CCA-attacks on lattice-based encryption-decryption schemes

This paper presents two distinct chosen-ciphertext attacks (CCA) against latticebased encryption and decryption schemes, in particular based on the LWE problem, a class of post-quantum cryptographic algorithms. First we attack fully homomorphic encryption-decryption schemes (FHE) exploiting the additional information that the small modulus reduction offers. We compare this attack with the CPA ^D attack presented in [1]. Afterwards we present an attack against the CPA secure encryption decryption Kyber scheme and another against the CPA Kyber KEM, and compare them to the previous Key Mismatch Attack in [2]. Our work remarks the importance of protecting the decryption function in the different implementations of these cryptographic schemes, and the importance of CCA security in nowadays cryptosystems. MSC Classification: 94a60