This article gives an overview on the T-310 encryption machine, which was widely used by East German state authorities in the 1980s. The T-310 was based on a U880 processor and it encrypted 5 bit words as used by the teleprinters of the time. The encryption algorithm the T-310 used is a stream cipher working on 5 bit units. A complete description of this previously unpublished algorithm is given in this article. Although most of the content of this publication is based on information found by Bernd Lippmann in a state-owned archive (Birthler authority archive), additional input came from the former military technician Jö rg Drobick, who maintained several T-310 copies in the 1980s.
Linear Cryptanalysis (LC) is an important code-breaking method which has become popular in the 1990s and has roots in earlier research [Shamir,Davies] in the 1980s. In this article we show evidence that Linear Cryptanalysis is even older. According to documents from the former Eastern German cipher authority ZCO, systematic study of linear characteristics for non-linear Boolean functions was routinely performed already in the 1970s. At the same period Eastern German cryptologists have produced an excessively complex set of requirements known as KT1, which the long term keys are required to satisfy and keys of this type were in widespread use to encrypt communications in the 1980s. An interesting question is then, to see if KT1 keys offer some level of protection against linear cryptanalysis. In this article we demonstrate that (strangely) not really. This is demonstrated by constructing specific counter-examples of pathologically weak keys which satisfy all the requirements of KT1.However, as T-310 is used 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 keys could lead to key recovery attacks on T-310.
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