Cryptology ePrint Archive: Report 2007/436

New Attacks on the Stream Cipher TPy6 and Design of New Ciphers the TPy6-A and the TPy6-B

Gautham Sekar and Souradyuti Paul and Bart Preneel

Abstract: The stream ciphers Py, Pypy and Py6 were designed by Biham and Seberry for the ECRYPT-eSTREAM project in 2005. The ciphers were promoted to the `Focus' ciphers of the Phase II of the eSTREAM project. However, due to some cryptanalytic results on the ciphers, strengthened versions of the ciphers, namely TPy, TPypy and TPy6 were built. So far there exists no attacks on TPy6. In this paper, we find hitherto unknown weaknesses in the keystream generation algorithms of the Py6 and of its stronger variant TPy6. Exploiting these weaknesses, a large number of distinguishing attacks are mounted on the ciphers, the best of which works with $2^{224.6}$ data and comparable time. In the second part, we present two new ciphers derived from the TPy6, namely TPy6-A and TPy6-B, whose performances are 2.65 cycles/byte and 4.4 cycles/byte on Pentium III. As a result, to the best of our knowledge, on Pentium platforms TPy6-A becomes the fastest stream cipher in the literature. Based on our security analysis, we conjecture that no attacks better than brute force are possible on the ciphers TPy6-A and TPy6-B.

Category / Keywords: secret-key cryptography / Stream cipher, Distinguishing attack, PRBG

Publication Info: This is the full version of the paper published in the proceedings of WEWoRC 2007.

Date: received 23 Nov 2007, last revised 29 Nov 2008

Contact author: Gautham Sekar at esat kuleuven be

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Version: 20081129:234040 (All versions of this report)

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