A group key distribution scheme with decentralised user join

Group key distribution schemes (GKDS) provide a common key for a dynamic group of users. By dynamic we mean that the authorised set of users changes in each session. In GKDS with dynamic controller, the group management is decentralised and any group member can act as a group controller to form a subgroup. In this paper, we propose a GKDS that has decentralised group management and is dynamic. That is it allows any user in U, the universe of users, to form a subgroup of existing users and also sponsor a new user to join the group that he initiates. A user v ∉ U that is sponsored by u ∈ U cannot participate in groups initiated by other users. However if enough users in U sponsor v, he will be permanently admitted to U and will have the same capabilities as others. This provides a powerful mechanism for groups to be self-sufficient and grow from a small subset of users without requiring a trusted third party. We also consider security requirement of join operation in GKDS and show that a recently proposed GKDS [1] although provides secure dynamic controller property but is insecure under its proposed method of user join (using a trusted third party). We give a modification of this system to provide secure user join. We also show that our proposed GKDS provides secure and flexible user sponsorship and join.