A new distinguisher for clock controlled stream ciphers

Abstract

In this paper we present a distinguisher targeting towards irregularly clocked filter generators. The attack is applied on the irregularly clocked stream cipher called LILI-II. LILI-II is the successor of the cipher LILI-128 and its design was published in [1]. There have been no known attacks better than exhaustive key search on LILI-II. Our attack is the first of this kind that distinguishes the cipher output from a random source using 2103 bits of keystream using computational complexity of approximately 2103 operations. © International Association for Cryptologic Research 2005.