Paper 2015/944 Original: eprint.iacr.org/2015/944 ↗

New Complexity Trade-Offs for the (Multiple) Number Field Sieve Algorithm in Non-Prime Fields

Abstract

The selection of polynomials to represent number fields crucially determines the efficiency of the Number Field Sieve (NFS) algorithm for solving the discrete logarithm in a finite field. An important recent work due to Barbulescu et al. builds upon existing works to propose two new methods for polynomial selection when the target field is a non-prime field. These methods are called the generalised Joux-Lercier (GJL) and the Conjugation methods. In this work, we propose a new method (which we denote as $\mathcal{A}$) for polynomial selection for the NFS algorithm in fields $\mathbb{F}_{Q}$, with $Q=p^n$ and $n>1$. The new method both subsumes and generalises the GJL and the Conjugation methods and provides new trade-offs for both $n$ composite and $n$ prime. Let us denote the variant of the (multiple) NFS algorithm using the polynomial selection method ``{X}" by (M)NFS-{X}. Asymptotic analysis is performed for both the NFS-$\mathcal{A}$ and the MNFS-$\mathcal{A}$ algorithms. In particular, when $p=L_Q(2/3,c_p)$, for $c_p\in [3.39,20.91]$, the complexity of NFS-$\mathcal{A}$ is better than the complexities of all previous algorithms whether classical or MNFS. The MNFS-$\mathcal{A}$ algorithm provides lower complexity compared to NFS-$\mathcal{A}$ algorithm; for $c_p\in (0, 1.12] \cup [1.45,3.15]$, the complexity of MNFS-$\mathcal{A}$ is the same as that of the MNFS-Conjugation and for $c_p\notin (0, 1.12] \cup [1.45,3.15]$, the complexity of MNFS-$\mathcal{A}$ is lower than that of all previous methods.