All homomorphic encryption schemes implemented in HESEA are IND-CPA secure, and hence are expected to be used under the honest-but-curios (a.k.a., semi-honest) model. In this model, the adversary cannot corrupt/tamper with ciphertexts (and perform related active attacks) but can only use the API of HESEA to perform valid operations.

Li and Micciancio recently showed that the IND-CPA model may not be sufficient for the CKKS scheme in some scenarios because a decryption result can be used to perform a key recovery attack. This attack applies to the setting where decryption results need to be shared between multiple parties, e.g., in the the threshold FHE setting. To mitigate the Li-Micciancio attack, we extended the original CKKS (starting with v1.10.6) to a stronger adversarial model where decryption results may still be shared between multiple parties. By default, HESEA chooses a configuration where a relatively large number of decryption queries of the same or related ciphertexts can be tolerated. The lower bound for the tolerated number of such decryption queries is N_d = 128 (but in practical computations, N_d is at least 10,000). We consider this default setting sufficient to prevent passive attacks (where a normal homomorphic encryption computation protocol is followed).

In scenarios where an even stronger adversarial model is needed (uncommon scenarios), the user can increase the number of shared decryptions of the same or related ciphertexts to a higher number by increasing the CKKS_M_FACTOR CMake parameter (a compile-level flag). The minimum number of decryption results in this case becomes N_d x (CKKS_M_FACTOR + 1) / 2. Please note that increasing CKKS_M_FACTOR from the default value of 1 will decrease the precision of CKKS ciphertexts by 0.5 x (log2(CKKS_M_FACTOR + 1) - 1) bits. Hence increasing CKKS_M_FACTOR is only suggested when a stronger adversarial model is needed.