Revisiting Stream-Cipher-Based Homomorphic Transciphering in the TFHE Era

Transciphering allows to workaround the large expansion of the size of FHE encrypted data, thanks to the use of symmetric cryptography. Transciphering is a recryption technique that delegates the effective homomorphic encryption to the cloud. As a result, a client only has to encrypt (once) a symmetric key $${\textsf {SYM}}.\!{\textsf {sk}}$$ under a homomorphic encryption system, while his payload data are encrypted under $${\textsf {SYM}}.\!{\textsf {sk}}$$ using the chosen symmetric encryption algorithm. In this work, we study the performances of some symmetric encryption algorithms in light of the TFHE cryptosystem and its properties. This allows us to unleash the use of additional existing symmetric algorithms which were not viable candidates for efficient encrypted domain execution with levelled-FHEs. In particular, we provide experimental evidences that Grain128-AEAD, a well established and well respected stream-cipher which is a finalist of the NIST competition for light-weight cryptography, is amenable to practical performances when run in the encrypted domain. As such, our work extends practical transciphering capabilities to include authenticated encryption for the first time.