Cryptanalysis of Unbalanced RSA with Small CRT-Exponent

  • Authors:
  • Alexander May

  • Affiliations:
  • -

  • Venue:
  • CRYPTO '02 Proceedings of the 22nd Annual International Cryptology Conference on Advances in Cryptology
  • Year:
  • 2002

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Abstract

We present lattice-based attacks on RSA with prime factors p and q of unbalanced size. In our scenario, the factor q is smaller than N脽 and the decryption exponent d is small modulo p - 1. We introduce two approaches that both use a modular bivariate polynomial equation with a small root. Extracting this root is in both methods equivalent to the factorization of the modulus N = pq. Applying a method of Coppersmith, one can construct from a bivariate modular equation a bivariate polynomial f(x, y) over Z that has the same small root. In our first method, we prove that one can extract the desired root of f(x, y) in polynomial time. This method works up to 脽 d modulo p - 1 provided that 脽 驴 0.23.