Low-Cost Client Puzzles Based on Modular Exponentiation

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Executive Summary

Client puzzles have been proposed as a useful mechanism for mitigating Denial of Service attacks on network protocols. While several puzzles have been proposed in recent years, most existing non-parallelizable puzzles are based on modular exponentiations. The main drawback of these puzzles is in the high cost that they incur on the puzzle generator (the verifier). In this paper, the authors propose cryptographic puzzles based on modular exponentiations that reduce this overhead. Their constructions are based on a reasonable intractability assumption in RSA: essentially the difficulty of computing a small private exponent when the public key is larger by several orders of magnitude than the semi-prime modulus.

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