| Summary: | © 2017 ACM. We develop two IND-CPA-secure multihop unidirectional Proxy Re-Encryption (PRE) schemes by applying the Ring-LWE (RLWE) key switching approach from the homomorphic encryption literature. Unidirectional PRE is ideal for secure publish-subscribe operations where a publisher encrypts information using a public key without knowing upfront who the subscriber will be and what private key will be used for decryption. The proposed PRE schemes provide a multihop capability, meaning that when PRE-encrypted information is published onto a PRE-enabled server, the server can either delegate access to specific clients or enable other servers the right to delegate access. Our first scheme (which we call NTRU-ABD-PRE) is based on a variant of the NTRU-RLWE homomorphic encryption scheme. Our second and main PRE scheme (which we call BV-PRE) is built on top of the Brakerski-Vaikuntanathan (BV) homomorphic encryption scheme and relies solely on the RLWE assumption. We present an open-source C++ implementation of both schemes and discuss several algorithmic and software optimizations. We examine parameter selection tradeoffs in the context of security, runtime/latency, throughput, ciphertext expansion, memory usage, and multihop capabilities. Our experimental analysis demonstrates that BV-PRE outperforms NTRU-ABD-PRE in both single-hop and multihop settings. The BVPRE scheme has a lower time and space complexity than existing IND-CPA-secure lattice-based PRE schemes and requires small concrete parameters, making the scheme computationally efficient for use on low-resource embedded systems while still providing 100 bits of security. We present practical recommendations for applying the PRE schemes to several use cases of ad hoc information sharing for publish-subscribe operations.
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