Conjunctive Broadcast and Attribute-Based Encryption

Abstract

Attribute-based encryption (ABE) system enables an access control mechanism over encrypted data by specifying access policies among private keys and ciphertexts. There are two flavors of ABE, namely key-policy and ciphertext-policy, depending on which of private keys or ciphertexts that access policies are associated with. In this paper we propose a new cryptosystem called Broadcast ABE for both flavors. Broadcast ABE can be used to construct ABE systems with direct revocation mechanism. Direct revocation has a useful property that revocation can be done without affecting any non-revoked users; in particular, it does not require users to update keys periodically. For key-policy variant, our systems appear to be the first fully-functional directly revocable schemes. For ciphertext-policy variant, our systems improve the efficiency from the previously best revocable schemes; in particular, one of our schemes admits ciphertext and private key sizes roughly the same as the currently best (non-revocable) ciphertext-policy ABE. Broadcast ABE can also be utilized to construct multi-authority ABE in the disjunctive setting.