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Byzantine agreement using partial authentication. (English) Zbl 1350.68033

Peleg, David (ed.), Distributed computing. 25th international symposium, DISC 2011, Rome, Italy, September 20–22, 2011. Proceedings. Berlin: Springer (ISBN 978-3-642-24099-7/pbk). Lecture Notes in Computer Science 6950, 389-403 (2011).
Summary: Three decades ago, M. Pease et al. introduced the problem of Byzantine Agreement [J. ACM 27, 228–234 (1980; Zbl 0434.68031)] where nodes need to maintain a consistent view of the world in spite of the challenge posed by Byzantine faults. Subsequently, it is well known that Byzantine agreement over a completely connected synchronous network of \(n\) nodes tolerating up to \(t\) faults is (efficiently) possible if and only if \(t < n/3\). Pease et al. further empowered the nodes with the ability to authenticate themselves and their messages and proved that agreement in this new model (popularly known as authenticated Byzantine agreement (ABA)) is possible if and only if \(t < n\). (which is a huge improvement over the bound of \(t < n/3\) in the absence of authentication for the same functionality).
To understand the utility, potential and limitations of using authentication in distributed protocols for agreement, A. Gupta et al. [Lect. Notes Comput. Sci. 5935, 79–91 (2010; Zbl 1350.68035)] studied ABA in new light. They generalize the existing models and thus, attempt to give a unified theory of agreements over the authenticated and non-authenticated domains. In this paper we extend their results to synchronous (undirected) networks and give a complete characterization of agreement protocols.
As a corollary, we show that agreement can be strictly easier than all-pair point-to-point communication. It is well known that in a synchronous network over \(n\) nodes of which up to any \(t\) are corrupted by a Byzantine adversary, BA is possible only if all pair point-to-point reliable communication is possible [D. Dolev et al., J. Algorithms 3, 14–30 (1982; Zbl 0495.68093); J. ACM 40, No. 1, 17–47 (1993; Zbl 0774.68017)]. Thus, a folklore in the area is that maintaining global consistency (agreement) is at least as hard as the problem of all pair point-to-point communication. Equivalently, it is widely believed that protocols for BA over incomplete networks exist only if it is possible to simulate an overlay-ed complete network. Surprisingly, we show that the folklore is not always true. Thus, it seems that agreement protocols may be more fundamental to distributed computing than reliable communication.
For the entire collection see [Zbl 1225.68024].

MSC:

68M12 Network protocols
68M14 Distributed systems
68W15 Distributed algorithms
94A62 Authentication, digital signatures and secret sharing
Full Text: DOI

References:

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