International Association for Cryptologic Research

International Association
for Cryptologic Research


Hai H. Nguyen


Secure Non-interactive Simulation: Feasibility \& Rate 📺
A natural solution to increase the efficiency of secure computation will be to non-interactively and securely transform diverse inexpensive-to-generate correlated randomness, like, joint samples from noise sources, into correlations useful for secure computation protocols. Motivated by this general application for secure computation, our work introduces the notion of {\em secure non-interactive simulation} (\snis). Parties receive samples of correlated randomness, and they, without any interaction, securely convert them into samples from another correlated randomness. Our work presents a simulation-based security definition for \snis and initiates the study of the feasibility and efficiency of \snis. We also study \snis among fundamental correlated randomnesses like random samples from the binary symmetric and binary erasure channels, represented by \BSC and \BEC, respectively. We show the impossibility of interconversion between \BSC and \BEC samples. Next, we prove that a \snis of a $\BEC(\eps')$ sample (a \BEC with noise characteristic $\eps'$) from $\BEC(\eps)$ is feasible if and only if $(1-\eps') = (1-\eps)^k$, for some $k\in\NN$. In this context, we prove that all \snis constructions must be linear. Furthermore, if $(1-\eps') = (1-\eps)^k$, then the rate of simulating multiple independent $\BEC(\eps')$ samples is at most $1/k$, which is also achievable using (block) linear constructions. Finally, we show that a \snis of a $\BSC(\eps')$ sample from $\BSC(\eps)$ samples is feasible if and only if $(1-2\eps')=(1-2\eps)^k$, for some $k\in\NN$. Interestingly, there are linear as well as non-linear \snis constructions. When $(1-2\eps')=(1-2\eps)^k$, we prove that the rate of a {\em perfectly secure} \snis is at most $1/k$, which is achievable using linear and non-linear constructions. Our technical approach algebraizes the definition of \snis and proceeds via Fourier analysis. Our work develops general analysis methodologies for Boolean functions, explicitly incorporating cryptographic security constraints. Our work also proves strong forms of {\em statistical-to-perfect security} transformations: one can error-correct a statistically secure \snis to make it perfectly secure. We show a connection of our research with {\em homogeneous Boolean functions} and {\em distance-invariant codes}, which may be of independent interest.
Leakage-resilience of the Shamir Secret-sharing Scheme against Physical-bit Leakages 📺
Efficient Reed-Solomon code reconstruction algorithms, for example, by Guruswami and Wooters (STOC--2016), translate into local leakage attacks on Shamir secret-sharing schemes over characteristic-2 fields. However, Benhamouda, Degwekar, Ishai, and Rabin (CRYPTO--2018) showed that the Shamir secret sharing scheme over prime-fields is leakage resilient to one-bit local leakage if the reconstruction threshold is roughly 0.87 times the total number of parties. In several application scenarios, like secure multi-party multiplication, the reconstruction threshold must be at most half the number of parties. Furthermore, the number of leakage bits that the Shamir secret sharing scheme is resilient to is also unclear. Towards this objective, we study the Shamir secret-sharing scheme's leakage-resilience over a prime-field $F$. The parties' secret-shares, which are elements in the finite field $F$, are naturally represented as $\lambda$-bit binary strings representing the elements $\{0,1,\dotsc,p-1\}$. In our leakage model, the adversary can independently probe $m$ bit-locations from each secret share. The inspiration for considering this leakage model stems from the impact that the study of oblivious transfer combiners had on general correlation extraction algorithms, and the significant influence of protecting circuits from probing attacks has on leakage-resilient secure computation. Consider arbitrary reconstruction threshold $k\geq 2$, physical bit-leakage parameter $m\geq 1$, and the number of parties $n\geq 1$. We prove that Shamir's secret-sharing scheme with random evaluation places is leakage-resilient with high probability when the order of the field $F$ is sufficiently large; ignoring polylogarithmic factors, one needs to ensure that $\log \abs F \geq n/k$. Our result, excluding polylogarithmic factors, states that Shamir's scheme is secure as long as the total amount of leakage $m\cdot n$ is less than the entropy $k\cdot\lambda$ introduced by the Shamir secret-sharing scheme. Note that our result holds even for small constant values of the reconstruction threshold $k$, which is essential to several application scenarios. To complement this positive result, we present a physical-bit leakage attack for $m=1$ physical bit-leakage from $n=k$ secret shares and any prime-field $F$ satisfying $\abs F=1\mod k$. In particular, there are (roughly) $\abs F^{n-k+1}$ such vulnerable choices for the $n$-tuple of evaluation places. We lower-bound the advantage of this attack for small values of the reconstruction threshold, like $k=2$ and $k=3$, and any $\abs F=1\mod k$. In general, we present a formula calculating our attack's advantage for every $k$ as $\abs F\rightarrow\infty.$ Technically, our positive result relies on Fourier analysis, analytic properties of proper rank-$r$ generalized arithmetic progressions, and B\'ezout's theorem to bound the number of solutions to an equation over finite fields. The analysis of our attack relies on determining the ``discrepancy'' of the Irwin-Hall distribution. A probability distribution's discrepancy is a new property of distributions that our work introduces, which is of potential independent interest.
Secure Computation Using Leaky Correlations (Asymptotically Optimal Constructions)
Most secure computation protocols can be effortlessly adapted to offload a significant fraction of their computationally and cryptographically expensive components to an offline phase so that the parties can run a fast online phase and perform their intended computation securely. During this offline phase, parties generate private shares of a sample generated from a particular joint distribution, referred to as the correlation. These shares, however, are susceptible to leakage attacks by adversarial parties, which can compromise the security of the secure computation protocol. The objective, therefore, is to preserve the security of the honest party despite the leakage performed by the adversary on her share.Prior solutions, starting with n-bit leaky shares, either used 4 messages or enabled the secure computation of only sub-linear size circuits. Our work presents the first 2-message secure computation protocol for 2-party functionalities that have $$\varTheta (n)$$ circuit-size despite $$\varTheta (n)$$-bits of leakage, a qualitatively optimal result. We compose a suitable 2-message secure computation protocol in parallel with our new 2-message correlation extractor. Correlation extractors, introduced by Ishai, Kushilevitz, Ostrovsky, and Sahai (FOCS–2009) as a natural generalization of privacy amplification and randomness extraction, recover “fresh” correlations from the leaky ones, which are subsequently used by other cryptographic protocols. We construct the first 2-message correlation extractor that produces $$\varTheta (n)$$-bit fresh correlations even after $$\varTheta (n)$$-bit leakage.Our principal technical contribution, which is of potential independent interest, is the construction of a family of multiplication-friendly linear secret sharing schemes that is simultaneously a family of small-bias distributions. We construct this family by randomly “twisting then permuting” appropriate Algebraic Geometry codes over constant-size fields.