A Unified Approach to Constructing Black-box UC Protocols in Trusted Setup Models

We present a unified framework for obtaining black-box constructions of Universal Composable (UC) protocol in trusted setup models. Our result is analogous to the unified framework of Lin, Pass, and Venkitasubramaniam [STOC\u2709, Asiacrypt\u2712] that, however, only yields non-black-box constructions of UC protocols. Our unified framework shows that to obtain black-box constructions of UC protocols, it suffices to implement a special purpose commitment scheme that is, in particular, concurrently extractable using a given trusted setup. Using our framework, we improve black-box constructions in the common reference string and tamper-proof hardware token models by weakening the underlying computational and setup assumptions

Improved Black-Box Constructions of Composable Secure Computation

We close the gap between black-box and non-black-box constructions of $\mathit{composable}$ secure multiparty computation in the plain model under the $\mathit{minimal}$ assumption of semi-honest oblivious transfer. The notion of protocol composition we target is $\mathit{angel\text{-}based}$ security, or more precisely, security with super-polynomial helpers. In this notion, both the simulator and the adversary are given access to an oracle called an $\mathit{angel}$ that can perform some predefined super-polynomial time task. Angel-based security maintains the attractive properties of the universal composition framework while providing meaningful security guarantees in complex environments without having to trust anyone. Angel-based security can be achieved using non-black-box constructions in $\max(R_{\mathsf{OT}},\widetilde{O}(\log n))$ rounds where $R_{\mathsf{OT}}$ is the round-complexity of the semi-honest oblivious transfer. However, currently, the best known $\mathit{black\text{-}box}$ constructions under the same assumption require $\max(R_{\mathsf{OT}},\widetilde{O}(\log^2 n))$ rounds. If $R_{\mathsf{OT}}$ is a constant, the gap between non-black-box and black-box constructions can be a multiplicative factor $\log n$. We close this gap by presenting a $\max(R_{\mathsf{OT}},\widetilde{O}(\log n))$-round black-box construction. We achieve this result by constructing constant-round 1-1 CCA-secure commitments assuming only black-box access to one-way functions

Classical Cryptographic Protocols in a Quantum World

Cryptographic protocols, such as protocols for secure function evaluation (SFE), have played a crucial role in the development of modern cryptography. The extensive theory of these protocols, however, deals almost exclusively with classical attackers. If we accept that quantum information processing is the most realistic model of physically feasible computation, then we must ask: what classical protocols remain secure against quantum attackers? Our main contribution is showing the existence of classical two-party protocols for the secure evaluation of any polynomial-time function under reasonable computational assumptions (for example, it suffices that the learning with errors problem be hard for quantum polynomial time). Our result shows that the basic two-party feasibility picture from classical cryptography remains unchanged in a quantum world.Comment: Full version of an old paper in Crypto'11. Invited to IJQI. This is authors' copy with different formattin

Universally Composable Quantum Multi-Party Computation

The Universal Composability model (UC) by Canetti (FOCS 2001) allows for secure composition of arbitrary protocols. We present a quantum version of the UC model which enjoys the same compositionality guarantees. We prove that in this model statistically secure oblivious transfer protocols can be constructed from commitments. Furthermore, we show that every statistically classically UC secure protocol is also statistically quantum UC secure. Such implications are not known for other quantum security definitions. As a corollary, we get that quantum UC secure protocols for general multi-party computation can be constructed from commitments

Composable Adaptive Secure Protocols without Setup under Polytime Assumptions

All previous constructions of general multiparty computation protocols that are secure against adaptive corruptions in the concurrent setting either require some form of setup or non-standard assumptions. In this paper we provide the first general construction of secure multi-party computation protocol without any setup that guarantees composable security in the presence of an adaptive adversary based on standard polynomial-time assumptions. We prove security under the notion of ``UC with super-polynomial helpers\u27\u27 introduced by Canetti et al. (FOCS 2010), which is closed under universal composition and implies ``super-polynomial-time simulation\u27\u27. Moreover, our construction relies on the underlying cryptographic primitives in a black-box manner. Next, we revisit the zero-one law for two-party secure functions evaluation initiated by the work of Maji, Prabhakaran and Rosulek (CRYPTO 2010). According to this law, every two-party functionality is either trivial (meaning, such functionalities can be reduced to any other functionality) or complete (meaning, any other functionality can be reduced to these functionalities) in the Universal Composability (UC) framework. As our second contribution, assuming the existence of a simulatable public-key encryption scheme, we establish a zero-one law in the adaptive setting. Our result implies that every two-party non-reactive functionality is either trivial or complete in the UC framework in the presence of adaptive, malicious adversaries

