Breaking '128-bit secure' supersingular binary curves: (Or how to solve discrete logarithms in double-struck F24.1223 and double-struck F212.367)

Abstract

In late 2012 and early 2013 the discrete logarithm problem (DLP) in finite fields of small characteristic underwent a dramatic series of breakthroughs, culminating in a heuristic quasi-polynomial time algorithm, due to Barbulescu, Gaudry, Joux and Thomé. Using these developments, Adj, Menezes, Oliveira and Rodríguez-Henríquez analysed the concrete security of the DLP, as it arises from pairings on (the Jacobians of) various genus one and two supersingular curves in the literature, which were originally thought to be 128-bit secure. In particular, they suggested that the new algorithms have no impact on the security of a genus one curve over double-struck F 21223, and reduce the security of a genus two curve over ouble-struck F2367 to 94.6 bits. In this paper we propose a new field representation and efficient general descent principles which together make the new techniques far more practical. Indeed, at the '128-bit security level' our analysis shows that the aforementioned genus one curve has approximately 59 bits of security, and we report a total break of the genus two curve. © 2014 International Association for Cryptologic Research.