Abstract
We consider the graph-theoretic problem of removing (few) nodes from a directed acyclic graph in order to reduce its depth. While this problem is intractable in the general case, we provide a variety of algorithms in the case where the graph is that of a circuit of fan-in (at most) two, and explore applications of these algorithms to secure multiparty computation with low communication. Over the past few years, a paradigm for low-communication secure multiparty computation has found success based on decomposing a circuit into low-depth “chunks”. This approach was however previously limited to circuits with a “layered” structure. Our graph-theoretic approach extends this paradigm to all circuits. In particular, we obtain the following contributions:
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Fractionally linear-communication MPC in the correlated randomness model. We provide an N-party protocol for computing an n-input, m-output \(\mathbb {F}\)-arithmetic circuit with s internal gates (over any basis of binary gates) with communication complexity \((\frac{2}{3}s + n + m)\cdot N\cdot \log |\mathbb {F}|\), which can be improved to \(((1+\epsilon )\cdot \frac{2}{5}s+n+m)\cdot N\cdot \log |\mathbb {F}|\) (at the cost of increasing the computational overhead from a small constant factor to a large one). Previously, comparable protocols either used more than \(s\cdot N\cdot \log |\mathbb {F}|\) bits of communication, required super-polynomial computation, were restricted to layered circuits, or tolerated a sub-optimal corruption threshold.
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Sublinear-Communication MPC. Assuming the existence of N-party Homomorphic Secret Sharing for logarithmic depth circuits (respectively doubly logarithmic depth circuits), we show there exists sublinear-communication secure N-party computation for all \(\log ^{1+o(1)}\)-depth (resp. \((\log \log )^{1+o(1)}\)-depth) circuits. Previously, this result was limited to \((\mathcal {O}(\log ))\)-depth (resp. \((\mathcal {O}(\log \log ))\)-depth) circuits, or to circuits with a specific structure (e.g. layered).
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The \(\boldsymbol{{N\atopwithdelims ()1}}\)-OT complexity of MPC. We introduce the “\(N\atopwithdelims ()1\)-OT complexity of MPC” of a function f, denoted \(C_N(f)\), as the number of oracle calls required to securely compute f in the \(N\atopwithdelims ()1\)-OT hybrid model. We establish the following upper bound: for every \(N\ge 2\), \(C_N(f) \le (1+g(N))\cdot \frac{2 |f|}{5}\), where g(N) is an explicit vanishing function.
We also obtain additional contributions to reducing the amount of bootstrapping for fully homomorphic encryption, and to other types of sublinear-communication MPC protocols such as those based on correlated symmetric private information retrieval.
P. Meyer—Most of this work was done while P. Meyer was a PhD student jointly at Reichman University, Herzliya, ISRAEL and Université Paris Cité, CNRS, IRIF, Paris, FRANCE.
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Notes
- 1.
If we lift this restriction, Beaver, Feigenbaum, Kilian, and Rogoway [BFKR91] showed that any function from \(\{0,1\} ^n\) to \(\{0,1\} \) can be securely computed in the presence of up to t corruptions by \(N = \mathcal {O}(t \cdot n/ \log n)\) computationally unbounded parties (which the protocol indeed requires to use an exponential amount of computation) using \(\textsf{poly}(n + N)\) communication. In addition, the protocol of [IKM+13], which we state as a result for securely computing functions with a polynomial-size look-up table, can be applied to general functions if the parties are computationally unbounded (and have access to a doubly exponential amount of correlated randomness).
- 2.
These HSS schemes support bounded-integer computation, which translates to low-communication secure computation over arbitrary fields using hybrid encryption.
- 3.
Note that this class is somewhat related to logarithmic-depth branching programs.
- 4.
The complexity we provide here is for when the depth is at most \((\log \log s)/4\), not \((\log \log s-\log \log \log s)\); this is done in order to simplify the expression and absorb some negligible terms.
- 5.
A synchronous circuit is a layered circuit whose input gates are all in the first layer.
- 6.
We note there is some inconsistency in the literature in how the vertex cover problem is extended to directed graphs.
- 7.
- 8.
In order to simplify the expressions of M, we assume none of the input gates are also outputs.
- 9.
- 10.
Beware that the literature is inconsistent on whether the length of a path should be counted in vertices or edges.
- 11.
We refer to Sect. 2.2 for what we mean by “output nodes of a DAG”.
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Acknowledgments
We thank Mikaël Rabie and Elette Boyle for helpful discussions and pointers to respectively degenerate graphs and memory-hard functions.
Geoffroy Couteau was supported by the French Agence Nationale de la Recherche (ANR), under grant ANR-20-CE39-0001 (project SCENE), and by the France 2030 ANR Project ANR22-PECY-003 SecureCompute. Pierre Meyer was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreements numbers 852952 (HSS) and 803096 (SPEC).
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Charbit, P., Couteau, G., Meyer, P., Naserasr, R. (2025). A Note on Low-Communication Secure Multiparty Computation via Circuit Depth-Reduction. In: Boyle, E., Mahmoody, M. (eds) Theory of Cryptography. TCC 2024. Lecture Notes in Computer Science, vol 15367. Springer, Cham. https://doi.org/10.1007/978-3-031-78023-3_6
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