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Timed-Release Cryptography

Mao, Wenbo


Keyword(s): timed-release cryptography; time-lock puzzles; timed commitments; efficient zero-knowledge protocols

Abstract: Please Note. This abstract contains mathematical formulae which cannot be represented here. Let n be a large composite number. Without t factoring n, the validation of a2 (mod n ) given a, t with gcd (a, n) = 1 and t < n can be done in t squarings modulo n. For t" n (e.g., n > 2 1024 and t < 2 100 ), no lower complexity than t squarings is known to fulfil this task (even considering massive parallelisation). Rivest et al suggested to use such constructions as good candidates for realising timed-release crypto problems. We argue the necessity for zero-knowledge proof of the correctness of such constructions and propose the first practically efficient protocol for a realisation. Our protocol proves, in log2 t standard crypto operations, the correctness of t (ae ) 2 (mod n ) with respect to ae where e is an RSA encryption exponent. With such a proof, a Timed-release RSA Encryption of a message M t can be given as a2 M (mod n ) with the assertion that the correct decryption of the RSA ciphertext Me (mod n ) can be obtained by performing t squarings modulo n starting from a. Timed-release RSA signatures can be constructed analogously.

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