International Association for Cryptologic Research

International Association
for Cryptologic Research


Satrajit Ghosh


Threshold Private Set Intersection with Better Communication Complexity
Satrajit Ghosh Mark Simkin
Given $\ell$ parties with sets $X_1, \dots, X_\ell$ of size $n$, we would like to securely compute the intersection $\cap_{i=1}^\ell X_i$, if it is larger than $n-t$ for some threshold $t$, without revealing any other additional information. It has previously been shown (Ghosh and Simkin, Crypto 2019) that this function can be securely computed with a communication complexity that only depends on $t$ and in particular does not depend on $n$. For small values of $t$, this results in protocols that have a communication complexity that is sublinear in the size of the inputs. Current protocols either rely on fully homomorphic encryption or have an at least quadratic dependency on the parameter $t$. In this work, we construct protocols with a quasilinear dependency on $t$ from simple assumptions like additively homomorphic encryption and oblivious transfer. All existing approaches, including ours, rely on protocols for computing a single bit, which indicates whether the intersection is larger than $n-t$ without actually computing it. Our key technical contribution, which may be of independent interest, takes any such protocol with secret shared outputs and communication complexity $\mathcal{O}(\lambda \ell \mathsf{poly}(t))$, where $\lambda$ is the security parameter, and transforms it into a protocol with communication complexity $\mathcal{O}(\lambda^2 \ell t \mathsf{polylog}(t))$.
Improved Primitives for MPC over Mixed Arithmetic-Binary Circuits 📺
This work introduces novel techniques to improve the translation between arithmetic and binary data types in multi-party computation. To this end, we introduce a new approach to performing these conversions, using what we call \emph{extended doubly-authenticated bits} (edaBits), which correspond to shared integers in the arithmetic domain whose bit decomposition is shared in the binary domain. These can be used to considerably increase the efficiency of non-linear operations such as truncation, secure comparison and bit-decomposition. Our eDaBits are similar to the \emph{daBits} technique introduced by Rotaru et al.~(Indocrypt 2019). However, our main observations are that (1) applications that benefit from daBits can also benefit from edaBits in the same way, and (2) we can generate edaBits directly in a much more efficeint way than computing them directly from a set of DaBits. Technically, the second contribution is much more challenging, and involves a novel cut and choose technique that may be of independent interest, and requires taking advantage of natural tamper-resilient properties of binary circuits that occur in our construction to obtain the best level of efficiency. Finally, we show how our eDaBits can be applied to efficiently implement various non-linear protocols of interest, and we thoroughly analyze their correctness for both signed and unsigned integers. The results of this work can be applied to any corruption threshold, although they seem best suited to dishonest majority protocols such as SPDZ. We implement and benchmark our constructions, and experimentally verify that our technique yield a substantial increase in effiency. Our eDaBits save in communication by a factor that lies between $2$ and $170$ for secure comparisons with respect to a purely arithmetic approach, and between $2$ and $60$ with respect to using daBits. Improvements in throughput per second are more subdued but still as high as a factor of $47$. We also apply our novel machinery to the tasks of biometric matching and convolutional neural networks, obtaining a noticeable improvement as well.
An Algebraic Approach to Maliciously Secure Private Set Intersection 📺
Satrajit Ghosh Tobias Nilges
Private set intersection (PSI) is an important area of research and has been the focus of many works over the past decades. It describes the problem of finding an intersection between the input sets of at least two parties without revealing anything about the input sets apart from their intersection.In this paper, we present a new approach to compute the intersection between sets based on a primitive called Oblivious Linear Function Evaluation (OLE). On an abstract level, we use this primitive to efficiently add two polynomials in a randomized way while preserving the roots of the added polynomials. Setting the roots of the input polynomials to be the elements of the input sets, this directly yields an intersection protocol with optimal asymptotic communication complexity $$O(m\kappa )$$. We highlight that the protocol is information-theoretically secure against a malicious adversary assuming OLE.We also present a natural generalization of the 2-party protocol for the fully malicious multi-party case. Our protocol does away with expensive (homomorphic) threshold encryption and zero-knowledge proofs. Instead, we use simple combinatorial techniques to ensure the security. As a result we get a UC-secure protocol with asymptotically optimal communication complexity $$O((n^2+nm)\kappa )$$, where n is the number of parties, m is the set size and $$\kappa $$ is the security parameter. Apart from yielding an asymptotic improvement over previous works, our protocols are also conceptually simple and require only simple field arithmetic. Along the way we develop techniques that might be of independent interest.
The Communication Complexity of Threshold Private Set Intersection 📺
Satrajit Ghosh Mark Simkin
Threshold private set intersection enables Alice and Bob who hold sets $$S_{\mathsf {A}}$$ and $$S_{\mathsf {B}}$$ of size n to compute the intersection $$S_{\mathsf {A}} \cap S_{\mathsf {B}} $$ if the sets do not differ by more than some threshold parameter $$t$$ . In this work, we investigate the communication complexity of this problem and we establish the first upper and lower bounds. We show that any protocol has to have a communication complexity of $$\varOmega (t)$$ . We show that an almost matching upper bound of $$\tilde{\mathcal {O}}(t)$$ can be obtained via fully homomorphic encryption. We present a computationally more efficient protocol based on weaker assumptions, namely additively homomorphic encryption, with a communication complexity of $$\tilde{\mathcal {O}}(t ^2)$$ . For applications like biometric authentication, where a given fingerprint has to have a large intersection with a fingerprint from a database, our protocols may result in significant communication savings.Prior to this work, all previous protocols had a communication complexity of $$\varOmega (n)$$ . Our protocols are the first ones with communication complexities that mainly depend on the threshold parameter $$t$$ and only logarithmically on the set size n.