Security Analysis of Gagne et al.´s Threshold Attribute-Based Signcryption Scheme

In SCN2010, Gagne, Narayan, and Safavi-Naini proposed attribute-based signcryption with threshold structure. As in cipher text-policy attribute-based encryption, a sender can specify the access structure of recipients, and as in attribute-based signature, each recipient can verify the sender´s attributes. In contrast to the access structure of recipients, the access structure of the sender needs to be fixed in the setup phase. In this paper, we show that Gagne et al.´s threshold attribute-based signcryption is not secure and give a concrete forgery attack. The attack can make use some keys to construct any other legal keys and at last totally break the scheme. We also show that the attack is effective on other attribute-based cryptographic schemes with the same key structure.