## CryptoDB

### Abhishek Jain

#### Publications

**Year**

**Venue**

**Title**

2021

EUROCRYPT

Non-Interactive Zero Knowledge from Sub-exponential DDH
Abstract

We provide the first constructions of non-interactive zero-knowledge and Zap arguments for NP based on the sub-exponential hardness of Decisional Diffie-Hellman against polynomial time adversaries (without use of groups with pairings).
Central to our results, and of independent interest, is a new notion of interactive trapdoor hashing protocols.

2021

EUROCRYPT

Order-C Secure Multiparty Computation for Highly Repetitive Circuits
Abstract

Running secure multiparty computation (MPC) protocols with hundreds or thousands of players would allow leveraging large volunteer networks (such as blockchains and Tor) and help justify honest majority assumptions. However, most existing protocols have at least a linear (multiplicative) dependence on the number of players, making scaling difficult. Known protocols with asymptotic efficiency independent of the number of parties (excluding additive factors) require expensive circuit transformations that induce large overheads.
We observe that the circuits used in many important applications of MPC such as training algorithms used to create machine learning models have a highly repetitive structure. We formalize this class of circuits and propose an MPC protocol that achieves O(|C|) total complexity for this class. We implement our protocol and show that it is practical and outperforms O(n|C|) protocols for modest numbers of players.

2021

EUROCRYPT

Unbounded Multi-Party Computation from Learning with Errors
Abstract

We consider the problem of round-optimal *unbounded MPC*: in the first round, parties publish a message that depends only on their input. In the second round, any subset of parties can jointly and securely compute any function $f$ over their inputs in a single round of broadcast. We do not impose any a priori bound on the number of parties nor on the size of the functions that can be computed.
Our main result is a semi-honest two-round protocol for unbounded MPC in the plain model from the hardness of the standard learning with errors (LWE) problem. Prior work in the same setting assumes the hardness of problems over bilinear maps. Thus, our protocol is the first example of unbounded MPC that is post-quantum secure.
The central ingredient of our protocol is a new scheme of attribute-based secure function evaluation (AB-SFE) with *public decryption*. Our construction combines techniques from the realm of homomorphic commitments with delegation of lattice basis. We believe that such a scheme may find further applications in the future.

2021

CRYPTO

Fluid MPC: Secure Multiparty Computation with Dynamic Participants
📺
Abstract

Existing approaches to secure multiparty computation (MPC) require all participants to commit to the entire duration of the protocol. As interest in MPC continues to grow, it is inevitable that there will be a desire to use it to evaluate increasingly complex functionalities, resulting in computations spanning several hours or days.
Such scenarios call for a *dynamic* participation model for MPC where participants have the flexibility to go offline as needed and (re)join when they have available computational resources. Such a model would also democratize access to privacy-preserving computation by facilitating an ``MPC-as-a-service'' paradigm --- the deployment of MPC in volunteer-operated networks (such as blockchains, where dynamism is inherent) that perform computation on behalf of clients.
In this work, we initiate the study of *fluid MPC*, where parties can dynamically join and leave the computation. The minimum commitment required from each participant is referred to as *fluidity*, measured in the number of rounds of communication that it must stay online. Our contributions are threefold:
- We provide a formal treatment of fluid MPC, exploring various possible modeling choices.
- We construct information-theoretic fluid MPC protocols in the honest-majority setting. Our protocols achieve *maximal fluidity*, meaning that a party can exit the computation after receiving and sending messages in one round.
- We implement our protocol and test it in multiple network settings.

2021

CRYPTO

Non-Interactive Batch Arguments for NP from Standard Assumptions
📺
Abstract

We study the problem of designing *non-interactive batch arguments* for NP. Such an argument system allows an efficient prover to prove multiple $\npol$ statements, with size much smaller than the combined witness length.
We provide the first construction of such an argument system for NP in the common reference string model based on standard cryptographic assumptions. Prior works either require non-falsifiable assumptions (or the random oracle model) or can only support private verification.
At the heart of our result is a new *dual mode* interactive batch argument system for NP. We show how to apply the correlation-intractability framework for Fiat-Shamir -- that has primarily been applied to proof systems -- to such interactive arguments.

2020

TCC

Round Optimal Secure Multiparty Computation from Minimal Assumptions
📺
Abstract

We construct a four round secure multiparty computation (MPC) protocol in the plain model that achieves security against any dishonest majority. The security of our protocol relies only on the existence of four round oblivious transfer. This culminates the long line of research on constructing round-efficient MPC from minimal assumptions (at least w.r.t. black-box simulation).

