Section: New Results

Reverse-engineering of communication systems

Participants : Marion Bellard, Nicolas Sendrier, Jean-Pierre Tillich, Audrey Tixier.

To assess the quality of a cryptographic algorithm, it is usually assumed that its specifications are public, as, in accordance with Kerckhoffs principle (Kerckhoffs stated that principle in a paper entitled La Cryptographie militaire, published in 1883.), it would be dangerous to rely, even partially, on the fact that the adversary does not know those specifications. However, this fundamental rule does not mean that the specifications are known to the attacker. In practice, before mounting a cryptanalysis, it is necessary to strip off the data. This reverse-engineering process is often subtle, even when the data formatting is not concealed on purpose. A typical case is interception; some raw data, not necessarily encrypted, are observed out of a noisy channel. To access the information, the whole communication system has first to be disassembled and every constituent reconstructed. Our activity within this domain, whose first aim is to establish the scientific and technical foundations of a discipline which does not exist yet at an academic level, has been supported by some industrial contracts driven by the Ministry of Defense.

Recent results:

• Reconstruction of the constellation labelling (i.e. used in the modulator of a communication system) in the presence of errors and when the underlying code is convolutional [10] .

• Reconstruction of a convolutional code. This reconstruction technique is based on a new method for detecting whether a given binary sequence is a noisy convolutional codeword obtained from an unknown convolutional code [45] .

• Reconstruction of the interleaver of a turbo-code from the knowledge of several noisy codewords [63] .