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**2008/348**

**Title:**

Iterative Probabilistic Reconstruction of RC4 Internal States

**Authors:**
Jovan Golic, Guglielmo Morgari

**Abstract:**

It is shown that an improved version of a previously proposed iterative probabilistic algorithm, based on forward and backward probability recursions along a short keystream segment, is capable of reconstructing the RC4 internal states from a relatively small number of known initial permutation entries. Given a modulus N, it is argued that about N/3 and N/10 known entries are sufficient for success, for consecutive and specially generated entries, respectively. The complexities of the corresponding guess-and-determine attacks are analyzed and, e.g., for N=256, the data and time complexities are (conservatively) estimated to be around D \approx 2^{41}, C \approx 2^{689} and D \approx 2^{211}, C \approx 2^{262}, for the two types of guessed entries considered, respectively.

**ePrint:**
https://eprint.iacr.org/2008/348

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