2020

EUROCRYPT

Statistical Zaps and New Oblivious Transfer Protocols
📺
Abstract

We study the problem of achieving statistical privacy in interactive proof systems and oblivious transfer -- two of the most well studied two-party protocols -- when limited rounds of interaction are available.
-- Statistical Zaps: We give the first construction of statistical Zaps, namely, two-round statistical witness-indistinguishable (WI) protocols with a public-coin verifier. Our construction achieves computational soundness based on the quasi-polynomial hardness of learning with errors assumption.
-- Three-Round Statistical Receiver-Private Oblivious Transfer: We give the first construction of a three-round oblivious transfer (OT) protocol -- in the plain model -- that achieves statistical privacy for receivers and computational privacy for senders against malicious adversaries, based on polynomial-time assumptions. The round-complexity of our protocol is optimal.
We obtain our first result by devising a public-coin approach to compress sigma protocols, without relying on trusted setup. To obtain our second result, we devise a general framework via a new notion of statistical hash commitments that may be of independent interest.

2020

TCC

Multi-key Fully-Homomorphic Encryption in the Plain Model
📺
Abstract

The notion of multi-key fully homomorphic encryption (multi-key FHE) [Lopez-Alt, Tromer, Vaikuntanathan, STOC'12] was proposed as a generalization of fully homomorphic encryption to the multiparty setting. In a multi-key FHE scheme for $n$ parties, each party can individually choose a key pair and use it to encrypt its own private input. Given n ciphertexts computed in this manner, the parties can homomorphically evaluate a circuit C over them to obtain a new ciphertext containing the output of C, which can then be decrypted via a decryption protocol. The key efficiency property is that the size of the (evaluated) ciphertext is independent of the size of the circuit.
Multi-key FHE with one-round decryption [Mukherjee and Wichs, Eurocrypt'16], has found several powerful applications in cryptography over the past few years. However, an important drawback of all such known schemes is that they require a trusted setup.
In this work, we address the problem of constructing multi-key FHE in the plain model. We obtain the following results:
- A multi-key FHE scheme with one-round decryption based on the hardness of learning with errors (LWE), ring LWE, and decisional small polynomial ratio (DSPR) problems.
- A variant of multi-key FHE where we relax the decryption algorithm to be non-compact -- i.e., where the decryption complexity can depend on the size of C -- based on the hardness of LWE. We call this variant multi-homomorphic encryption (MHE). We observe that MHE is already sufficient for some of the applications of multi-key FHE.

2020

ASIACRYPT

Towards Efficiency-Preserving Round Compression in MPC: Do fewer rounds mean more computation?
📺
Abstract

Reducing the rounds of interaction in secure multiparty computation (MPC) protocols has been the topic of study of many works. One popular approach to reduce rounds is to construct {\em round compression compilers}. A round compression compiler is one that takes a highly interactive protocol and transforms it into a protocol with far fewer rounds. The design of round compression compilers has traditionally focused on preserving the security properties of the underlying protocol and in particular, not much attention has been given towards preserving their computational and communication efficiency. Indeed, the recent round compression compilers that yield round-optimal MPC protocols incur large computational and communication overhead.
In this work, we initiate the study of {\em efficiency-preserving} round compression compilers, i.e. compilers that translate the efficiency benefits of the underlying highly interactive protocols to the fewer round setting. Focusing on the honest majority setting (with near-optimal corruption threshold $\frac{1}{2} - \varepsilon$, for any $\varepsilon > 0$), we devise a new compiler that yields two round (i.e., round optimal) semi-honest MPC with similar communication efficiency as the underlying (arbitrary round) protocol. By applying our compiler on the most efficient known MPC protocols, we obtain a two-round semi-honest protocol based on one-way functions, with total communication (and per-party computation) cost $\widetilde{O}(s+n^4)$ -- a significant improvement over prior two-round protocols with cost $\widetilde{O}(n^\tau s+n^{\tau+1}d)$, where $\tau\geq 2$, $s$ is the size of the circuit computing the function and $d$ the corresponding depth. Our result can also be extended to handle malicious adversaries, either using stronger assumptions in the public key infrastructure (PKI) model, or in the plain model using an extra round.
An artifact of our approach is that the resultant protocol is ``unbalanced'' in the amount of computation performed by different parties. We give evidence that this is {\em necessary} in our setting. Our impossibility result makes novel use of the ``MPC-in-the-head" paradigm which has typically been used to demonstrate feasibility results.

