Cryptology ePrint Archive: Report 2017/430

Improving TFHE: faster packed homomorphic operations and efficient circuit bootstrapping

Ilaria Chillotti and Nicolas Gama and Mariya Georgieva and Malika Izabachène

Abstract: In this paper, we present several methods to improve the evaluation of homomorphic functions, both for fully and for leveled homomorphic encryption. We propose two packing methods, in order to decrease the expansion factor and optimize the evaluation of look-up tables and random functions in TRGSW-based homomorphic schemes. We also extend the automata logic, introduced in [19, 12], to the efficient leveled evaluation of weighted automata, and present a new homomorphic counter called TBSR, that supports all the elementary operations that occur in a multiplication. These improvements speed-up the evaluation of most arithmetic functions in a packed leveled mode, with a noise overhead that remains additive. We finally present a new circuit bootstrapping that converts TLWE into low-noise TRGSW ciphertexts in just 137ms, which makes the leveled mode of TFHE composable, and which is fast enough to speed-up arithmetic functions, compared to the gate-by-gate bootstrapping given in [12]. Finally, we propose concrete parameter sets and timing comparison for all our constructions.

Category / Keywords: public-key cryptography / FHE, leveled, bootstrapping, LWE, GSW, packing, weighted automata, arithmetic

Date: received 18 May 2017

Contact author: ilaria chillotti at uvsq fr

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Version: 20170522:213506 (All versions of this report)

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