Authenticated Key Agreement in Dynamic Groups

Abstract

Multicast security poses interesting challenges in the area of key management. Designing a good protocol for key agreement in dynamic multicast groups involves a thorough understanding of the trade-offs that exist among storage, communication and computation overhead. The contribution of this thesis is a verifiable protocol for authenticated key agreement based on a distributed key generation scheme. The underlying key generation scheme has shown promise in being natural for collaborative group applications. The protocol can then be tailored to particular applications once we understand the communication, storage and computation constraints specific to the application. To handle group membership changes in dynamic groups, an auxiliary key agreement protocol is introduced. The auxiliary protocol re-uses contributions to the key in the previous round, to form the new key. The key shares of the members contributing fresh values in the current round are more susceptible to discovery by colluding group members (not outsiders). The auxiliary protocol does not introduce any other security weakness. A protocol that starts from the scratch on membership change is going to be expensive, slow and unsuitable for most applications.We use the well-known Logical Key Tree (LKH) structure to allow the key management (distribution) part of the protocol to scale to large groups. The key tree structure helps to localize the effect of membership change and as a result, reduces the communication overhead to form the new session key.