An evaluation of the Klimov-Shamir keystream generator
Stream ciphers are valuable in applications where efficiency and security are both needed. Linear feedback shift register sequences have been the mainstay of stream ciphers in the past. However, Alexander Klimov and Adi Shamir recently proposed a class of invertible T-functions as a possible source of cryptographic building blocks for stream ciphers. In particular, they present the mapping xf = x + (x² V 5) (mod 2[superscript n]) which is a permutation with a single cycle modulo 2[superscript n] for any n. We discuss traditional stream cipher constructions and the desired properties of sequences produced by pseudorandom keystream generators. We then utilize these desired properties to analyze the aforementioned Klimov-Shamir keystream generator. Finally, we propose a possible construction for a keystream generator that combines a traditional stream cipher construction and the function proposed by Klimov and Shamir in order to produce a keystream that adheres to the desired properties we set forth.