Abstract: The use of constraint solvers, such as SAT- or Pseudo- Boolean-solvers, allows the extraction of the secret key from one or two side-channel traces. However, to use such a solver the cipher must be rep- resented at bit-level. For byte-oriented ciphers this produces very large and unwieldy instances, leading to unpredictable, and often very long, run times. In this paper we describe a specialized byte-oriented constraint solver for side channel cryptanalysis. The user only needs to supply code snippets for the native operations of the cipher, arranged in a flow graph that models the dependence between the side channel leaks. Our frame- work uses a soft decision mechanism which overcomes realistic measure- ment noise and decoder classification errors, through a novel method for reconciling multiple probability distributions. On the DPA v4 contest dataset our framework is able to extract the correct key from one or two power traces in under 9 seconds with a success rate of over 79%.

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