A new index calculus algorithm with complexity L(1/4 + o(1)) in small characteristic

International audienceIn this paper, we describe a new algorithm for discrete logarithms in small characteristic. This algorithm is based on index calculus and includes two new contributions. The first is a new method for generating multiplicative relations among elements of a small smoothness basis. The second is a new descent strategy that allows us to express the logarithm of an arbitrary finite field element in terms of the logarithm of elements from the smoothness basis. For a small characteristic finite field of size Q = pn, this algorithm achieves heuristic complexity LQ(1/4 + o(1)). For technical reasons, unless is already a composite with factors of the right size, this is done by embedding double-struck FQ in a small extension with double-struck FQe with e ≤ 2⌈logpn⌉. © 2014 Springer-Verlag