Modern Cryptanalysis of Schlüsselgerät 41

The Schlusselgerat 41 was an highly secure encryption machine developed by Fritz Menzer and used from 1944 by the Abwehr. Bletchley Park could not decipher its traffic. In this article, we provide a functional description of the SG-41 and present a novel cryptanalytic method to recover the key settings from ciphertext and knownplaintext. This attack requires extensive computing power, a testimony to the resilience of the SG-41 even against modern cryptanalysis. We also present an alternative method, based on acoustic cryptanalysis, which allows for the recovery of the key settings in minutes.