  Attribute-based Encryption (ABE) is an information centric security solution
that moves beyond traditional restrictions of point-to-point encryption by
allowing for flexible, fine-grain policy-based and content-based access control
that is cryptographically enforced. As the original ABE systems are managed by
a single authority, several efforts have decentralized different ABE schemes to
address the key escrow problem, where the authority can issue secret keys to
itself to decrypt all the ciphertext. However, decentralized ABE (DABE) schemes
raise the issue of collusion attacks. In this paper, we review two existing
types of collusion attacks on DABE systems, and introduce a new type of
collusion among authorities and data users. We show that six existing DABE
systems are vulnerable to the newly introduced collusion and propose a model to
secure one of the DABE schemes.

Cryptography and Security