## CryptoDB

### Lawrence Roy

#### Publications

**Year**

**Venue**

**Title**

2024

EUROCRYPT

Succinct Homomorphic Secret Sharing
Abstract

This work introduces homomorphic secret sharing (HSS) with succinct share size. In HSS, private inputs are shared between parties, who can then homomorphically evaluate a function on their shares, obtaining a share of the function output. In succinct HSS, a portion of the inputs can be distributed using shares whose size is sublinear in the number of such inputs. The parties can then locally evaluate a function f on the shares, with the restriction that f must be linear in the succinctly shared inputs.
We construct succinct, two-party HSS for branching programs, based on either the decisional composite residuosity assumption, a DDH-like assumption in class groups, or learning with errors with a superpolynomial modulus-to-noise ratio. We then give several applications of succinct HSS, which were only previously known using fully homomorphic encryption, or stronger tools:
1. Succinct vector oblivious linear evaluation (VOLE): Two parties can obtain secret shares of a long, arbitrary vector x, multiplied by a scalar ∆, with communication sublinear in the size of the vector.
2. Batch, multi-party distributed point functions: A protocol for distributing a batch of secret, random point functions among N parties, for any polynomial N, with communication sublinear in the number of DPFs.
3. Sublinear MPC for any number of parties: Two new constructions of MPC with sublinear communication complexity, with N parties for any polynomial N: (1) For general layered Boolean circuits of size s, with communication O(N s/log log s), and (2) For layered, sufficiently wide Boolean circuits, with communication O(N s/log s).

2023

PKC

A Universally Composable PAKE with Zero Communication Cost (And Why It Shouldn’t Be Considered UC-Secure)
Abstract

A Password-Authenticated Key Exchange (PAKE) protocol allows two parties to agree upon a cryptographic key, when the only information shared in advance is a low-entropy password. The standard security notion for PAKE (Canetti et al., Eurocrypt 2005) is in the Universally Composable (UC) framework. We show that unlike most UC security notions, UC PAKE does not imply correctness. While Canetti et al. seems to have implicitly noticed this issue, it has yet to be explicitly identified by the PAKE literature. We present a comprehensive study of correctness in UC PAKE:
1. We show that TrivialPAKE, a no-message protocol that does not satisfy correctness, is a UC PAKE;
2. We propose nine approaches to guaranteeing correctness in the UC security notion of PAKE, and show that seven of them are equivalent, whereas the other two are unachievable;
3. We prove that a direct solution, namely changing the UC PAKE functionality to incorporate correctness, is impossible;
4. Finally, we show how to naturally incorporate correctness by changing the model — we view PAKE as a three-party protocol, with the man-in-the-middle adversary as the third party.
In this way, we hope to shed some light on the very nature of UC-security in the man-in-the-middle setting.

2023

CRYPTO

Two-Round Stateless Deterministic Two-Party Schnorr Signatures From Pseudorandom Correlation Functions
Abstract

Schnorr signatures are a popular choice due to their simplicity, provable security, and linear structure that enables relatively easy threshold signing protocols. The deterministic variant of Schnorr (where the nonce is derived in a stateless manner using a PRF from the message and a long term secret) is more popular in practice since it mitigates the threats of a faulty or poor randomness generator (which in Schnorr leads to catastrophic breaches of security). Unfortunately, threshold protocols for the deterministic variant of Schnorr have so far been quite inefficient, as they make non black-box use of the PRF involved in the nonce generation.
In this paper, we present the first two-party threshold protocol for the determistic variant of Schnorr signatures, which only makes black-box use of the underlying cryptographic algorithms.
We present a protocol from general assumptions which achieves covert security and a protocol that achieves full active security under factoring-like assumptions. Our protocols make crucial use of recent advances within the field of pseudorandom correlation functions (PCFs).
As an additional benefit, only two-rounds are needed to perform distributed signing in our protocol, connecting our work to a recent line of research on the trade-offs between round complexity and computational assumptions for threshold Schnorr signatures.

2023

CRYPTO

Publicly Verifiable Zero-Knowledge and Post-Quantum Signatures From VOLE-in-the-Head
Abstract

We present a new method for transforming zero-knowledge protocols in the designated verifier setting into public-coin protocols, which can be made non-interactive and publicly verifiable.
Our transformation applies to a large class of ZK protocols based on oblivious transfer.
In particular, we show that it can be applied to recent, fast protocols based on vector oblivious linear evaluation (VOLE), with a technique we call VOLE-in-the-head, upgrading these protocols to support public verifiability.
Our resulting ZK protocols have linear proof size, and are simpler, smaller and faster than related approaches based on MPC-in-the-head.
To build VOLE-in-the-head while supporting both binary circuits and large finite fields, we develop several new technical tools.
One of these is a new proof of security for the SoftSpokenOT protocol (Crypto 2022), which generalizes it to produce certain types of VOLE correlations over large fields.
Secondly, we present a new ZK protocol that is tailored to take advantage of this form of VOLE, which leads to a publicly verifiable VOLE-in-the-head protocol with only 2x more communication than the best, designated-verifier VOLE-based protocols.
We analyze the soundness of our approach when made non-interactive using the Fiat-Shamir transform, using round-by-round soundness.
As an application of the resulting NIZK, we present FAEST, a post-quantum signature scheme based on AES.
FAEST is the first AES-based signature scheme to be smaller than SPHINCS+, with signature sizes between 5.6 and 6.6kB at the 128-bit security level.
Compared with the smallest version of SPHINCS+ (7.9kB), FAEST verification is slower, but the signing times are between 8x and 40x faster.

