Stream Ciphers: A Practical Solution for Efficient Homomorphic-Ciphertext Compression

Abstract

International audienceIn typical applications of homomorphic encryption, the rststep consists for Alice to encrypt some plaintext m under Bob's publickey pk and to send the ciphertext c = HEpk(m) to some third-partyevaluator Charlie. This paper specically considers that rst step, i.e.the problem of transmitting c as eciently as possible from Alice toCharlie. As previously noted, a form of compression is achieved usinghybrid encryption. Given a symmetric encryption scheme E, Alice picks arandom key k and sends a much smaller ciphertext c0 = (HEpk(k); Ek(m))that Charlie decompresses homomorphically into the original c using adecryption circuit C(E^{-1}).In this paper, we revisit that paradigm in light of its concrete implementationconstraints; in particular E is chosen to be an additive IV-basedstream cipher. We investigate the performances oered in this contextby Trivium, which belongs to the eSTREAM portfolio, and we also proposea variant with 128-bit security: Kreyvium. We show that Trivium,whose security has been rmly established for over a decade, and thenew variant Kreyvium have an excellent performance