An Analysis of Serpent-p and Serpent-p-ns

Serpent is the AES candidate with the largest safety margins. In this paper we try to analyze how these security margins a#ect the strength of the cipher. We adopt a simpli#ed Linear Transformation #proposed by the designers of Serpent as their preliminary candidate#, and show that even this weakened variant requires at least 2 121 known plaintext to be attacked. We conclude that Serpent is secure, even with much smaller number of rounds. 1 Introduction Serpent#1, 2# is one of the more promising AES candidate, due to large safety margins. It was introduced by Eli Biham, Ross J. Anderson and Lars R. Knudsen. The Serpent family has two variants: original published variant Serpent-0 #1#, and the AES submission variant Serpent-1 #2#. By convention when wesay just Serpentwe mean Serpent-1. In the history remarks in #1,2# a preliminary linear transformation is mentioned, which rotates three of the words in the bitslice implementation, rather than performing a more complex mixing.