Cryptology ePrint Archive: Report 2017/319

Encrypt-Augment-Recover: Function Private Predicate Encryption from Minimal Assumptions in the Public-Key Setting

Sikhar Patranabis and Debdeep Mukhopadhyay

Abstract: We present new public-key predicate encryption schemes in the standard model that are provably function private under standard computational assumptions. A large class of existing function private constructions in the public-key setting impose highly stringent requirements on the min-entropy of predicate distributions, thereby limiting their applicability in the context of real-world predicates. Other existing constructions are either secure only in the generic group model, or require strong assumptions such as indistinguishability obfuscation. Our constructions, on the other hand, are function private for predicate distributions that satisfy more realistic min-entropy requirements, and avoid the need for strong assumptions such as obfuscation.

In order to prove function privacy, we adopt the indistinguishability-based framework proposed by Boneh, Raghunathan and Segev in Crypto'13. The framework requires a secret-key corresponding to a predicate sampled from a distribution with min-entropy super logarithmic in the security parameter $\lambda$, to be computationally indistinguishable from another secret-key corresponding to a uniformly and independently sampled predicate. Within this framework, we develop a novel approach, denoted as {encrypt-augment-recover}, that takes an existing predicate encryption scheme and transforms it into a computationally function private one while retaining its original data privacy guarantees. Our approach leads to public-key constructions for identity-based encryption (IBE) and inner-product encryption (IPE) that are computationally function private in the standard model under a family of weaker variants of the DLIN assumption.

Category / Keywords: Predicate Encryption, Public-Key, Function Privacy, Computational Indistinguishability, Min-Entropy, Identity-Based Encryption, Inner-Product Encryption

Date: received 11 Apr 2017, last revised 24 May 2017

Contact author: sikhar patranabis at iitkgp ac in

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Note: The paper is augmented with a revised related work section

Version: 20170525:040531 (All versions of this report)

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