Contributions to group key distribution schemes

Group-oriented communication has grown considerably with the wide use of broadcasting and multicasting of media content. In most group-oriented applications, access to the communicated content must be restricted to authorised users. These applications include news feeds, Pay-TV and private teleconferencing systems. A commonly used solution for controlling access in a group communication is to encrypt the content using a group key (session key). The group key is only known to the users in the authorized group. A group is called dynamic if the set of authorised group members changes in each session. The group key must be updated in each session to ensure only authorised users of the session can access the content. A group key distribution scheme provides algorithms to establish and maintain the group key. The challenge is to design secure and efficient group key distribution schemes. Security means that the collusion of unauthorised users cannot obtain the group key. Efficiency is measured in terms of the required secure storage, communication bandwidth and computation effort to update the group key. Diverse group applications pose new challenges and designing group key distribution schemes that are tailored to specific group communication scenarios is of high importance. In this thesis, we propose methods of constructing secure and efficient group key distribution schemes with several properties of high interest. We consider group key distribution schemes for completely decentralised environments, and propose secure and efficient constructions for group key distribution schemes where group management operations can be performed by either any group member or a collaboration of several group members. Both these settings have many applications in modern group communication systems. We show correctness of the proposed constructions, prove their security and assess their performance.