2022

CRYPTO

SoftSpokenOT: Quieter OT Extension From Small-Field Silent VOLE in the Minicrypt Model
Abstract

Given a small number of base oblivious transfers (OTs), how does one generate a large number of extended OTs as efficiently as possible? The answer has long been the seminal work of IKNP (Ishai et al., Crypto 2003) and the family of protocols it inspired, which only use Minicrypt assumptions. Recently, Boyle et al. (Crypto 2019) proposed the Silent-OT technique that improves on IKNP, but at the cost of a much stronger, non-Minicrypt assumption: the learning parity with noise (LPN) assumption. We present SoftSpokenOT, the first OT extension to improve on IKNP's communication cost in the Minicrypt model. While IKNP requires security parameter $\lambda$ bits of communication for each OT, SoftSpokenOT only needs $\lambda / k$ bits, for any $k$, at the expense of requiring $2^{k-1} / k$ times the computation. For small values of $k$, this tradeoff is favorable since IKNP-style protocols are network-bound. We implemented SoftSpokenOT and found that our protocol gives almost a $5 \times$ speedup over IKNP in the LAN setting.
Our technique is based on a novel silent protocol for vector oblivious linear evaluation (VOLE) over polynomial-sized fields. We created a framework to build maliciously secure 1-of-N OT extension from this VOLE, revisiting and improving the existing work for each step. Along the way, we found several flaws in the existing work, including a practical attack against the consistency check of Patra et al. (NDSS 2017).

2021

CRYPTO

Three Halves Make a Whole? Beating the Half-Gates Lower Bound for Garbled Circuits
📺 ★
Abstract

We describe a garbling scheme for boolean circuits, in which XOR gates are free and AND gates require communication of $1.5\kappa + 5$ bits. This improves over the state-of-the-art ``half-gates'' scheme of Zahur, Rosulek, and Evans (Eurocrypt 2015), in which XOR gates are free and AND gates cost $2\kappa$ bits. The half-gates paper proved a lower bound of $2\kappa$ bits per AND gate, in a model that captured all known garbling techniques at the time. We bypass this lower bound with a novel technique that we call \textbf{slicing and dicing}, which involves slicing wire labels in half and operating separately on those halves. Ours is the first to bypass the lower bound while being fully compatible with free-XOR, making it a drop-in replacement for half-gates. Our construction is proven secure from a similar assumption to prior free-XOR garbling (circular correlation-robust hash), and uses only slightly more computation than half-gates.

2021

CRYPTO

Large Message Homomorphic Secret Sharing from DCR and Applications
📺
Abstract

We present the first homomorphic secret sharing (HSS) construction that simultaneously (1) has negligible correctness error, (2) supports integers from an exponentially large range, and (3) relies on an assumption not known to imply FHE --- specifically, the Decisional Composite Residuosity (DCR) assumption. This resolves an open question posed by Boyle, Gilboa, and Ishai (Crypto 2016). Homomorphic secret sharing is analogous to fully-homomorphic encryption, except the ciphertexts are shared across two non-colluding evaluators. Previous constructions of HSS either had non-negligible correctness error and polynomial-size plaintext space or were based on the stronger LWE assumption. We also present two applications of our technique: a multi-server ORAM with constant bandwidth overhead, and a rate-$1$ trapdoor hash function with negligible error rate.

2021

ASIACRYPT

Batching Base Oblivious Transfers
📺
Abstract

Protocols that make use of oblivious transfer (OT) rarely require just one instance. Usually a batch of OTs is required — notably, when generating base OTs for OT extension. There is a natural way to optimize 2-round OT protocols when generating a batch, by reusing certain protocol messages across all instances. In this work we show that this batch optimization is error-prone. We catalog many implementations and papers that have an incorrect treatment of this batch optimization, some of them leading to catastrophic leakage in OT extension protocols. We provide a full treatment of how to properly optimize recent 2-round OT protocols for the batch setting. Along the way we show several performance improvements to the OT protocol of McQuoid, Rosulek, and Roy (ACM CCS 2020). In particular, we show an extremely simple OT construction that may be of pedagogical interest.

#### Program Committees

- Crypto 2024

#### Coauthors

- Damiano Abram (1)
- Carsten Baum (1)
- Lennart Braun (1)
- Cyprien Delpech de Saint Guilhem (1)
- Michael Klooß (1)
- Yashvanth Kondi (1)
- Ian McQuoid (1)
- Claudio Orlandi (1)
- Emmanuela Orsini (1)
- Mike Rosulek (2)
- Peter Scholl (2)
- Jaspal Singh (1)
- Jiayu Xu (1)