Security analysis on authorized group signature

In the study of the well known ElGamal encryption variant which underlies some authorized group signature schemes, some vulnerabilities are found. And the concept of strong anonymity of group signature is proposed based on the security analysis of such an authorized group signature which is based on the ElGamal encryption variant. To show how to use the vulnerabilities, three anonymity attack methods aiming to the authorized group signature are proposed. Additionally, the Simmons I and II type subliminal channels which allow an authority manager to leak information to signers are also found in the group signature scheme. And the latent harm of the subliminal channels is also analyzed. Finally, the authority demise attack is found in the group signature.