Paper 2024/1689

Homomorphic Encryption with Authority

Abstract

Fully homomorphic encryption enables computations over encrypted data, which allows privacy-preserving services to be held between a server and a client. However, real-world applications demand practical considerations, especially concerning public safety and legal investigations. Existing FHE schemes focus solely on privacy, neglecting the societal risks posed by criminal activities utilizing privacy-preserving services. This paper introduces Homomorphic Encryption with Authority (HEwA), a novel framework that balances data privacy with public safety by incorporating an "authority" party. The proposed HEwA system operates in two phases: a normal phase, where client data privacy is protected, and an investigative phase, where the authority referring to a legally authorized entity such as government agencies exerts the right to recover suspicious client’s data. We formalize the security model for HEwA, ensuring that client privacy is protected during the normal phase while enabling authorities to recover encrypted data in the investigative phase. As a concrete example, we design an efficient HEwA system solely based on the CKKS homomorphic encryption scheme, which supports approximate computations over real-number data, making it highly suitable for fruitful applications in AI such as secure genomic analysis. We further provide rigorous security proofs. This new approach addresses the tension between privacy and public safety in cloud services, paving the way for responsible use of homomorphic encryption in practice.