On Some Sequences of the Secret Pseudo-random Index j in RC4 Key Scheduling

  • Authors:
  • Riddhipratim Basu;Subhamoy Maitra;Goutam Paul;Tanmoy Talukdar

  • Affiliations:
  • Indian Statistical Institute, Kolkata, India 700 108;Indian Statistical Institute, Kolkata, India 700 108;Department of Computer Science and Engineering, Jadavpur University, Kolkata 700032;Indian Statistical Institute, Kolkata, India 700 108

  • Venue:
  • AAECC-18 '09 Proceedings of the 18th International Symposium on Applied Algebra, Algebraic Algorithms and Error-Correcting Codes
  • Year:
  • 2009

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Abstract

RC4 Key Scheduling Algorithm (KSA) uses a secret pseudo-random index j which is dependent on the secret key. Let S N be the permutation after the complete KSA of RC4. It is known that the value of j in round y + 1 can be predicted with high probability from S N [y ] for the initial values of y and from $S^{-1}_N[y]$ for the final values of y . This fact has been exploited in several recent works on secret key recovery from S N . In this paper, we perform extensive analysis of some special sequences of indices corresponding to the j values that leak useful information for key recovery. We present new theoretical results on the probability and the number of such sequences. As an application, we explain a new secret key recovery algorithm that can recover a 16 bytes secret key with a success probability of 0.1409. Our strategy has high time complexity at this point and requires further improvement to be feasible in practice.