CCA-attacks on lattice-based encryption-decryption schemes

This paper presents two distinct chosen-ciphertext attacks (CCA) against lattice-based 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 attack presented in Checri (2024). Afterwards we present an attack against Kyber.CPAPKE, and another against a weakened version of Kyber.KEM where the cyphertext is not checked for correctness; and compare them to the previous Key Mismatch Attack in Qin (2021). 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.