2019

EUROCRYPT

Two Round Information-Theoretic MPC with Malicious Security
📺
Abstract

We provide the first constructions of two round information-theoretic (IT) secure multiparty computation (MPC) protocols in the plain model that tolerate any $$t<n/2$$t<n/2 malicious corruptions. Our protocols satisfy the strongest achievable standard notions of security in two rounds in different communication models.Previously, IT-MPC protocols in the plain model either required a larger number of rounds, or a smaller minority of corruptions.

2019

EUROCRYPT

Founding Secure Computation on Blockchains
📺
Abstract

We study the foundations of secure computation in the blockchain-hybrid model, where a blockchain – modeled as a global functionality – is available as an Oracle to all the participants of a cryptographic protocol. We demonstrate both destructive and constructive applications of blockchains:We show that classical rewinding-based simulation techniques used in many security proofs fail against blockchain-active adversaries that have read and post access to a global blockchain. In particular, we show that zero-knowledge (ZK) proofs with black-box simulation are impossible against blockchain-active adversaries.Nevertheless, we show that achieving security against blockchain-active adversaries is possible if the honest parties are also blockchain active. We construct an $$\omega (1)$$-round ZK protocol with black-box simulation. We show that this result is tight by proving the impossibility of constant-round ZK with black-box simulation.Finally, we demonstrate a novel application of blockchains to overcome the known impossibility results for concurrent secure computation in the plain model. We construct a concurrent self-composable secure computation protocol for general functionalities in the blockchain-hybrid model based on standard cryptographic assumptions.
We develop a suite of techniques for constructing secure protocols in the blockchain-hybrid model that we hope will find applications to future research in this area.

2019

TCC

Interactive Non-malleable Codes
Abstract

Non-malleable codes (NMC) introduced by Dziembowski et al. [ICS’10] allow one to encode “passive” data in such a manner that when a codeword is tampered, the original data either remains completely intact or is essentially destroyed.In this work, we initiate the study of interactive non-malleable codes (INMCs) that allow for encoding “active communication” rather than passive data. An INMC allows two parties to engage in an interactive protocol such that an adversary who is able to tamper with the protocol messages either leaves the original transcript intact (i.e., the parties are able to reconstruct the original transcript) or the transcript is completely destroyed and replaced with an unrelated one.We formalize a tampering model for interactive protocols and put forward the notion of INMCs. Since constructing INMCs for general adversaries is impossible (as in the case of non-malleable codes), we construct INMCs for several specific classes of tampering functions. These include bounded state, split state, and fragmented sliding window tampering functions. We also obtain lower bounds for threshold tampering functions via a connection to interactive coding. All of our results are unconditional.

2019

ASIACRYPT

The Broadcast Message Complexity of Secure Multiparty Computation
Abstract

We study the broadcast message complexity of secure multiparty computation (MPC), namely, the total number of messages that are required for securely computing any functionality in the broadcast model of communication.MPC protocols are traditionally designed in the simultaneous broadcast model, where each round consists of every party broadcasting a message to the other parties. We show that this method of communication is sub-optimal; specifically, by eliminating simultaneity, it is, in fact, possible to reduce the broadcast message complexity of MPC.More specifically, we establish tight lower and upper bounds on the broadcast message complexity of n-party MPC for every $$t<n$$ corruption threshold, both in the plain model as well as common setup models. For example, our results show that the optimal broadcast message complexity of semi-honest MPC can be much lower than 2n, but necessarily requires at least three rounds of communication. We also extend our results to the malicious setting in setup models.

2019

ASIACRYPT

UC-Secure Multiparty Computation from One-Way Functions Using Stateless Tokens
Abstract

We revisit the problem of universally composable (UC) secure multiparty computation in the stateless hardware token model.
We construct a three round multi-party computation protocol for general functions based on one-way functions where each party sends two tokens to every other party. Relaxing to the two-party case, we also construct a two round protocol based on one-way functions where each party sends a single token to the other party, and at the end of the protocol, both parties learn the output.One of the key components in the above constructions is a new two-round oblivious transfer protocol based on one-way functions using only one token, which can be reused an unbounded polynomial number of times.
All prior constructions required either stronger complexity assumptions, or larger number of rounds, or a larger number of tokens.

