Improved Differential Cryptanalysis of Serpent

Serpent is one of the five AES finalist. In 2001, Biham, Dunkelman and Keller present a differential cryptanalysis of Serpent up to 8 rounds with 256-key bits by using 2¹⁴ 6-round differential characteristics with probability 2⁹³. In this paper we present a new 6-round differential characteristic Ω_A with the same probability 2^-93. By changing the input differences of Ω_A, we get a characteristic Ω_B with probability 2^-95. We give an improved differential attack on 8-round Serpent with 256-bit keys by using 2^19.62 6-round characteristics with probability 2^-95. Similarly, we present an improved differential attack on 8-round Serpent with 256-bit keys by using 2^25.24 6-round characteristics with probability 2^-97.