Paper 2025/894

Achieving "beyond CCA1" security for linearly homomorphic encryption, without SNARKs?

Abstract

In the wake of Manulis and Nguyen's Eurocrypt'24 paper, new CCA security notions, vCCA and vCCAD, and associated construction blueprints have been proposed to leverage either CPA or CPAD secure FHE beyond the CCA1 security barrier. These two notions are the strongest CCA security notions so far achievable, respectively, by correct and approximate homomorphic schemes. However, the only known construction strategies intimately require advanced SNARK machinery, undermining their practicality. In this context, this paper is an attempt to achieve these advanced CCA security notions in the restricted case of linearly homomorphic encryption, without resorting to SNARKs. To do so, we investigate the relationship between the Linear-Only Homomorphism (LOH) assumption, an assumption that has been used for more than a decade at the core of several proof-of-knowledge constructions, and these two recent security notions (vCCA and vCCAD). On the bright side, when working under the correctness assumption, we establish that the LOH property is sufficient to achieve vCCA security in both the private and public key settings. In the public key setting, we further show that a surprisingly simple and previously known Paillier-based construction also achieves this level of security, at only twice the cost of the baseline scheme. We then turn our attention to LWE-based schemes for which the Pandora box of decryption errors opens up. In the private key setting, we are able to achieve CPAD and vCCAD security but only in a fairly restrictive non-adaptive setting, in which vCCAD collapses onto a weak relaxation of CCA1. Finally, we eventually achieve adaptive vCCAD security provided that the number of ciphertexts given to the adversary is suitably restricted. While bridging the gap towards credible practicality requires further work, this is a first step towards obtaining linear homomorphic schemes achieving these recent CCA security notions by means only of relatively lightweight machinery.

Note: Added on p. 17 (end of Sect. 3.2) positioning with respect to the results in [55].