An ID-based group signature

Suppose that there are n departments in a company Ω and any dept. has the right to sign a message on behalf of the company. The group signature made by the dept. should be agreed by the members whose number is over the threshold. The Receiver can verify the validity of the signature but can't distinguish which dept. the signature comes from. When the dispute occurs, only the Authority can "open" the group signature to distinguish where the signature comes from. Motivated by this consideration, with the combination of vector space secret sharing and bilinear pairing, a new group signature is presented in this paper. The member can be added or deleted after the scheme founded and the public key length is independent. The "open" process in the presented group signature is realized by subliminal channel.