LSTM-attack on polyalphabetic cyphers with known plaintext : Case study on the Hagelin C-38 and Siemens and Halske T52

Abstract: Polyalphabetic cyphers have been used for centuries and well into the 1970s to transmit all kinds of messages. Since then, computers and modern cryptography have taken over making bruteforce attacks unfeasible when designed properly. However, there was a time where mechanical machines, built to operate in harsh conditions and sometimes even without power, were the state of the art for keeping secrets secret. During World War II both Axis and Allied powers used different machines to ensure their dominance. To communicate with occupied territories, Germany installed the Siemens & Halske T52, also known as Geheimschreiber (the secret teleprinter), on telex lines running through Sweden. On the allied side, the United States of America adopted the Hagelin C-38 (known as the M-209) of Swedish invention for field operation. While both machines are inherently different as to what tasks they perform and how, they can both be considered complex polyalphabetic cyphers. Many different methods and attacks on these kind of cyphers have been developed, some relying on inner knowledge and some on mechanical devices. However, the application of Machine Learning to extract key information from intercepts is not a well researched area yet. This thesis aims to demonstrate the potential of LSTM networks on known plaintext attacks against different classical as well as stream cyphers. The techniques used have proven to be effective on the Vigenère as well as with the Hagelin C-38 while being partially successful on the Geheimschreiber with crib lengths of only 15 characters.