This paper presents a concrete democratic group signature scheme which holds （t, n）-threshold traceability. In the scheme, the capability of tracing the actual signer is distributed among n group members. It gives a valid democratic group signature such that any subset with more than t members can jointly reconstruct a secret and reveal the identity of the signer. Any active adversary cannot do this even if he can corrupt up to t - 1 group members.
The universal composability framework is a new approach for designing and analyzing the security of cryptographic protocols.In this framework,the security of protocols is maintained under a general protocol composition operation.In the paper,we propose the universal composability framework for the analysis of proxy threshold signature and present a universally composable secure proxy threshold signature scheme which is the first one in this area.The proposed scheme is suitable for the mobile agents,which should migrate across different environment through network.Furthermore,we give the concrete analysis of the reduction to prove the security of the proposed scheme.
Democratic group signatures （DGSs） attract many researchers due to their appealing properties, i.e., anonymity, traceability and no group manager. Security results of existing work are based on decisional Diffie-Hellman （DDH） assumption. In this paper, we present a democratic group signature scheme based on any gap Diffie-Hellman （GDH） group where DDH problem is easily but computational Diffe-Hellman （CDH） problem is hard to be solved. Besides the properties of ordinary DGSs, our scheme also provides the property of linkability, i.e., any public verifier can tell whether two group signatures are generated using the same private key. Security properties of our scheme employ a new and independently interesting decisional product Diffie-Hellman （DPDH） assumption which is weaker than DDH one.
