Decoded-Output Conformance Need Not Establish CKKS Execution

Technical preprint on conformance testing for CKKS fully homomorphic encryption.

The paper studies whether a verifier that observes only decoded numerical outputs can establish that CKKS evaluation was actually performed. It shows that, for the examined known-answer conformance protocol, a respondent can reproduce accepted outputs using ordinary double-precision arithmetic, without performing homomorphic encryption, ring arithmetic, or key generation.

The work formalizes conditions under which decoded-output conformance fails to establish scheme execution, demonstrates the attack experimentally, measures the cost advantage of the alternative computation, and examines several natural defenses including precision thresholds, timing, and increased probe volume.

A blind protocol is also evaluated in which the verifier generates the FHE keys, encrypts the probes, and receives ciphertext results. This removes the demonstrated plaintext-reconstruction attack, but the paper does not claim general execution soundness or proof of device provenance.

The accompanying implementation includes the two protocols, attack code, multiple CKKS backends, and scripts used to reproduce the reported results.