International Association
for Cryptologic Research

This paper introduces registered functional encryption (RFE) that eliminates trust on the central authority for handling secrets. Unlike standard functional encryption (FE), in an RFE scheme, users create their secret keys themselves and then register the associated public keys to a key curator along with the functions they wish to compute on the encrypted data. The key curator aggregates the public keys from the different users into a single compact master public key. To decrypt, users occasionally need to obtain helper decryption keys from the key curator which they combine with their own secret keys. We require that the size of the aggregated public key, the helper decryption keys, the ciphertexts, as well as the encryption/decryption time to be polylogarithmic in the number of registered users. Moreover, the key curator is entirely transparent and maintains no secrets. RFE generalizes the notions of registration-based encryption (RBE) introduced by Garg et al. (TCC 2018) and registered attribute-based encryption introduced by Hohenberger et al. (EUROCRYPT 2023) who dealt with the “all-or-nothing” variants of FE. We present an RFE scheme for general functions and an arbitrary number of users from indistinguishability obfuscation and somewhere statistically binding hash functions. Surprisingly, our construction is achieved via only a minor tweak applied to the registered ABE of Hohenberger et al.