Paper 2022/1438

Plug-and-play sanitization for TFHE

Abstract

Fully Homomorphic encryption allows the evaluation of any circuits over encrypted data while preserving the privacy of the data. However, without any additional properties, no guarantee is provided for the privacy of the circuits which are evaluated. A sanitization algorithm allows to destroy all previous information about how a ciphertext was obtained, ensuring that the circuit which was evaluated remains secret. In this paper, we present two new techniques to randomize RLWE ciphertexts, and show how they can be used to achieve ciphertext sanitization for the TFHE scheme proposed by Chillotti et al. (Asiacrypt 2016), by modifying the bootstrapping procedure internally. Our first technique relies on a generalization of the strategy proposed by Bourse et al. (Crypto 2016) to the ring setting. Using a backward induction over the circuit size, we also improve on their proof technique to avoid randomization at each step of the computation, enabling faster randomization and smaller noise growth. While this first approach adapts well in theory, we show evidence that it fails to provide a practical solution and propose a second solution with more realistic parameters at the cost of using an additional public key. As an additional contribution, we improve on the prohibitive size of the public key by relaxing the circuit privacy property to its computational counterpart, and build an efficient public randomizer composed of an RLWE-based public key encryption with additional properties on the ciphertexts distribution. We show that this randomizer can be used in the soak-and-spin paradigm of Ducas and Stehlé (Eurocrypt 2016) as well, and that it yields a significant improvement, mainly in the size of the public key. As a proof of concept, we provide a C implementation of our sanitization strategy, which shows that a sanitized LWE ciphertext can be obtained almost for free compared to a bootstrapped LWE ciphertext assuming many discrete Gaussian samples at hand.