Computing inverses over a shared secret modulus

  • Authors:
  • Dario Catalano;Rosario Gennaro;Shai Halevi

  • Affiliations:
  • Dipartimento di Matematica e Informatica, Università di Catania, Catania;IBM T.J. Watson Research Center, Yorktown Heights, New York;IBM T.J. Watson Research Center, Yorktown Heights, New York

  • Venue:
  • EUROCRYPT'00 Proceedings of the 19th international conference on Theory and application of cryptographic techniques
  • Year:
  • 2000

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Abstract

We discuss the following problem: Given an integer φ shared secretly among n players and a prime number e, how can the players efficiently compute a sharing of e-1 mod φ. The most interesting case is when φ is the Euler function of a known RSA modulus N, φ = φ(N). The problem has several applications, among which the construction of threshold variants for two recent signature schemes proposed by Gennaro-Halevi-Rabin and Cramer-Shoup. We present new and efficient protocols to solve this problem, improving over previous solutions by Boneh-Franklin and Frankel et al. Our basic protocol (secure against honest but curious players) requires only two rounds of communication and a single GCD computation. The robust protocol (secure against malicious players) adds only a couple of rounds and a few modular exponentiations to the computation.