Paper 2024/1673

Proteus: A Fully Homomorphic Authenticated Transciphering Protocol

Abstract

Fully Homomorphic Encryption (FHE) is a powerful technology that allows a cloud server to perform computations directly on ciphertexts. To overcome the overhead of sending and storing large FHE ciphertexts, the concept of FHE transciphering was introduced, allowing symmetric key encrypted ciphertexts to be transformed into FHE ciphertexts by deploying symmetric key decryption homomorphically. However, existing FHE transciphering schemes remain unauthenticated and malleable, allowing attackers to manipulate data and remain undetected. This work introduces Proteus, a new methodology for authenticated transciphering, which enables oblivious access control, preventing users from downloading unauthenticated or malicious data. Our protocol implementation adopts ASCON, NIST's new standard for lightweight cryptography, to enable homomorphic hashing and authenticated transciphering. Our ASCON transcipher is paired with the TFHE encryption scheme, which is well suited to perform encrypted rotation and bitwise operations. We evaluate our approach with a variety of real-life privacy-preserving applications, including URL phishing detection, private content moderation of hate speech, and biometric authentication.