A Unified Framework for Small Secret Exponent Attack on RSA⋆

Abstract. We address a lattice based method on small secret exponent attack on RSA scheme. Boneh and Durfee reduced the attack into finding small roots of a bivariate modular equation: x(N+1+y)+1 ≡ 0 ( mod e), where N is an RSA moduli and e is the RSA public key. Boneh and Dur-fee proposed a lattice based algorithm for solving the problem. When the secret exponent d is less than N0.292, their method breaks RSA scheme. Since the lattice used in the analysis is not full-rank, the analysis is not easy. Blömer and May gave an alternative algorithm. Although their bound d ≤ N0.290 is worse than Boneh–Durfee result, their method used a full rank lattice. However, the proof for their bound is still complicated. Herrmann and May gave an elementary proof for the Boneh–Durfee’s bound: d ≤ N0.292. In this paper, we first give an elementary proof for achieving the bound of Blömer–May: d ≤ N0.290. Our proof employs un-ravelled linearization technique introduced by Herrmann and May and is rather simpler than Blömer–May’s proof. Then, we provide a unified framework to construct a lattice that are used for solving the problem, which includes two previous method: Herrmann–May and Blömer–May methods as a special case. Furthermore, we prove that the bound of Boneh–Durfee: d ≤ N0.292 is still optimal in our unified framework