Round-Efficient Byzantine Agreement and Multi-Party Computation with Asynchronous Fallback

Giovanni Deligios, Martin Hirt, and Chen-Da Liu Zhang

Theory of Cryptography — TCC 2021, LNCS, Springer International Publishing, vol. 13042, pp. 623–653, Nov 2021.

Protocols for Byzantine agreement (BA) and secure multi-party computation (MPC) can be classified according to the underlying communication model. The two most commonly considered models are the synchronous one and the asynchronous one. Synchronous protocols typically lose their security guarantees as soon as the network violates the synchrony assumptions. Asynchronous protocols remain secure regardless of the network conditions, but achieve weaker security guarantees even when the network is synchronous.

Recent works by Blum, Katz and Loss [TCC’19], and Blum, Liu-Zhang and Loss [CRYPTO’20] introduced BA and MPC protocols achieving security guarantees in both settings: security up to 𝑡𝑠 corruptions in a synchronous network, and up to 𝑡𝑎 corruptions in an asynchronous network, under the provably optimal threshold trade-offs $t_{a} \leq t_{s}$ and $t_{a} + 2 t_{s} < n$ . However, current solutions incur a high synchronous round complexity when compared to state-of-the-art purely synchronous protocols. When the network is synchronous, the round complexity of BA protocols is linear in the number of parties, and the round complexity of MPC protocols also depends linearly on the depth of the circuit to evaluate.

In this work, we provide round-efficient constructions for both primitives with optimal resilience: fixed-round and expected constant-round BA protocols, and an MPC protocol whose round complexity is independent of the circuit depth.

BibTeX Citation

@inproceedings{DeHiLi21,
    author       = {Giovanni Deligios and Martin Hirt and {Chen-Da} {Liu Zhang}},
    title        = {Round-Efficient Byzantine Agreement and Multi-Party Computation with Asynchronous Fallback},
    editor       = {Nissim, Kobbi and Waters, Brent},
    booktitle    = {Theory of Cryptography --- TCC 2021},
    pages        = {623--653},
    series       = {LNCS},
    volume       = {13042},
    year         = {2021},
    month        = {11},
    address      = {Cham},
    publisher    = {Springer International Publishing},
}