Analysis and Forgery Attack to Authorised Group Signature Scheme;

A new forgery attack on message recovery signatures;

A class of threshold signature scheme based on ElGamal is improved,in which dealer right can be restricted,it can prevent forgery attack for security.

Based on security analysis on Hsu scheme,an improved threshold proxy signature scheme was proposed,which effectively overcomed the disadvantage of Hsu scheme which couldn t stand the internal conspiracy attack and the forge attack.

There is forge attack in the scheme.

They considerd their scheme possible to satisfy multed security requirements to resisit coalition attacks.

And the scheme does not satisfy the properties of against coalition attack.

A coalition attack that the manager and the revocation center could conspire to generate valid group signatures and impose them on an arbitrary member of the group was presented.

This new scheme not only solves the problem that QCX scheme has not the property of strong identifiablity through introducing the set ASI in the threshold proxy signature but also overcomes the security leaks such as collusion attack,the original forgeable attack in the congener schemes.

Two forgery attacks and their corresponding conspiracy attacks are proposed against the(t,n) threshold signature scheme without a trusted party worked out by Wang Bin and Li Jian-hua(the WL Scheme for short).

The results show that the proposed scheme can not only resist conspiracy attacks and forgery attacks essentially,but also provide anonymity and traceability simultaneously.