Distinguishing attack against TPypy

Abstract

TPypy is a tweaked version of the Py stream cipher algorithm submitted to eSTREAM. Py uses a kind of processing referred to as a 'rolling array', the mixing of two types of array and one variable, to generate the keystream. TPypy is proposed as a highly secure stream cipher that fixes all of the previously identified weaknesses of Py. This paper reports a significant bias in the pseudo-random generation algorithm of TPypy that can be exploited to distinguish the keystream obtained from multiple arbitrary secret key and initial vector pairs from a truly random number sequence using about 2199 words. © Springer-Verlag Berlin Heidelberg 2007.