Faulty Group Protocols

A group protocol is a protocol whereby the group participants securely exchange information in order to achieve a common goal or application, e.g. agreeing on a contract or on a session key. Groups are typically dynamic and relatively small in size, of the order of a hundred members [2]. To enable dynamic changes to group constitution, a group protocol may be defined by several sub-protocols for joining and leaving, and may appeal to a trusted third party, called the server for short. The design of a group protocol may address several requirements, including efficiency (e.g., the crypt-algorithm used, the number of messages exchanged), security guarantee (e.g., implicit key agreement, exchange fairness) the type of channel (private, public), the group topology (e.g., ring), the communication technology (e.g., wired, wireless), the scheme used in key generation (centralised, distributed), the level of involvement of the server, if any (e.g., online or offline), the communication paradigm (e.g., RPC, connection-oriented) and trust hierarchy. Thus, a protocol may be correct in some scenarios but not in others. Similarly, an attack may be valid in some scenarios but not in others. In what follows, we overview the several faulty group protocols from the literature. Protocols will be given by a sequence of steps, each of the form: