On non-negligible bias of the first output byte of RC4 towards the first three bytes of the secret key

  • Authors:
  • Goutam Paul;Siddheshwar Rathi;Subhamoy Maitra

  • Affiliations:
  • Department of Computer Science and Engineering, Jadavpur University, Kolkata, India 700 032;, Kolkata, India;Applied Statistics Unit, Indian Statistical Institute, Kolkata, India 700 108

  • Venue:
  • Designs, Codes and Cryptography
  • Year:
  • 2008

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Abstract

In this paper, we show that the first byte of the keystream output of RC4 has non-negligible bias towards the sum of the first three bytes of the secret key. This result is based on our observation that the index, where the first byte of the keystream output is chosen from, is approximately twice more likely to be 2 than any other value. Our technique is further used to theoretically prove Roos's experimental observation (A class of weak keys in the RC4 stream cipher, 1995) related to weak keys.