Cryptology ePrint Archive: Report 2015/047

Linearly Homomorphic Encryption from DDH

Guilhem Castagnos and Fabien Laguillaumie

Abstract: We design a linearly homomorphic encryption scheme whose security relies on the hardness of the decisional Diffie-Hellman problem. Our approach requires some special features of the underlying group. In particular, its order is unknown and it contains a subgroup in which the discrete logarithm problem is tractable. Therefore, our instantiation holds in the class group of a non maximal order of an imaginary quadratic field. Its algebraic structure makes it possible to obtain such a linearly homomorphic scheme whose message space is the whole set of integers modulo a prime p and which supports an unbounded number of additions modulo p from the ciphertexts. A notable difference with previous works is that, for the first time, the security does not depend on the hardness of the factorization of integers. As a consequence, under some conditions, the prime p can be scaled to fit the application needs.

Category / Keywords: public-key cryptography / Linearly Homomorphic Encryption, Orders of Quadratic Fields, Diffie-Hellman Assumptions

Original Publication (with major differences): Proc. of CT-RSA 2015

Date: received 20 Jan 2015, last revised 26 Jan 2015

Contact author: guilhem castagnos at math u-bordeaux1 fr

Available format(s): PDF | BibTeX Citation

Note: An extended abstract of this paper will be published in the proceedings of CT-RSA 2015. This is the full version.

Version: 20150126:121521 (All versions of this report)

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