## CryptoDB

### Saikrishna Badrinarayanan

#### Affiliation: University of California, Los Angeles

#### Publications

**Year**

**Venue**

**Title**

2020

EUROCRYPT

Statistical ZAP Arguments
📺
Abstract

Dwork and Naor (FOCS'00) first introduced and constructed two message public coin witness indistinguishable proofs (ZAPs) for NP based on trapdoor permutations. Since then, ZAPs have also been obtained based on the decisional linear assumption on bilinear maps, and indistinguishability obfuscation, and have proven extremely useful in the design of several cryptographic primitives.
However, all known constructions of two-message public coin (or even publicly verifiable) proof systems only guarantee witness indistinguishability against computationally bounded verifiers.
In this paper, we construct the first public coin two message witness indistinguishable (WI) arguments for NP with {\em statistical} privacy, assuming quasi-polynomial hardness of the learning with errors (LWE) assumption. We also show that the same protocol has a super-polynomial simulator (SPS), which yields the first public-coin SPS statistical zero knowledge argument.
Prior to this, there were no known constructions of two-message publicly verifiable WI protocols under lattice assumptions, even satisfying the weaker notion of computational witness indistinguishability.

2019

EUROCRYPT

Revisiting Non-Malleable Secret Sharing
📺
Abstract

A threshold secret sharing scheme (with threshold t) allows a dealer to share a secret among a set of parties such that any group of t or more parties can recover the secret and no group of at most
$$t-1$$
t-1 parties learn any information about the secret. A non-malleable threshold secret sharing scheme, introduced in the recent work of Goyal and Kumar (STOC’18), additionally protects a threshold secret sharing scheme when its shares are subject to tampering attacks. Specifically, it guarantees that the reconstructed secret from the tampered shares is either the original secret or something that is unrelated to the original secret.In this work, we continue the study of threshold non-malleable secret sharing against the class of tampering functions that tamper each share independently. We focus on achieving greater efficiency and guaranteeing a stronger security property. We obtain the following results:Rate Improvement. We give the first construction of a threshold non-malleable secret sharing scheme that has rate
$$> 0$$
>0. Specifically, for every
$$n,t \ge 4$$
n,t≥4, we give a construction of a t-out-of-n non-malleable secret sharing scheme with rate
$$\varTheta (\frac{1}{t\log ^2 n})$$
Θ(1tlog2n). In the prior constructions, the rate was
$$\varTheta (\frac{1}{n\log m})$$
Θ(1nlogm) where m is the length of the secret and thus, the rate tends to 0 as
$$m \rightarrow \infty $$
m→∞. Furthermore, we also optimize the parameters of our construction and give a concretely efficient scheme.Multiple Tampering. We give the first construction of a threshold non-malleable secret sharing scheme secure in the stronger setting of bounded tampering wherein the shares are tampered by multiple (but bounded in number) possibly different tampering functions. The rate of such a scheme is
$$\varTheta (\frac{1}{k^3t\log ^2 n})$$
Θ(1k3tlog2n) where k is an apriori bound on the number of tamperings. We complement this positive result by proving that it is impossible to have a threshold non-malleable secret sharing scheme that is secure in the presence of an apriori unbounded number of tamperings.General Access Structures. We extend our results beyond threshold secret sharing and give constructions of rate-efficient, non-malleable secret sharing schemes for more general monotone access structures that are secure against multiple (bounded) tampering attacks.

2019

TCC

From FE Combiners to Secure MPC and Back
Abstract

Cryptographic combiners allow one to combine many candidates for a cryptographic primitive, possibly based on different computational assumptions, into another candidate with the guarantee that the resulting candidate is secure as long as at least one of the original candidates is secure. While the original motivation of cryptographic combiners was to reduce trust on existing candidates, in this work, we study a rather surprising implication of combiners to constructing secure multiparty computation protocols. Specifically, we initiate the study of functional encryption combiners and show its connection to secure multiparty computation.Functional encryption (FE) has incredible applications towards computing on encrypted data. However, constructing the most general form of this primitive has remained elusive. Although some candidate constructions exist, they rely on nonstandard assumptions, and thus, their security has been questioned. An FE combiner attempts to make use of these candidates while minimizing the trust placed on any individual FE candidate. Informally, an FE combiner takes in a set of FE candidates and outputs a secure FE scheme if at least one of the candidates is secure.Another fundamental area in cryptography is secure multi-party computation (MPC), which has been extensively studied for several decades. In this work, we initiate a formal study of the relationship between functional encryption (FE) combiners and secure multi-party computation (MPC). In particular, we show implications in both directions between these primitives. As a consequence of these implications, we obtain the following main results.
A two-round semi-honest MPC protocol in the plain model secure against up to $$n-1$$ corruptions with communication complexity proportional only to the depth of the circuit being computed assuming learning with errors (LWE). Prior two round protocols based on standard assumptions that achieved this communication complexity required trust assumptions, namely, a common reference string.A functional encryption combiner based on pseudorandom generators (PRGs) in $$\mathsf {NC}^1$$. This is a weak assumption as such PRGs are implied by many concrete intractability problems commonly used in cryptography, such as ones related to factoring, discrete logarithm, and lattice problems [11]. Previous constructions of FE combiners, implicit in [7], were known only from LWE. Using this result, we build a universal construction of functional encryption: an explicit construction of functional encryption based only on the assumptions that functional encryption exists and PRGs in $$\mathsf {NC}^1$$.

