Server-Aided Revocable Attribute-Based Encryption Resilient to Decryption Key Exposure

Attribute-based encryption (ABE) is a promising approach that enables scalable access control on encrypted data. However, one of the main efficiency drawbacks of ABE is the lack of practical user revocation mechanisms. In CCS 2008, Boldyreva, Goyal and Kumar put forward an efficient way to revoke users. But, it requires each data user storing a (non-constant) number of long-term private keys and periodically communicating with the key generation center to update his/her decryption keys. In ESORICS 2016, Cui et al. proposed the first server-aided revocable ABE scheme to address the above two issues. It involves an untrusted server to transform any non-revoked user’s ABE ciphertexts into short ciphertexts using user’s short-term transformation keys. The data user can fully decrypt the transformed ciphertexts using his/her local decryption keys. Cui et al. also introduced the decryption key exposure (DKE) attacks on transformation keys. However, if the untrusted server colludes with an adversary, the scheme may be insecure against DKE attacks on user’s local decryption keys. In this paper, we first revisit Cui et al. security model, and enhance it by capturing the DKE attacks on user’s local decryption keys and allowing the adversary to fully corrupt the server simultaneously. We then construct a server-aided revocable ABE based on Rouselakis-Waters ciphertext-policy ABE (CCS 2013). We show that our scheme is secure against local decryption key exposure attacks, and maintains the outstanding properties of efficient user revocation, short local ciphertext size and fast local decryption.