International Association for Cryptologic Research

International Association
for Cryptologic Research


Mingyuan Wang


Computational Hardness of Optimal Fair Computation: Beyond Minicrypt 📺
Hemanta K. Maji Mingyuan Wang
Secure multi-party computation allows mutually distrusting parties to compute securely over their private data. However, guaranteeing output delivery to honest parties when the adversarial parties may abort the protocol has been a challenging objective. As a representative task, this work considers two-party coin-tossing protocols with guaranteed output delivery, a.k.a., fair coin-tossing. In the information-theoretic plain model, as in two-party zero-sum games, one of the parties can force an output with certainty. In the commitment-hybrid, any $r$-message coin-tossing protocol is ${1/\sqrt r}$-unfair, i.e., the adversary can change the honest party's output distribution by $1/\sqrt r$ in the statistical distance. Moran, Naor, and Segev (TCC--2009) constructed the first $1/r$-unfair protocol in the oblivious transfer-hybrid. No further security improvement is possible because Cleve (STOC--1986) proved that $1/r$-unfairness is unavoidable. Therefore, Moran, Naor, and Segev's coin-tossing protocol is optimal. However, is oblivious transfer necessary for optimal fair coin-tossing? Maji and Wang (CRYPTO--2020) proved that any coin-tossing protocol using one-way functions in a black-box manner is at least $1/\sqrt r$-unfair. That is, optimal fair coin-tossing is impossible in Minicrypt. Our work focuses on tightly characterizing the hardness of computation assumption necessary and sufficient for optimal fair coin-tossing within Cryptomania, outside Minicrypt. Haitner, Makriyannia, Nissim, Omri, Shaltiel, and Silbak (FOCS--2018 and TCC--2018) proved that better than $1/\sqrt r$-unfairness, for any constant $r$, implies the existence of a key-agreement protocol. We prove that any coin-tossing protocol using public-key encryption (or, multi-round key agreement protocols) in a black-box manner must be $1/\sqrt r$-unfair. Next, our work entirely characterizes the additional power of secure function evaluation functionalities for optimal fair coin-tossing. We augment the model with an idealized secure function evaluation of $f$, \aka, the $f$-hybrid. If $f$ is complete, that is, oblivious transfer is possible in the $f$-hybrid, then optimal fair coin-tossing is also possible in the $f$-hybrid. On the other hand, if $f$ is not complete, then a coin-tossing protocol using public-key encryption in a black-box manner in the $f$-hybrid is at least $1/\sqrt r$-unfair.
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.
Constructing Locally Leakage-resilient Linear Secret-sharing Schemes 📺
Innovative side-channel attacks have repeatedly falsified the assumption that cryptographic implementations are opaque black-boxes. Therefore, it is essential to ensure cryptographic constructions' security even when information leaks via unforeseen avenues. One such fundamental cryptographic primitive is the secret-sharing schemes, which underlies nearly all threshold cryptography. Our understanding of the leakage-resilience of secret-sharing schemes is still in its preliminary stage. This work studies locally leakage-resilient linear secret-sharing schemes. An adversary can leak $m$ bits of arbitrary local leakage from each $n$ secret shares. However, in a locally leakage-resilient secret-sharing scheme, the leakage's joint distribution reveals no additional information about the secret. For every constant $m$, we prove that the Massey secret-sharing scheme corresponding to a random linear code of dimension $k$ (over sufficiently large prime fields) is locally leakage-resilient, where $k/n > 1/2$ is a constant. The previous best construction by Benhamouda, Degwekar, Ishai, Rabin (CRYPTO--2018) needed $k/n > 0.907$. A technical challenge arises because the number of all possible $m$-bit local leakage functions is exponentially larger than the number of random linear codes. Our technical innovation begins with identifying an appropriate pseudorandomness-inspired family of tests; passing them suffices to ensure leakage-resilience. We show that most linear codes pass all tests in this family. This Monte-Carlo construction of linear secret-sharing scheme that is locally leakage-resilient has applications to leakage-resilient secure computation. Furthermore, we highlight a crucial bottleneck for all the analytical approaches in this line of work. Benhamouda et al. introduced an analytical proxy to study the leakage-resilience of secret-sharing schemes; if the proxy is small, then the scheme is leakage-resilient. However, we present a one-bit local leakage function demonstrating that the converse is false, motivating the need for new analytically well-behaved functions that capture leakage-resilience more accurately. Technically, the analysis involves probabilistic and combinatorial techniques and (discrete) Fourier analysis. The family of new ``tests'' capturing local leakage functions, we believe, is of independent and broader interest.
