Cache Timing Analysis of RC4

In this paper we present an attack that recovers the whole internal state of RC4 using a cache timing attack model first introduced in the cache timing attack of Osvik, Shamir and Tromer against some highly efficient AES implementations. In this model, the adversary can obtain some information related to the elements of a secret state used during the encryption process. Zenner formalized this model for LFSRbased stream ciphers. In this theoretical model inspired from practical attacks, we propose a new state recovery analysis on RC4 using a belief propagation algorithm. The algorithm works well and its soundness is proved for known or unknown plaintext and only requires that the attacker queries the RC4 encryption process byte by byte for a practical attack. Depending on the processor, our simulations show that we need between 300 to 1,300 keystream bytes and a computation time of less than a minute.