When constant-time source yields variable-time binary: Exploiting curve25519-donna built with MSVC 2015

Abstract

The elliptic curve Curve25519 has been presented as protected against state-of-the-art timing attacks [2]. This paper shows that a timing attack is still achievable against a particular X25519 implementation which follows the RFC 7748 requirements [10]. The attack allows the retrieval of the complete private key used in the ECDH protocol. This is achieved due to timing leakage during Montgomery ladder execution and relies on a conditional branch in the Windows runtime library 2015. The attack can be applied remotely.