Ciphertext-only attacks and weak long-term keys in T-310

T-310 is an important Cold War cipher (Schmeh 2006 Schmeh, K. 2006. The East German encryption machine T-310 and the algorithm it used. Cryptologia, 30(3):251–257.[Taylor &; Francis Online] , [Google Scholar]). It was the principal encryption algorithm used to protect various state communication lines in Eastern Germany in the 1980s. The cipher is quite robust, and it outputs extremely few bits from the internal state. In this article, the authors study the choice of the long-term key in T-310. They show that if a key is faulty, for example if they omit to check just one condition which the keys should satisfy, and more or less each time the round function is not bijective, communications can be decrypted in a ciphertext-only scenario. The authors provide mathematical proofs that the main historical key classes KT1 and KT2 are secure against such attacks.     

Slide attacks and LC-weak keys in T-310

T-310 is an important Cold War cipher (Cryptologia 2006). In a recent article (Cryptologia 2018), researchers show that, in spite of specifying numerous very technical requirements, the designers do not protect the cipher against linear cryptanalysis and some 3% of the keys are very weak. However, such a weakness does not necessarily allow breaking the cipher because it is extremely complex and extremely few bits from the internal state are used for the actual encryption. In this article, we finally show a method that allows recovering a part of the secret key for about half of such weak keys in a quasi-realistic setting. For this purpose, we revisit another recent article from Cryptologia from 2018 and introduce a new peculiar variant of the decryption oracle slide attack with d?=?0.     

Linear cryptanalysis and block cipher design in East Germany in the 1970s

Linear cryptanalysis (LC) is an important codebreaking method that became popular in the 1990s and has roots in the earlier research of Shamir in the 1980s. In this article we show evidence that linear cryptanalysis is even older. According to documents from the former East Germany cipher authority ZCO, the systematic study of linear characteristics for nonlinear Boolean functions was routinely performed in the 1970s. At the same time East German cryptologists produced an excessively complex set of requirements known as KT1, which requirements were in particular satisfied by known historical used in the 1980s. An interesting line of inquiry, then, is to see if KT1 keys offer some level of protection against linear cryptanalysis. In this article we demonstrate that, strangely, this is not really the case. This is demonstrated by constructing specific counterexamples of pathologically weak keys that satisfy all the requirements of KT1. However, because we use T-310 in a stream cipher mode that uses only a tiny part of the internal state for actual encryption, it remains unclear whether this type of weak key could lead to key recovery attacks on T-310.