Abstract
Broadcast is an essential primitive for secure computation. We focus in this paper on optimal resilience (i.e., when the number of corrupted parties t is less than a third of the computing parties n), and with no setup or cryptographic assumptions.
While broadcast with worst case t rounds is impossible, it has been shown [Feldman and Micali STOC’88, Katz and Koo CRYPTO’06] how to construct protocols with expected constant number of rounds in the private channel model. However, those constructions have large communication complexity, specifically \({\mathcal {O}}(n^2L+n^6\log n)\) expected number of bits transmitted for broadcasting a message of length L. This leads to a significant communication blowup in secure computation protocols in this setting.
In this paper, we substantially improve the communication complexity of broadcast in constant expected time. Specifically, the expected communication complexity of our protocol is \({\mathcal {O}}(nL+n^4\log n)\). For messages of length \(L=\varOmega (n^3 \log n)\), our broadcast has no asymptotic overhead (up to expectation), as each party has to send or receive \({\mathcal {O}}(n^3 \log n)\) bits. We also consider parallel broadcast, where n parties wish to broadcast L bit messages in parallel. Our protocol has no asymptotic overhead for \(L=\varOmega (n^2\log n)\), which is a common communication pattern in perfectly secure MPC protocols. For instance, it is common that all parties share their inputs simultaneously at the same round, and verifiable secret sharing protocols require the dealer to broadcast a total of \({\mathcal {O}}(n^2\log n)\) bits.
As an independent interest, our broadcast is achieved by a packed verifiable secret sharing, a new notion that we introduce. We show a protocol that verifies \({\mathcal {O}}(n)\) secrets simultaneously with the same cost of verifying just a single secret. This improves by a factor of n the state-of-the-art.
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Notes
- 1.
Broadcast extension protocols handle long messages efficiently at the cost of a small number of single-bit broadcasts.
- 2.
Using broadcast extension of [46] we can bring the asymptotic cost to \({\mathcal {O}}(nL)+E({\mathcal {O}}(n^7\log {n}))\) bits. However, the minimum message size to achieve this \(L = \varOmega (n^6\log {n})\). This is prohibitively high even for \(n = 100\).
- 3.
In fact, in each round of the protocol, each party performs \({\mathcal {O}}(n)\) verifiable secret sharings (VSSs), i.e., it has to broadcast \({\mathcal {O}}(n^3 \log n)\) bits. In [2] it has been shown how to reduce it to \({\mathcal {O}}(1)\) VSSs per party, i.e., each party might have to broadcast \({\mathcal {O}}(n^2 \log n)\).
- 4.
We call a bivariate polynomial where the degree in x is 2t and in y is t, i.e., \(S(x,y)=\sum _{i=0}^{2t}\sum _{j=0}^{t}a_{i,j}x^iy^j\) as a (2t, t)-bivariate polynomial.
- 5.
The \(\textsf{happy}\) bits will be used later for Multi-Moderated VSS in Sect. 6.
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Acknowledgements
Gilad Asharov is sponsored by the Israel Science Foundation (grant No. 2439/20), by JPM Faculty Research Award, by the BIU Center for Research in Applied Cryptography and Cyber Security in conjunction with the Israel National Cyber Bureau in the Prime Minister’s Office, and by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 891234. Shravani Patil would like to acknowledge the support of DST National Mission on Interdisciplinary Cyber-Physical Systems (NM-ICPS) 2020-2025. Arpita Patra would like to acknowledge the support of DST National Mission on Interdisciplinary Cyber-Physical Systems (NM-ICPS) 2020-2025, Google India Faculty Award, and SERB MATRICS (Theoretical Sciences) Grant 2020-2023.
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Abraham, I., Asharov, G., Patil, S., Patra, A. (2022). Asymptotically Free Broadcast in Constant Expected Time via Packed VSS. In: Kiltz, E., Vaikuntanathan, V. (eds) Theory of Cryptography. TCC 2022. Lecture Notes in Computer Science, vol 13747. Springer, Cham. https://doi.org/10.1007/978-3-031-22318-1_14
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