Active Security in a Two-Server Homomorphic Encryption Based Outsourced Protocol

252 / BBB

2026-03-17

Till Caspar

15:45

Homomorphic encryption based outsourced computation allows functions to be evaluated on encrypted data and has been studied as a promising approach for privacypreserving computation. However, so far, most approaches only achieve limited suitability for a real-world web service setting. This thesis extends one protocol, which is suitable for such a setting with some constraints, the practical non-interactive outsourced computation based on black-box homomorphic encryption of Beskorovajnov et al. [5]. First, the protocol is strengthened to achieve security against active adversaries by incorporating non-interactive zero-knowledge proofs and commitment schemes. These mechanisms restrict arbitrary deviations from the protocol specification and allow the protocol to be proven secure in the Universal Composability framework. Second, in a further extension, the non-collusion assumption between the calculation server and the decryption server is relaxed by replacing the single decryption server with a threshold decryption scheme. In this setting, security is preserved as long as less than a threshold number of decryption parties are corrupted and the calculation server, the decryption server and the input client do not collude simultaneously. Both extensions are formally proven to UC-realize a ideal functionality. While the resulting protocols introduce additional cryptographic overhead, they strengthen security guarantees and reduce trust assumptions, thereby narrowing the gap between theoretical security models and more realistic deployment scenarios.