Rounding LLL: Finding Faster Small Roots of Univariate Polynomial Congruences

In a seminal work at EUROCRYPT ’96, Coppersmith showed how to find all small roots of a univariate polynomial congruence in polynomial time: this has found many applications in public-key cryptanalysis and in a few security proofs. However, the running time of the algorithm is a high-degree polynomial, which limits experiments: the bottleneck is an LLL reduction of a high-dimensional matrix with extra-large coefficients. We present in this paper a polynomial speedup over Coppersmith’s algorithm. Our improvement is based on a special property of the matrices used by Coppersmith’s algorithm, which allows us to speed up the LLL reduction by rounding. The exact speedup depends on the LLL algorithm used: for instance, the speedup is quadratic in the bit-size of the small-root bound if one uses the Nguyen-Stehlé L algorithm.