International Association
for Cryptologic Research

Oblivious Transfer (OT) is at the heart of secure computation and is a foundation for many applications in cryptography. Over two decades of work have led to extremely efficient protocols for efficiently evaluating OT instances in the preprocessing model, through a paradigm called OT extension. A few OT instances generated in an offline phase can be used to perform many OTs in an online phase efficiently, i.e., with very low communication and computational overheads. Specifically, traditional OT extension uses a small number of “base” OTs, generated using any black-box OT protocol, and convert them into many OT instances using only lightweight symmetric-key primitives. Recently, a new paradigm of OT with a public-key setup has emerged, which replaces the base OTs with a non-interactive setup: Using only the public key of the other party, two parties can efficiently compute a virtually unbounded number of OT instances on-the-fly. In this paper, we put forth a novel framework for OT extension with a public-key setup (henceforth, “public-key OT”) and concretely efficient instantiations. Implementations of our framework are 30–100× faster when compared to the previous state-of-the-art public-key OT protocols, and remain competitive even when compared to OT protocols that do not offer a public-key setup. Additionally, our instantiations result in the first public-key schemes with plausible post-quantum security. In summary, this paper contributes: - QuietOT: A framework for OT extension with public-key setup that uses fast, symmetric-key primitives to generate OT instances following a one-time public-key setup, and offering additional features such as precomputability. - A public-key setup for QuietOT from the RingLWE assumption, resulting in the first post-quantum construction of OT extension with a public-key setup. - An optimized, open-source implementation of our construction that can generate up to 1M OT extensions per second on commodity hardware. In contrast, the state-of-the-art public-key OT protocol is limited to at most 20K OTs per second. - The first formal treatment of the security of OT with a public-key setup in a multi-party setting, which addresses several subtleties that were overlooked in prior work.

We propose and study a new privacy definition, termed Probably Approximately Correct (PAC) Privacy. PAC Privacy characterizes the information-theoretic hardness to recover sensitive data given arbitrary information disclosure/leakage during/after any processing. Unlike the classic cryptographic definition and Differential Privacy (DP), which consider the adversarial input-independent worst case}, PAC Privacy is a simulatable metric that quantifies the instance-based impossibility of inference. A fully automatic analysis and proof generation framework is proposed: security parameters can be produced with arbitrarily high confidence via Monte-Carlo simulation for any black-box data processing oracle. This appealing automation property enables analysis of complicated data processing, where the worst-case proof in the classic privacy regime could be loose or even intractable. Moreover, we show that the produced PAC Privacy guarantees enjoy simple composition bounds and the automatic analysis framework can be implemented in an online fashion to analyze the composite PAC Privacy loss even under correlated randomness. On the utility side, the magnitude of (necessary) perturbation required in PAC Privacy is not lower bounded by Theta(\sqrt{d}) for a d-dimensional release but could be O(1) for many practical data processing tasks, which is in contrast to the input-independent worst-case information-theoretic lower bound. Example applications of PAC Privacy are included with comparisons to existing works.

The round complexity of Byzantine Broadcast (BB) has been a central question in distributed systems and cryptography. In the honest majority setting, expected constant round protocols have been known for decades even in the presence of a strongly adaptive adversary. In the corrupt majority setting, however, no protocol with sublinear round complexity is known, even when the adversary is allowed to {\it strongly adaptively} corrupt only 51\% of the players, and even under reasonable setup or cryptographic assumptions. Recall that a strongly adaptive adversary can examine what original message an honest player would have wanted to send in some round, adaptively corrupt the player in the same round and make it send a completely different message instead. In this paper, we are the first to construct a BB protocol with sublinear round complexity in the corrupt majority setting. Specifically, assuming the existence of time-lock puzzles with suitable hardness parameters and other standard cryptographic assumptions, we show how to achieve BB in $(\frac{n}{n-f})^2 \cdot \poly\log \lambda$ rounds with $1-\negl(\lambda)$ probability, where $n$ denotes the total number of players, $f$ denotes the maximum number of corrupt players, and $\lambda$ is the security parameter. Our protocol completes in polylogarithmically many rounds even when 99\% of the players can be corrupt.