Improved cryptanalysis of Py

Paul Crowley

Paper 2006/030

Improved cryptanalysis of Py

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.

Metadata

Available format(s): PDF
Category: Secret-key cryptography
Publication info: Published elsewhere. SASC 2006 workshop without proceedings
Keywords: Py symmetric cryptanalysis hidden Markov model
Contact author(s): paul @ ciphergoth org
History: 2006-01-27: received
Short URL: https://ia.cr/2006/030
License: CC BY

BibTeX

@misc{cryptoeprint:2006/030,
      author = {Paul Crowley},
      title = {Improved cryptanalysis of Py},
      howpublished = {Cryptology {ePrint} Archive, Paper 2006/030},
      year = {2006},
      url = {https://eprint.iacr.org/2006/030}
}