Multi-hop Fine-Grained Proxy Re-encryption

Proxy re-encryption (PRE) allows a proxy to transform a ciphertext intended for Alice (delegator) to another ciphertext intended for Bob (delegatee) without revealing the underlying message. Recently, a new variant of PRE, namely fine-grained PRE (FPRE), was proposed in [Zhou et al., Asiacrypt 2023]. Generally, FPRE is designed for a function family \(\mathcal {F}\) : each re-encryption key \(\textsf{rk}_{A\rightarrow B}^f\) is associated with a function \(f\in \mathcal {F}\) , and with \(\textsf{rk}_{A\rightarrow B}^f\) , a proxy can transform Alice’s ciphertext encrypting m to Bob’s ciphertext encrypting f(m). However, their scheme only supports single-hop re-encryption and achieves only CPA security. In this paper, we formalize multi-hop FPRE (mFPRE) that supports multi-hop re-encryptions in the fine-grained setting, and propose two mFPRE schemes achieving CPA security and stronger HRA security (security against honest re-encryption attacks), respectively.