International Association
for Cryptologic Research

Registered encryption (Garg {\em et al.}, TCC'18) is an emerging paradigm that tackles the key-escrow problem associated with identity-based encryption by replacing the private-key generator with a much weaker entity known as the key curator. The key curator holds no secret information, and is responsible to: (i) update the master public key whenever a new user registers its own public key to the system; (ii) provide helper decryption keys to the users already in the system, in order to make them still able to decrypt. For practical purposes, tasks (i) and (ii) need to be efficient, in the sense that the size of the public parameters, of the master public key, and of the helper decryption keys, as well as the running time for key generation and user registration, and the number of updates, must be small.

In this paper, we generalize the notion of registered encryption to the setting of functional encryption (FE). Our contributions are twofold: On the one hand, we show that registered FE exists assuming indistinguishability obfuscation and somewhere statistically binding hash functions. On the other hand, we show an efficient construction of registered FE for the special case of inner-product predicates, over asymmetric bilinear groups of prime order, with provable security in the generic group model.