and inventor of considerable standing, invented Enigma in 1918 [14]. Subsequently it was improved and then used by all three parts of the German armed forces, namely army (Heer), air force (Luftwaffe), and military navy (Kriegsmarine), for enciphering and deciphering of their secret messages.