A Framework for Efficient Adaptively Secure Composable Oblivious Transfer in the ROM

Oblivious Transfer (OT) is a fundamental cryptographic protocol that finds a number of applications, in particular, as an essential building block for two-party and multi-party computation. We construct a round-optimal (2 rounds) universally composable (UC) protocol for oblivious transfer secure against active adaptive adversaries from any OW-CPA secure public-key encryption scheme with certain properties in the random oracle model (ROM). In terms of computation, our protocol only requires the generation of a public/secret-key pair, two encryption operations and one decryption operation, apart from a few calls to the random oracle. In~terms of communication, our protocol only requires the transfer of one public-key, two ciphertexts, and three binary strings of roughly the same size as the message. Next, we show how to instantiate our construction under the low noise LPN, McEliece, QC-MDPC, LWE, and CDH assumptions. Our instantiations based on the low noise LPN, McEliece, and QC-MDPC assumptions are the first UC-secure OT protocols based on coding assumptions to achieve: 1) adaptive security, 2) optimal round complexity, 3) low communication and computational complexities. Previous results in this setting only achieved static security and used costly cut-and-choose techniques.Our instantiation based on CDH achieves adaptive security at the small cost of communicating only two more group elements as compared to the gap-DH based Simplest OT protocol of Chou and Orlandi (Latincrypt 15), which only achieves static security in the ROM

On Black-Box Complexity of Universally Composable Security in the CRS model

In this work, we study the intrinsic complexity of black-box Universally Composable (UC) secure computation based on general assumptions. We present a thorough study in various corruption modelings while focusing on achieving security in the common reference string (CRS) model. Our results involve the following: 1. Static UC secure computation. Designing the first static UC oblivious transfer protocol based on public-key encryption and stand-alone semi-honest oblivious transfer. As a corollary we obtain the first black-box constructions of UC secure computation assuming only two-round semi-honest oblivious transfer. 2. One-sided UC secure computation. Designing adaptive UC two-party computation with single corruptions assuming public-key encryption with oblivious ciphertext generation. 3. Adaptive UC secure computation. Designing adaptively secure UC commitment scheme assuming only public-key encryption with oblivious ciphertext generation. As a corollary we obtain the first black-box constructions of adaptive UC secure computation assuming only (trapdoor) simulatable public-key encryption (as well as a variety of concrete assumptions). We remark that such a result was not known even under non-black-box constructions

Applying Secure Multi-party Computation in Practice

In this work, we present solutions for technical difficulties in deploying secure multi-party computation in real-world applications. We will first give a brief overview of the current state of the art, bring out several shortcomings and address them. The main contribution of this work is an end-to-end process description of deploying secure multi-party computation for the first large-scale registry-based statistical study on linked databases. Involving large stakeholders like government institutions introduces also some non-technical requirements like signing contracts and negotiating with the Data Protection Agency

SnarkPack: Practical SNARK Aggregation

Zero-knowledge SNARKs (zk-SNARKs) are non-interactive proof systems with short and efficiently verifiable proofs that do not reveal anything more than the correctness of the statement. zk-SNARKs are widely used in decentralised systems to address privacy and scalability concerns. One of the main applications is the blockchain, were SNARKs are used to prove computations with private inputs and reduce on-chain footprint verification and transaction sizes. A major drawback of such proof systems in practice is the requirement to run a trusted setup for the public parameters. Moreover, these parameters set an upper bound to the sizeof the computations or statement to be proven, which results in new scalability problems. We design and implement SnarkPack, a new argument that further reduces the size of SNARK proofs by means of aggregation. Our goal is to provide an off-the-shelf solution that is practical in the following sense: (1) it is compatible with existing deployed SNARK systems, (2) it does not require any extra trusted setup. SnarkPack is designed to work with Groth16 scheme and has logarithmic size proofs and a verifier that runs in logarithmic time in the number of proofs to be aggregated. Most importantly, SnarkPack reuses the public parameters from Groth16 system. SnarkPack can aggregate 8192 proofs in 8.7s and verify them in 163ms, yielding a verification mechanism that is exponentially faster than batching and previous solutions in the field.SnarkPack can be deployed in blockchain applications that rely on many SNARK proofs such as Proof-of-Space or roll-up solutions