Paper 2010/011

Related Key Cryptanalysis of the LEX Stream Cipher

Mainack Mondal and Debdeep Mukhopadhyay

Abstract

LEX is a stream cipher proposed by Alex Biryukov. It was selected to phase 3 of the eSTREAM competition. LEX is based on the Advanced Encryption Standard (AES) block cipher and uses a methodology called "Leak Extraction", proposed by Biryukov himself. In this paper, we cryptanalyze LEX using two related keys. We have mounted a key recovery attack on LEX, which using $2^{54.3}$ key streams yields a complete round key with $2^{102}$ operations. This improves the existing best cryptanalysis of LEX which needs $2^{112}$ operations to ascertain the key.

Note: We are withdrawing this paper due to technical errors in it. Please remove it from the achive.