Faster individual discrete logarithms in finite fields of composite extension degree

TL;DR

This paper introduces an improved method for the initial splitting phase in discrete logarithm computations over finite fields, especially effective for fields with composite extension degrees, enhancing overall efficiency.

Contribution

It presents a novel approach that exploits subfields to achieve more smooth decompositions, reducing complexity in the last step of discrete logarithm algorithms.

Findings

Significant reduction in initial splitting complexity.

Enhanced efficiency in discrete logarithm computations for composite extension degrees.

Reduced size and height of the descent tree.

Abstract

Computing discrete logarithms in finite fields is a main concern in cryptography. The best algorithms in large and medium characteristic fields (e.g., {GF}$(p^2)$, {GF}$(p^{12})$) are the Number Field Sieve and its variants (special, high-degree, tower). The best algorithms in small characteristic finite fields (e.g., {GF}$(3^{6 \cdot 509})$) are the Function Field Sieve, Joux's algorithm, and the quasipolynomial-time algorithm. The last step of this family of algorithms is the individual logarithm computation. It computes a smooth decomposition of a given target in two phases: an initial splitting, then a descent tree. While new improvements have been made to reduce the complexity of the dominating relation collection and linear algebra steps, resulting in a smaller factor basis (database of known logarithms of small elements), the last step remains at the same level of difficulty. Indeed, we have to find a smooth decomposition of a typically large element in the finite field. This work improves the initial splitting phase and applies to any nonprime finite field. It is very efficient when the extension degree is composite. It exploits the proper subfields, resulting in a much more smooth decomposition of the target. This leads to a new trade-off between the initial splitting step and the descent step in small characteristic. Moreover it reduces the width and the height of the subsequent descent tree.