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

How Much Does GenoGuard Really "Guard"? An Empirical Analysis of Long-Term Security for Genomic Data

Due to its hereditary nature, genomic data is not only linked to its owner but to that of close relatives as well. As a result, its sensitivity does not really degrade over time; in fact, the relevance of a genomic sequence is likely to be longer than the security provided by encryption. This prompts the need for specialized techniques providing long-term security for genomic data, yet the only available tool for this purpose is GenoGuard~\citehuang_genoguard:_2015. By relying on \em Honey Encryption, GenoGuard is secure against an adversary that can brute force all possible keys; i.e., whenever an attacker tries to decrypt using an incorrect password, she will obtain an incorrect but plausible looking decoy sequence. In this paper, we set to analyze the real-world security guarantees provided by GenoGuard; specifically, assess how much more information does access to a ciphertext encrypted using GenoGuard yield, compared to one that was not. Overall, we find that, if the adversary has access to side information in the form of partial information from the target sequence, the use of GenoGuard does appreciably increase her power in determining the rest of the sequence. We show that, in the case of a sequence encrypted using an easily guessable (low-entropy) password, the adversary is able to rule out most decoy sequences, and obtain the target sequence with just 2.5% of it available as side information. In the case of a harder-to-guess (high-entropy) password, we show that the adversary still obtains, on average, better accuracy in guessing the rest of the target sequences than using state-of-the-art genomic sequence inference methods, obtaining up to 15% improvement in accuracy

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

Cryptographic Security Analysis of T-310

T-310 is an important Cold War cipher. It was the principal encryption algorithm used to protect various state communication lines in Eastern Germany throughout the 1980s. The cipher seems to be quite robust, and until now, no cryptography researcher has proposed an attack on T-310. In this paper we provide a detailed analysis of T-310 in the context of modern cryptography research and other important or similar ciphers developed in the same period. We introduce new notations which show the peculiar internal structure of this cipher in a new light. We point out a number of significant strong and weak properties of this cipher. Finally we propose several new attacks on T-310