History and Modern Cryptanalysis of Enigma's Pluggable Reflector

Abstract

The development history of Umkehrwalze Dora, 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 Umkehrwalze Dora 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:

• cryptanalysis
• Enigma
• UKWD
• Umkehrwalze Dora
• Uncle Dick
• unsolved ciphers

Acknowledgments

We thank Bob Lord, Ingvar Eriksson, and Dirk Rijmenants for their generosity in allowing us to use their photos and illustration for our article. Very special thanks are directed to Bernhard Richt for many valuable crypto talks and Ralph Erskine for his thorough review. The second author especially thanks the staff of the Politisches Archiv des Auswärtigen Amts in Berlin whose arrangements and friendly help made his research there a great success.

Notes

¹It was also called Uncle Dora, Uncle D, or simply UD.

²Also known as “reversal wheel,” Umkehrwalze in German, and abbreviated UKW.

³Other expressions used for the scrambler are rotor set or wheel maze.

⁴The documents referred to in this section are part of the TICOM (Target Intelligence Committee) collections T 1715, T1716, T1717, and T1718 containing original documents from ChiMaAG and Chiffriermaschinen Gesellschaft Heimsoeth und Rinke (H&R). The collections are in the Politisches Archiv des Auswärtigen Amts (PAAA), Berlin [21].

⁵Many original German patent documents, including all secret patents, were destroyed in a fire started by Regierungsrat Franke on 30 March 1945 in an underground patent storage facility at Heringen in Hessen. Already in February 1945, when the building of the Reichspatentamt in Berlin-Kreuzberg was bombed, many documents were destroyed [9].

⁶For information about Hans Thilo Schmidt, see Hugh Sebag-Montefiore [19].

⁷The German expression is Anzapfung Umkehrwalze, which refers to connecting plugs to the UKW's internal wiring.

⁸The Abwehr was the German military intelligence service under the command of Admiral Wilhelm Canaris, and the Militärisches Amt was the office within SD-Ausland, the foreign security service belonging to Himmler's Reichssicherheitshauptamt (RSHA), which took over some of the functions of the Abwehr after the 20 July plot in 1944.

⁹Waffenamt (WaA) was the German Army Weapons Agency responsible for research and development of the army's weapons and equipment.

¹⁰Käfigwalze (cage wheel) was a new naval Enigma wheel construction that was introduced with Model M3 from serial number M 1822 and onward. The wired core of the wheel was constructed as an insert that, with the removal of one screw, allowed the core to be removed from the rest of the wheel, its cage.

¹¹The messages can be found at http://www.ilord.com/bp-decrypts.html (accessed 8 December 2014).

Additional information

Notes on contributors

Olaf Ostwald

Olaf Ostwald is a microwave engineer working with Rohde & Schwarz in Munich on the design of electronic measuring equipment. He is interested in historical cryptography and cryptanalysis, especially in the techniques of breaking the Enigma.

Frode Weierud

Frode Weierud is a retired electronics engineer formerly employed by the European Organization for Particle Physics (CERN) in Geneva where he worked as a programmer in one of the equipment groups. Cryptography has been his main interest for more than 40 years. His cryptological research is focused on cipher machines and cryptanalytical techniques.