A new distinguisher for clock controlled stream ciphers

  • Authors:
  • Håkan Englund;Thomas Johansson

  • Affiliations:
  • Dept. of Information Technology, Lund University, Lund, Sweden;Dept. of Information Technology, Lund University, Lund, Sweden

  • Venue:
  • FSE'05 Proceedings of the 12th international conference on Fast Software Encryption
  • Year:
  • 2005

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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.