New Weaknesses in the Keystream Generation Algorithms of the Stream Ciphers TPy and Py

Abstract

The stream ciphers Py, Py6 designed by Biham and Seberry were promising candidates in the ECRYPT-eSTREAM project because of their impressive speed. Since their publication in April 2005, a number of cryptanalytic weaknesses of the ciphers have been discovered. As a result, a strengthened version Pypy was developed to repair these weaknesses; it was included in the category of ‘Focus ciphers’ of the Phase II of the eSTREAM competition. However, even the new cipher Pypy was not free from flaws, resulting in a second redesign. This led to the generation of three new ciphers TPypy, TPy and TPy6. The designers claimed that TPy would be secure with a key size up to 256 bytes, i.e., 2048 bits. In February 2007, Sekar et al. published an attack on TPy with 2²⁸¹ data and comparable time. This paper shows how to build a distinguisher with 2²⁷⁵ key/IVs and one outputword per each key (i.e., the distinguisher can be constructed within the design specifications); it uses a different set of weak states of the TPy. Our results show that distinguishing attacks with complexity lower than the brute force exist if the key size of TPy is longer than 275 bits. Furthermore, we discover a large number of similar bias-producing states of TPy and provide a general framework to compute them. The attacks on TPy are also shown to be effective on Py.

The first author is supported by an IWT SoBeNeT project. The second author is funded by the IBBT (Interdisciplinary Institute for BroadBand Technology), a research institute founded by the Flemish Government in 2004. The information in this document reflects only the authors’ views, is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.