Breaking and building of Group Inside Signature

Group Inside Signature (GIS) is a signature scheme that allows the signer to designate his signature to be verified by a group of people. Members other than the designated group cannot verify the signature generated by the signer. In Broadcast Group Oriented Signature (BGOS), a user from one group can designate his signature to be verified by members of another group. An Adaptable Designated Group Signature (ADGS), is one in which an user can designate his signature to be verified by a selected set of members who are from different groups. The two GIS schemes [5], [6] and the BGOS scheme [7], we consider are certificateless schemes and the ADGS scheme [8] which we consider here is an identity based scheme. In this paper, we present the cryptanalysis of all the four schemes that appeared in [5], [6], [7] and [8]. We also present a new identity based ADGS (N-ADGS) scheme and prove its security in the random oracle model. The existing model described in [8] for ADGS did not consider unlinkability which is one of the key properties required for ADGS. We provide the security model for unlinkability and also prove our scheme is unlinkable.© Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 2010.