eprint.iacr.orgpaper

Reaction Attack on CKKS Deployments with Asymptotically Optimal Query Complexity

Shows a protocol-level reaction attack on CKKS homomorphic-encryption deployments that publish coefficient-to-slot (C2S) evaluation keys and let the client return a 1-bit validity response. A malicious evaluator shifts a public encryption key, uses the public C2S layer to repack it into a ciphertext whose validity depends on one secret coefficient, and recovers a ternary secret key with O(H) queries, within a factor 2/log2(3) of the entropy lower bound. Each query is re-randomized with a fresh encryption of zero to evade trivial detection. The authors implemented it end to end in Lattigo v6.2.0 and recovered full keys across four standard CKKS parameter sets. It does not break RLWE or CKKS IND-CPA security.