Cryptology ePrint Archive: Report 2010/514

Semi-Homomorphic Encryption and Multiparty Computation

Rikke Bendlin and Ivan Damgård and Claudio Orlandi and Sarah Zakarias

Abstract: An additively-homomorphic encryption scheme enables us to compute linear functions of an encrypted input by manipulating only the ciphertexts. We define the relaxed notion of a semi-homomorphic encryption scheme, where the plaintext can be recovered as long as the computed function does not increase the size of the input "too much". We show that a number of existing cryptosystems are captured by our relaxed notion. In particular, we give examples of semi-homomorphic encryption schemes based on lattices, subset sum and factoring. We then demonstrate how semi-homomorphic encryption schemes allow us to construct an efficient multiparty computation protocol for arithmetic circuits, UC-secure against a dishonest majority. The protocol consists of a preprocessing phase and an online phase. Neither the inputs nor the function to be computed have to be known during preprocessing. Moreover, the online phase is extremely efficient as it requires no cryptographic operations: the parties only need to exchange additive shares and verify information theoretic MACs. Our contribution is therefore twofold: from a theoretical point of view, we can base multiparty computation on a variety of different assumptions, while on the practical side we offer a protocol with better efficiency than any previous solution.

Category / Keywords: cryptographic protocols / multiparty computation, homomorphic encryption

Publication Info: This is the full verion of the paper appearing in EuroCrypt 2011

Date: received 7 Oct 2010, last revised 6 May 2011

Contact author: zarah at cs au dk

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Version: 20110506:083511 (All versions of this report)

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