A Framework for Robust Group Key Agreement

Stanisław Jarecki, Jihye Kim, Gene Tsudik

A robust group key agreement protocol (GKA) allows a set of players to establish a shared secret key, regardless of network/node failures. Current constant-round GKA protocols are either efficient and nonrobust or robust but not efficient; assuming a reliable broadcast communication medium, the standard encryption-based group key agreement protocol can be robust against arbitrary number of node faults, but the size of the messages broadcast by every player is proportional to the number of players. In contrast, nonrobust group key agreement can be achieved with each player broadcasting just constant-sized messages. We propose a novel 2-round group key agreement protocol, which tolerates up to T node failures, using O(T)-sized messages for any T. We show that the new protocol implies a fully-robust group key agreement with logarithmic-sized messages and expected round complexity close to 2, assuming random node faults. The protocol can be extended to withstand malicious insiders at small constant factor increases in bandwidth and computation. The proposed protocol is secure under the (standard) Decisional Square Diffie-Hellman assumption.

Jens-Matthias Bohli, Benjamin Glas, Rainer Steinwandt

Gene Tsudik, Stanisław Jarecki, Jihye Kim

The last decade has witnessed a sharp spike in the popularity of group applications, such as multi-media conferencing, collaborative workspaces, as well as dynamic coalitions common in law enforcement and disaster rescue scenarios. Standard security services required in such group settings (such as confidentiality, integrity and authentication) can be efficiently achieved if all group members share a secret key. A group key agreement protocol (GKA) provide a technique for any set of participating players to collaboratively establish a shared key. In this thesis, we tackle two challenging issues in GKA: robustness and privacy-preserving authentication. Robustness improves performance by guaranteeing completion of GKA protocols despite participating player or communication failures. Affiliation-hiding authenticated GKA prevents from man-in-the-middle-attacks by authenticating players in a privacy-preserving manner. We propose a two-round group key agreement protocol, which offers a natural trade-off between message size and the desired level of fault-tolerance. We demonstrate that in a realistic setting of random player faults, the new protocol has logarithmic-sized messages and expected round complexity close to two. We generalize a robust group key agreement protocol to support heterogeneous players for a general environment. We prove the security of the proposed protocols under the standard cryptographic assumptions. We present the formal definition of the affiliation-hiding GKA in a standard PKI model with re-usable certificates. The model includes strong security properties, called perfect forward secrecy (PFS) and linkable affiliation-hiding (LAH)—that an key agreement protocol might provide in the face of player corruptions and session compromises. We construct two variants of optimal-cost AH-AGKA protocols, which satisfy PFS and LAH notions under the standard cryptographic assumptions in the random oracle model. The protocol yields minimal costs: three rounds of interaction matches the round complexity of non-private PFS authenticated GKA and its computation costs are comparable to the cost of unauthenticated GKA.

Junghyun Nam, Seokhyang Cho, Seungjoo Kim +1 more

Jianjie Zhao, Dawu Gu, Yali Li

Yong Chen, Mingxing He, Shengke Zeng +1 more

We analyze the security of asymmetric group key agreement (ASGKA) protocol in the universal composability (UC) framework. An ideal functionality that is suitable for ASGKA protocol is proposed. And we show that the ASGKA protocol securely realizes the proposed ideal functionality that can resist attacks by malicious insiders. The UC framework allows the protocols to guarantee their security under arbitrary protocol environment.