2019

ASIACRYPT

Output Compression, MPC, and iO for Turing Machines
Abstract

In this work, we study the fascinating notion of output-compressing randomized encodings for Turing Machines, in a shared randomness model. In this model, the encoder and decoder have access to a shared random string, and the efficiency requirement is, the size of the encoding must be independent of the running time and output length of the Turing Machine on the given input, while the length of the shared random string is allowed to grow with the length of the output. We show how to construct output-compressing randomized encodings for Turing machines in the shared randomness model, assuming iO for circuits and any assumption in the set
$$\{$$
LWE, DDH, N
$$^{th}$$
Residuosity
$$\}$$
.We then show interesting implications of the above result to basic feasibility questions in the areas of secure multiparty computation (MPC) and indistinguishability obfuscation (iO): 1.Compact MPC for Turing Machines in the Random Oracle Model. In the context of MPC, we consider the following basic feasibility question: does there exist a malicious-secure MPC protocol for Turing Machines whose communication complexity is independent of the running time and output length of the Turing Machine when executed on the combined inputs of all parties? We call such a protocol as a compact MPC protocol. Hubácek and Wichs [HW15] showed via an incompressibility argument, that, even for the restricted setting of circuits, it is impossible to construct a malicious secure two party computation protocol in the plain model where the communication complexity is independent of the output length. In this work, we show how to evade this impossibility by compiling any (non-compact) MPC protocol in the plain model to a compact MPC protocol for Turing Machines in the Random Oracle Model, assuming output-compressing randomized encodings in the shared randomness model.2.Succinct iO for Turing Machines in the Shared Randomness Model. In all existing constructions of iO for Turing Machines, the size of the obfuscated program grows with a bound on the input length. In this work, we show how to construct an iO scheme for Turing Machines in the shared randomness model where the size of the obfuscated program is independent of a bound on the input length, assuming iO for circuits and any assumption in the set
$$\{$$
LWE, DDH, N
$$^{th}$$
Residuosity
$$\}$$
.

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.

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

TCC

Upgrading to Functional Encryption
Abstract

The notion of Functional Encryption (FE) has recently emerged as a strong primitive with several exciting applications. In this work, we initiate the study of the following question: Can existing public key encryption schemes be “upgraded” to Functional Encryption schemes without changing their public keys or the encryption algorithm? We call a public-key encryption scheme with this property to be FE-compatible. Indeed, assuming ideal obfuscation, it is easy to see that every CCA-secure public-key encryption scheme is FE-compatible. Despite the recent success in using indistinguishability obfuscation to replace ideal obfuscation for many applications, we show that this phenomenon most likely will not apply here. We show that assuming fully homomorphic encryption and the learning with errors (LWE) assumption, there exists a CCA-secure encryption scheme that is provably not FE-compatible. We also show that a large class of natural CCA-secure encryption schemes proven secure in the random oracle model are not FE-compatible in the random oracle model.Nevertheless, we identify a key structure that, if present, is sufficient to provide FE-compatibility. Specifically, we show that assuming sub-exponentially secure iO and sub-exponentially secure one way functions, there exists a class of public key encryption schemes which we call Special-CCA secure encryption schemes that are in fact, FE-compatible. In particular, each of the following popular CCA secure encryption schemes (some of which existed even before the notion of FE was introduced) fall into the class of Special-CCA secure encryption schemes and are thus FE-compatible:1.[CHK04] when instantiated with the IBE scheme of [BB04].2.[CHK04] when instantiated with any Hierarchical IBE scheme.3.[PW08] when instantiated with any Lossy Trapdoor Function.

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

ASIACRYPT

#### Coauthors

- Shweta Agrawal (1)
- Shashank Agrawal (1)
- Prabhanjan Ananth (1)
- Rex Fernando (2)
- Sanjam Garg (1)
- Vipul Goyal (3)
- Divya Gupta (1)
- Yuval Ishai (1)
- Abhishek Jain (5)
- Aayush Jain (3)
- Yael Tauman Kalai (1)
- Dakshita Khurana (5)
- Venkata Koppula (1)
- Abishek Kumarasubramanian (1)
- Nathan Manohar (1)
- Eric Miles (2)
- Rafail Ostrovsky (3)
- Manoj Prabhakaran (1)
- Amit Sahai (12)
- Akshayaram Srinivasan (1)
- Ivan Visconti (3)
- Akshay Wadia (1)
- Brent Waters (2)
- Mark Zhandry (2)