2019

ASIACRYPT

Public-Key Function-Private Hidden Vector Encryption (and More)
Abstract

We construct public-key function-private predicate encryption for the “small superset functionality,” recently introduced by Beullens and Wee (PKC 2019). This functionality captures several important classes of predicates:Point functions. For point function predicates, our construction is equivalent to public-key function-private anonymous identity-based encryption.Conjunctions. If the predicate computes a conjunction, our construction is a public-key function-private hidden vector encryption scheme. This addresses an open problem posed by Boneh, Raghunathan, and Segev (ASIACRYPT 2013).d-CNFs and read-once conjunctions of d-disjunctions for constant-size d.
Our construction extends the group-based obfuscation schemes of Bishop et al. (CRYPTO 2018), Beullens and Wee (PKC 2019), and Bartusek et al. (EUROCRYPT 2019) to the setting of public-key function-private predicate encryption. We achieve an average-case notion of function privacy, which guarantees that a decryption key
$$\mathsf {sk} _f$$
reveals nothing about f as long as f is drawn from a distribution with sufficient entropy. We formalize this security notion as a generalization of the (enhanced) real-or-random function privacy definition of Boneh, Raghunathan, and Segev (CRYPTO 2013). Our construction relies on bilinear groups, and we prove security in the generic bilinear group model.

2018

CRYPTO

Round-Optimal Secure Multiparty Computation with Honest Majority
📺
Abstract

We study the exact round complexity of secure multiparty computation (MPC) in the honest majority setting. We construct several round-optimaln-party protocols, tolerating any $$t<\frac{n}{2}$$ corruptions.
1.Security with abort: We give the first construction of two round MPC for general functions that achieves security with abort against malicious adversaries in the plain model. The security of our protocol only relies on one-way functions.2.Guaranteed output delivery: We also construct protocols that achieve security with guaranteed output delivery: (i) Against fail-stop adversaries, we construct two round MPC either in the (bare) public-key infrastructure model with no additional assumptions, or in the plain model assuming two-round semi-honest oblivious transfer. In three rounds, however, we can achieve security assuming only one-way functions. (ii) Against malicious adversaries, we construct three round MPC in the plain model, assuming public-key encryption and Zaps.Previously, such protocols were only known based on specific learning assumptions and required the use of common reference strings.
All of our results are obtained via general compilers that may be of independent interest.

2018

CRYPTO

Promise Zero Knowledge and Its Applications to Round Optimal MPC
📺
Abstract

We devise a new partitioned simulation technique for MPC where the simulator uses different strategies for simulating the view of aborting adversaries and non-aborting adversaries. The protagonist of this technique is a new notion of promise zero knowledge (ZK) where the ZK property only holds against non-aborting verifiers. We show how to realize promise ZK in three rounds in the simultaneous-message model assuming polynomially hard DDH (or QR or N$$^{th}$$-Residuosity).We demonstrate the following applications of our new technique:We construct the first round-optimal (i.e., four round) MPC protocol for general functions based on polynomially hard DDH (or QR or N$$^{th}$$-Residuosity).We further show how to overcome the four-round barrier for MPC by constructing a three-round protocol for “list coin-tossing” – a slight relaxation of coin-tossing that suffices for most conceivable applications – based on polynomially hard DDH (or QR or N$$^{th}$$-Residuosity). This result generalizes to randomized input-less functionalities.
Previously, four round MPC protocols required sub-exponential-time hardness assumptions and no multi-party three-round protocols were known for any relaxed security notions with polynomial-time simulation against malicious adversaries.In order to base security on polynomial-time standard assumptions, we also rely upon a leveled rewinding security technique that can be viewed as a polynomial-time alternative to leveled complexity leveraging for achieving “non-malleability” across different primitives.

2018

ASIACRYPT

Non-interactive Secure Computation from One-Way Functions
Abstract

The notion of non-interactive secure computation (NISC) first introduced in the work of Ishai et al. [EUROCRYPT 2011] studies the following problem: Suppose a receiver R wishes to publish an encryption of her secret input y so that any sender S with input x can then send a message m that reveals f(x, y) to R (for some function f). Here, m can be viewed as an encryption of f(x, y) that can be decrypted by R. NISC requires security against both malicious senders and receivers, and also requires the receiver’s message to be reusable across multiple computations (w.r.t. a fixed input of the receiver).All previous solutions to this problem necessarily rely upon OT (or specific number-theoretic assumptions) even in the common reference string model or the random oracle model or to achieve weaker notions of security such as super-polynomial-time simulation.In this work, we construct a NISC protocol based on the minimal assumption of one way functions, in the stateless hardware token model. Our construction achieves UC security and requires a single token sent by the receiver to the sender.

