Lattice Klepto Revisited

Abstract

Kleptography introduced by Young and Yung is about using an embedded backdoor to perform attacks on a cryptosystems. At SAC'17, Kwantet al. proposed a kleptographic backdoor on NTRU encryption scheme and thought that the backdoor can not be detected. However, in this paper we show that the user can detect the backdoor very efficiently and hence the problem of constructing a kleptographic backdoor on NTRU stays open. Moreover, we also design a universal method to embed a kleptographic backdoor for RLWE-based scheme, such as NewHope. Our construction is shown to be strongly undetectable, which reveals the threats of the kleptographic attacks on lattice-based schemes.