[Resource Topic] 2006/030: Improved cryptanalysis of Py

Welcome to the resource topic for 2006/030

Title:
Improved cryptanalysis of Py

Authors: Paul Crowley

Abstract:

We improve on the best known cryptanalysis of the
stream cipher Py by using a hidden Markov model for the
carry bits in addition operations where a certain
distinguishing event takes place, and constructing from
it an “optimal distinguisher” for the bias in the output
bits which makes more use of the information available.
We provide a general means to efficiently measure the
efficacy of such a hidden Markov model based
distinguisher, and show that our attack improves on the
previous distinguisher by a factor of 2^16 in the number of
samples needed. Given 2^72 bytes of output we can
distinguish Py from random with advantage greater than 1/2, or given only a single stream of 2^64 bytes we have
advantage 0.03.

ePrint: https://eprint.iacr.org/2006/030

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