**Iterative Probabilistic Reconstruction of RC4 Internal States**

*Jovan Golic and 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.

**Category / Keywords: **secret-key cryptography/stream ciphers, RC4, iterative probabilistic cryptanalysis, guess-and-determine attacks

**Date: **received 8 Aug 2008, last revised 8 Aug 2008

**Contact author: **jovan golic at telecomitalia it

**Available format(s): **Postscript (PS) | Compressed Postscript (PS.GZ) | BibTeX Citation

**Version: **20080811:071214 (All versions of this report)

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