### Attaining GOD Beyond Honest Majority With Friends and Foes

##### Abstract

In the classical notion of multiparty computation (MPC), an honest party learning private inputs of others, either as a part of protocol specification or due to a malicious party's unspecified messages, is not considered a potential breach. Several works in the literature exploit this seemingly minor loophole to achieve the strongest security of guaranteed output delivery via a trusted third party, which nullifies the purpose of MPC. Alon et al. (CRYPTO 2020) presented the notion of Friends and Foes ($\mathtt{FaF}$) security, which accounts for such undesired leakage towards honest parties by modelling them as semi-honest (friends) who do not collude with malicious parties (foes). With real-world applications in mind, it's more realistic to assume parties are semi-honest rather than completely honest, hence it is imperative to design efficient protocols conforming to the $\mathtt{FaF}$ security model. Our contributions are not only motivated by the practical viewpoint, but also consider the theoretical aspects of $\mathtt{FaF}$ security. We prove the necessity of semi-honest oblivious transfer for $\mathtt{FaF}$-secure protocols with optimal resiliency. On the practical side, we present QuadSquad, a ring-based 4PC protocol, which achieves fairness and GOD in the $\mathtt{FaF}$ model, with an optimal corruption of $1$ malicious and $1$ semi-honest party. QuadSquad is, to the best of our knowledge, the first practically efficient $\mathtt{FaF}$ secure protocol with optimal resiliency. Its performance is comparable to the state-of-the-art dishonest majority protocols while improving the security guarantee from abort to fairness and GOD. Further, QuadSquad elevates the security by tackling a stronger adversarial model over the state-of-the-art honest-majority protocols, while offering a comparable performance for the input-dependent computation. We corroborate these claims by benchmarking the performance of QuadSquad. We also consider the application of liquidity matching that deals with highly sensitive financial transaction data, where $\mathtt{FaF}$ security is apt. We design a range of $\mathtt{FaF}$ secure building blocks to securely realize liquidity matching as well as other popular applications such as privacy-preserving machine learning (PPML). Inclusion of these blocks makes QuadSquad a comprehensive framework.

Available format(s)
Category
Cryptographic protocols
Publication info
A major revision of an IACR publication in ASIACRYPT 2022
Keywords
Friends and Foes Multiparty Computation Oblivious Transfer
Contact author(s)
ahegde @ cs jhu edu
kotis @ iisc ac in
varshak @ iisc ac in
shravanip @ iisc ac in
arpita @ iisc ac in
protikpaul @ iisc ac in
History
2022-09-14: approved
See all versions
Short URL
https://ia.cr/2022/1207

CC BY

BibTeX

@misc{cryptoeprint:2022/1207,
author = {Aditya Hegde and Nishat Koti and Varsha Bhat Kukkala and Shravani Patil and Arpita Patra and Protik Paul},
title = {Attaining GOD Beyond Honest Majority With Friends and Foes},
howpublished = {Cryptology ePrint Archive, Paper 2022/1207},
year = {2022},
note = {\url{https://eprint.iacr.org/2022/1207}},
url = {https://eprint.iacr.org/2022/1207}
}

Note: In order to protect the privacy of readers, eprint.iacr.org does not use cookies or embedded third party content.