Hierarchical Functional Encryption for Linear Transformations

In contrast to the conventional all-or-nothing encryption, functional encryption (FE) allows partial revelation of encrypted information based on the keys associated with different functionalities. Extending FE with key delegation ability, hierarchical functional encryption (HFE) enables a secret key holder to delegate a portion of its decryption ability to others and the delegation can be done hierarchically. All HFE schemes in the literature are for general functionalities and not very practical. In this paper, we focus on the functionality of linear transformations (i.e. matrix product evaluation). We refine the definition of HFE and further extend the delegation to accept multiple keys. We also propose a generic HFE construction for linear transformations with IND-CPA security in the standard model from hash proof systems. In addition, we give two instantiations from the DDH and DCR assumptions which to the best of our knowledge are the first practical concrete HFE constructions.