2017

CRYPTO

2015

EPRINT

2014

CRYPTO

2012

EUROCRYPT

2012

CRYPTO

2010

EPRINT

On the Round Complexity of Covert Computation
Abstract

In STOC'05, von Ahn, Hopper and Langford introduced the notion of covert computation. In covert computation, a party runs a secure computation protocol over a covert (or steganographic) channel without knowing if the other parties are participating as well or not. At the end of the protocol, if all parties participated in the protocol and if the function output is "favorable" to all parties, then the output is revealed (along with the fact that everyone participated). All covert computation protocols known so far require a large polynomial number of rounds. In this work, we first study the question of the round complexity of covert computation and obtain the following results:
1) There does not exist a constant round covert computation protocol with respect to black box simulation even for the case of two parties. (In comparison, such protocols are known even for the multi-party case if there is no covertness requirement.)
2) By relying on the two slot non-black-box simulation technique of Pass (STOC'04) and techniques from cryptography in NC^0 (Applebaum et al, FOCS'04), we obtain a construction of a constant round covert multi-party computation protocol.
Put together, the above adds one more example to the growing list of tasks for which non-black-box simulation techniques (introduced in the work of Barak in FOCS'01) are necessary.
Finally, we study the problem of covert multi-party computation in the setting where the parties only have point to point (covert) communication channels. We observe that our covert computation protocol for the broadcast channel inherits, from the protocol of Pass, the property of secure composition in the bounded concurrent setting. Then, as an application of this protocol, somewhat surprisingly we show the existence of covert multi-party computation with point to point channels (assuming that the number of parties is a constant).

#### Program Committees

- Eurocrypt 2020
- TCC 2019
- PKC 2017
- Eurocrypt 2016

#### Coauthors

- Shweta Agrawal (1)
- Prabhanjan Ananth (14)
- Gilad Asharov (1)
- Saikrishna Badrinarayanan (5)
- James Bartusek (1)
- Allison Bishop (2)
- Nir Bitansky (2)
- Elette Boyle (1)
- Ran Canetti (3)
- Brent Carmer (1)
- Angelo De Caro (1)
- David Cash (2)
- Arka Rai Choudhuri (8)
- Michele Ciampi (1)
- Aloni Cohen (1)
- Dana Dachman-Soled (1)
- Yevgeniy Dodis (1)
- Nils Fleischhacker (2)
- Sanjam Garg (6)
- Aarushi Goel (6)
- Shafi Goldwasser (2)
- S. Dov Gordon (1)
- Vipul Goyal (22)
- Matthew Green (1)
- Divya Gupta (3)
- Shai Halevi (1)
- Vincenzo Iovino (1)
- Yuval Ishai (1)
- Zhengzhong Jin (6)
- Yael Tauman Kalai (4)
- Gabriel Kaptchuk (2)
- Jonathan Katz (1)
- Dakshita Khurana (3)
- Eike Kiltz (2)
- Ilan Komargodski (1)
- Venkata Koppula (1)
- Lucas Kowalczyk (2)
- Stephan Krenn (1)
- Tancrède Lepoint (1)
- Feng-Hao Liu (1)
- Adriana López-Alt (2)
- Fermi Ma (1)
- Giulio Malavolta (3)
- Tal Malkin (1)
- Alex J. Malozemoff (1)
- Tal Moran (1)
- Adam O'Neill (1)
- Rafail Ostrovsky (6)
- Omer Paneth (3)
- Anat Paskin-Cherniavsky (1)
- Giuseppe Persiano (1)
- Krzysztof Pietrzak (5)
- Manoj Prabhakaran (1)
- Slava Radune (1)
- Mariana Raykova (1)
- Silas Richelson (2)
- Ron D. Rothblum (1)
- Amit Sahai (15)
- Grant Schoenebeck (1)
- Elaine Shi (1)
- Aris Tentes (2)
- Eran Tromer (1)
- Vinod Vaikuntanathan (2)
- Daniele Venturi (2)
- Ivan Visconti (4)
- Brent Waters (1)
- Daniel Wichs (3)
- Eylon Yogev (1)
- Hong-Sheng Zhou (1)