Nonsmooth cryptanalysis, with an application to the stream cipher MICKEY

Abstract

A new approach to the cryptanalysis of symmetric algorithms based on non- smooth optimisation is presented. We develop this technique as a novel way of dealing with nonlinearity over F2 by modeling the equations corresponding to the algorithm as a continuous optimisation problem that avoids terms of higher degree. The resulting prob- lems are not continuously differentiable, but can be approached with techniques from non- smooth analysis. Applied to the stream cipher MICKEY, which is part of the eSTREAM final portfolio, this method can solve instances corresponding to the full cipher, although with time complexity greater than brute force. Finally, we compare this approach to clas- sical pseudo-Boolean programming.