Forgery Attack on An Outsourced Attribute-Based Signature Scheme

Attribute-based signatures have been widely used in many applications including anonymous authentication and fine-grained access control. Unfortunately, existing attribute-based signature schemes have high computational complexity which usually makes them unsuitable for resource-constrained devices. One approach to get around this problem is outsourcing heavy computation through cloud servers. But then, we have to take extra measures to ensure that the server performs computations honestly and that it cannot forge any user’s signature. Recently, Liu et al. proposed an outsourced attribute-based signature scheme. The goal of this paper is to show that this signature is vulnerable against forgery attack so that an attacker can forge a signature without the knowledge of the secret keys. We further present an improved method and show that it resolves the attack.