Abstract
A classic approach to MPC uses preprocessed multiplication triples to evaluate arbitrary Boolean circuits. If the target circuit features conditional branching, e.g. as the result of a IF program statement, then triples are wasted: one triple is consumed per \(\mathtt {AND}\) gate, even if the output of the gate is entirely discarded by the circuit’s conditional behavior.
In this work, we show that multiplication triples can be re-used across conditional branches. For a circuit with b branches, each having n \(\mathtt {AND}\) gates, we need only a total of n triples, rather than the typically required \(b\cdot n\). Because preprocessing triples is often the most expensive step in protocols that use them, this significantly improves performance.
Prior work similarly amortized oblivious transfers across branches in the classic GMW protocol (Heath et al., Asiacrypt 2020, [HKP20]). In addition to demonstrating conditional improvements are possible for a different class of protocols, we also concretely improve over [HKP20]: their maximum improvement is bounded by the topology of the circuit. Our protocol yields improvement independent of topology: we need triples proportional to the size of the program’s longest execution path, regardless of the structure of the program branches.
We implemented our approach in C++. Our experiments show that we significantly improve over a “naïve” protocol and over prior work: for a circuit with 16 branches and in terms of total communication, we improved over naïve by \(12\times \) and over [HKP20] by an average of \(2.6\times \).
Our protocol is secure against the semi-honest corruption of \(p-1\) parties.
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Notes
- 1.
We emphasize communication improvement because multiplication triples are often constructed from communication-expensive oblivious transfers (OTs). Silent OT [BCG+19] is an exciting new primitive that largely removes the communication overhead of OT. The trade off is increased computation: the classic OT extension of [IKNP03], which uses relatively little computation, is still preferable to Silent OT in most settings. This said, even if we were to use Silent OT, our improvement would be beneficial: we greatly reduce the needed number of OTs and hence would significantly reduce Silent OT computation overhead.
- 2.
\(\mathtt {MOTIF}\) does not natively support \(\mathtt {NOT}\) gates because they would break the invariant: \(\mathtt {NOT}\) maps \([\![0 ]\!]\) to \([\![1 ]\!]\). \(\mathtt {NOT}\) gates can be implemented by \(\mathtt {XOR}\) gates together with a distinguished wire that holds \([\![1 ]\!]\) on the active branch and \([\![0 ]\!]\) on the inactive branch.
- 3.
We cannot support \(\varPi _{\mathtt {Base}}\) as a black-box because it is possible to implement a protocol that securely handles netlists, but that is insecure when given masked triples. For example, the parties could out-of-band check the validity of triples and reveal all inputs if the triples are found invalid. Therefore, \(\varPi _{\mathtt {Base}}\) is a white-box protocol.
- 4.
One caveat is that broadcasts used to reconstruct the circuit’s outputs must \(\mathtt {XOR}\) to the correct output value. The simulator must arrange the simulated output broadcasts such that they appropriately add up. This is typical in MPC proofs and is easy to set up.
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Acknowledgements
This work was supported in part by NSF award #1909769, by a Facebook research award, and by Georgia Tech’s IISP cybersecurity seed funding (CSF) award.
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Heath, D., Kolesnikov, V., Peceny, S. (2021). Masked Triples. In: Garay, J.A. (eds) Public-Key Cryptography – PKC 2021. PKC 2021. Lecture Notes in Computer Science(), vol 12711. Springer, Cham. https://doi.org/10.1007/978-3-030-75248-4_12
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