Paper 2023/457

Registered FE beyond Predicates: (Attribute-Based) Linear Functions and more

Abstract

This paper introduces the first registered functional encryption RFE scheme tailored for linear functions. Distinctly different from classical functional encryption (FE), RFE addresses the key-escrow issue and negates the master key exfiltration attack. Instead of relying on a centralized trusted authority, it introduces a “key curator” - a fully transparent entity that does not retain secrets. In an RFE framework, users independently generate secret keys and subsequently register their respective public keys, along with their authorized functions, with the key curator. This curator consolidates public keys from various users into a unified, concise master public key. For decryption, users occasionally secure helper decryption keys from the key curator, which they use in conjunction way with their private keys. It is imperative that the aggregate public key, helper decryption keys, ciphertexts, and the times for encryption/decryption are polylogarithmic in the number of registered users. All existing RFE designs were confined to predicates where given the correct credentials a user can retrieve the entire payload from a ciphertext or gain no information about it otherwise. Contrarily, our RFE scheme facilitates the computation of linear functions on encrypted content and extraction of only the computation results. Recognizing potential leaks from linear functions, we further enhance our RFE by incorporating an attribute-based access control mechanism. The outcome is the first registered attribute-based linear FE (RABIPFE), which supports access policies depicted as linear secret sharing schemes LSSS. Our proposed schemes are realized in the common reference string (CRS) model as introduced by Hohenberger et al.[EUROCRYPT 2023], employ simple tools and black-box methods. Specifically, our constructs operate in asymmetric prime-order bilinear group regime setting and are proven secure in the generic bilinear group model. Aligning with all pre-existing black-box RFE designs within the CRS model, our schemes cater to a predetermined maximum user count. A notable variant of our RABIPFE scheme also yields the first efficient register ABE (RABE) system for LSSS access policies in asymmetric prime-order bilinear groups. Conclusively, demonstrating feasibility, we formulated an RFE blueprint that supports general functionalities and an infinite user base, leveraging indistinguishability obfuscation and one-way functions.

Note: The updated version additionally includes constructions of registered (attribute-based) inner product functional encryption using pairing.