Black-box use of One-way Functions is Useless for Optimal Fair Coin-Tossing 📺
Hemanta K. Maji Mingyuan Wang
A two-party fair coin-tossing protocol guarantees output delivery to the honest party even when the other party aborts during the protocol execution. Cleve (STOC--1986) demonstrated that a computationally bounded fail-stop adversary could alter the output distribution of the honest party by (roughly) $1/r$ (in the statistical distance) in an $r$-message coin-tossing protocol. An optimal fair coin-tossing protocol ensures that no adversary can alter the output distribution beyond $1/r$. In a seminal result, Moran, Naor, and Segev (TCC--2009) constructed the first optimal fair coin-tossing protocol using (unfair) oblivious transfer protocols. Whether the existence of oblivious transfer protocols is a necessary hardness of computation assumption for optimal fair coin-tossing remains among the most fundamental open problems in theoretical cryptography. The results of Impagliazzo and Luby (FOCS–1989) and Cleve and Impagliazzo (1993) prove that optimal fair coin-tossing implies the necessity of one-way functions' existence; a significantly weaker hardness of computation assumption compared to the existence of secure oblivious transfer protocols. However, the sufficiency of the existence of one-way functions is not known. Towards this research endeavor, our work proves a black-box separation of optimal fair coin-tossing from the existence of one-way functions. That is, the black-box use of one-way functions cannot enable optimal fair coin-tossing. Following the standard Impagliazzo and Rudich (STOC--1989) approach of proving black-box separations, our work considers any $r$-message fair coin-tossing protocol in the random oracle model where the parties have unbounded computational power. We demonstrate a fail-stop attack strategy for one of the parties to alter the honest party's output distribution by $1/\sqrt r$ by making polynomially-many additional queries to the random oracle. As a consequence, our result proves that the $r$-message coin-tossing protocol of Blum (COMPCON--1982) and Cleve (STOC--1986), which uses one-way functions in a black-box manner, is the best possible protocol because an adversary cannot change the honest party's output distribution by more than $1/\sqrt r$. Several previous works, for example, Dachman--Soled, Lindell, Mahmoody, and Malkin (TCC--2011), Haitner, Omri, and Zarosim (TCC--2013), and Dachman--Soled, Mahmoody, and Malkin (TCC--2014), made partial progress on proving this black-box separation assuming some restrictions on the coin-tossing protocol. Our work diverges significantly from these previous approaches to prove this black-box separation in its full generality. The starting point is the recently introduced potential-based inductive proof techniques for demonstrating large gaps in martingales in the information-theoretic plain model. Our technical contribution lies in identifying a global invariant of communication protocols in the random oracle model that enables the extension of this technique to the random oracle model.
Explicit Rate-1 Non-malleable Codes for Local Tampering 📺
This paper constructs high-rate non-malleable codes in the information-theoretic plain model against tampering functions with bounded locality. We consider $$\delta $$-local tampering functions; namely, each output bit of the tampering function is a function of (at most) $$\delta $$ input bits. This work presents the first explicit and efficient rate-1 non-malleable code for $$\delta $$-local tampering functions, where $$\delta =\xi \lg n$$ and $$\xi <1$$ is any positive constant. As a corollary, we construct the first explicit rate-1 non-malleable code against NC$$^0$$ tampering functions.Before our work, no explicit construction for a constant-rate non-malleable code was known even for the simplest 1-local tampering functions. Ball et al. (EUROCRYPT–2016), and Chattopadhyay and Li (STOC–2017) provided the first explicit non-malleable codes against $$\delta $$-local tampering functions. However, these constructions are rate-0 even when the tampering functions have 1-locality. In the CRS model, Faust et al. (EUROCRYPT–2014) constructed efficient rate-1 non-malleable codes for $$\delta = O(\log n)$$ local tampering functions.Our main result is a general compiler that bootstraps a rate-0 non-malleable code against leaky input and output local tampering functions to construct a rate-1 non-malleable code against $$\xi \lg n$$-local tampering functions, for any positive constant $$\xi < 1$$. Our explicit construction instantiates this compiler using an appropriate encoding by Ball et al. (EUROCRYPT–2016).