Chosen-ciphertext secure multi-hop identity-based conditional proxy re-encryption with constant-size ciphertexts

Proxy Re-Encryption (PRE) allows one user to delegate the decryption rights of his/her ciphertexts to another user. Since the introduction of Multi-Hop Identity-Based PRE (MH-IBPRE) by Green and Ateniese, the ciphertext size and the decryption complexity grow linearly in the number of re-encryption "hops". In this paper, for the first time, we propose an MH-IBPRE that maintains the (constant) ciphertext size and computational complexity regardless of the number of re-encryption hops. Moreover, our scheme is bidirectional and also supports conditional re-encryption. The scheme is proven secure against selective identity and chosen-ciphertext attacks and collusion resistant in the standard model. As of independent interest, we also show that the conditional re-encryption can also be extended to a set of conditions.