## CryptoDB

### Ngoc Khanh Nguyen

#### Publications

**Year**

**Venue**

**Title**

2021

ASIACRYPT

Shorter Lattice-Based Group Signatures via ``Almost Free'' Encryption and Other Optimizations
Abstract

We present an improved lattice-based group signature scheme whose parameter sizes and running times are independent of the group size. The signature length in our scheme is around $200$KB, which is approximately a $3$X reduction over the previously most compact such scheme, based on any quantum-safe assumption, of del Pino et al. (CCS 2018). The improvement comes via several optimizations of some basic cryptographic components that make up group signature schemes, and we think that they will find other applications in privacy-based lattice cryptography.

2021

PKC

Shorter Lattice-Based Zero-Knowledge Proofs via One-Time Commitments
📺
Abstract

There has been a lot of recent progress in constructing efficient zero-knowledge proofs for showing knowledge of an $\vec{\bm{s}}$ with small coefficients satisfying $\bm{A}\vec{\bm{s}}=\vec{\bm{t}}$. For typical parameters, the proof sizes have gone down from several megabytes to a bit under $50$KB (Esgin et al., Asiacrypt 2020). These are now within an order of magnitude of the sizes of lattice-based signatures, which themselves constitute proof systems which demonstrate knowledge of something weaker than the aforementioned equation. One can therefore see that this line of research is approaching optimality. In this paper, we modify a key component of these proofs, as well as apply several other tweaks, to achieve a further reduction of around $30\%$ in the proof output size. We also show that this savings propagates itself when these proofs are used in a general framework to construct more complex protocols.

2021

CRYPTO

SMILE: Set Membership from Ideal Lattices with Applications to Ring Signatures and Confidential Transactions
📺
Abstract

In a set membership proof, the public information consists of a set of elements and a commitment. The prover then produces a zero-knowledge proof showing that the commitment is indeed to some element from the set. This primitive is closely related to concepts like ring signatures and ``one-out-of-many'' proofs that underlie many anonymity and privacy protocols. The main result of this work is a new succinct lattice-based set membership proof whose size is logarithmic in the size of the set.
We also give transformations of our set membership proof to a ring signature scheme and to a confidential transaction payment system. The ring signature size is also logarithmic in the size of the public key set and has size $16$~KB for a set of $2^5$ elements, and $22$~KB for a set of size $2^{25}$. At an approximately $128$-bit security level, these outputs are between 1.5X and 7X smaller than the current state of the art succinct ring signatures of Beullens et al. (Asiacrypt 2020) and Esgin et al. (CCS 2019).
We then show that our ring signature, combined with a few other techniques and optimizations, can be turned into a fairly efficient Monero-like confidential transaction system based on the MatRiCT framework of Esgin et al. (CCS 2019). With our new techniques, we are able to reduce the transaction proof size by factors of about 4X - 10X over the aforementioned work. For example, a transaction with two inputs and two outputs, where each input is hidden among $2^{15}$ other accounts, requires approximately $30$KB in our protocol.

2020

CRYPTO

A non-PCP Approach to Succinct Quantum-Safe Zero-Knowledge
📺
Abstract

Today's most compact zero-knowledge arguments are based on the hardness of the discrete logarithm problem and related classical assumptions. If one is interested in quantum-safe solutions, then all of the known techniques stem from the PCP-based framework of Kilian (STOC 92) which can be instantiated based on the hardness of any collision-resistant hash function. Both approaches produce asymptotically logarithmic sized arguments but, by exploiting extra algebraic structure, the discrete logarithm arguments are a few orders of magnitude more compact in practice than the generic constructions.\\
In this work, we present the first (poly)-logarithmic \emph{post-quantum} zero-knowledge arguments that deviate from the PCP approach. At the core of succinct zero-knowledge proofs are succinct commitment schemes (in which the commitment and the opening proof are sub-linear in the message size), and we propose two such constructions based on the hardness of the (Ring)-Short Integer Solution (Ring-SIS) problem, each having certain trade-offs. For commitments to $N$ secret values, the communication complexity of our first scheme is $\tilde{O}(N^{1/c})$ for any positive integer $c$, and $O(\log^2 N)$ for the second. %Both of our protocols have somewhat large \emph{slack}, which in lattice constructions is the ratio of the norm of the extracted secrets to the norm of the secrets that the honest prover uses in the proof. The lower this factor, the smaller we can choose the practical parameters. For a fixed value of this factor, our $\tilde{O}(N^{1/c})$-argument actually achieves lower communication complexity.
Both of these are a significant theoretical improvement over the previously best lattice construction by Bootle et al. (CRYPTO 2018) which gave $O(\sqrt{N})$-sized proofs.

2020

CRYPTO

Lattice-Based Blind Signatures, Revisited
📺
Abstract

We observe that all previously known lattice-based blind signatures schemes contain subtle flaws in their security proofs (e.g.,~Rückert, ASIACRYPT '08) or can be attacked (e.g., BLAZE by Alkadri et al., FC~'20). Motivated by this, we revisit the problem of constructing blind signatures from standard lattice assumptions. We propose a new three-round lattice-based blind signature scheme whose security can be proved, in the random oracle model, from the standard SIS assumption. Our starting point is a modified version of the insecure three-round BLAZE scheme, which itself is based Lyubashevsky's three-round identification scheme combined with a new aborting technique to reduce the correctness error. Our proof builds upon and extends the recent modular framework for blind signatures of Hauck, Kiltz, and Loss (EUROCRYPT~'19). It also introduces several new techniques to overcome the additional challenges posed by the correctness error which is inherent to all lattice-based constructions.
While our construction is mostly of theoretical interest, we believe it to be an important stepping stone for future works in this area.

2020

ASIACRYPT

Practical Exact Proofs from Lattices: New Techniques to Exploit Fully-Splitting Rings
📺
Abstract

We propose a lattice-based zero-knowledge proof system for exactly proving knowledge of a ternary solution $\vec{s} \in \{-1,0,1\}^n$ to a linear equation $A\vec{s}=\vec{u}$ over $\mathbb{Z}_q$, which improves upon the protocol by Bootle, Lyubashevsky and Seiler (CRYPTO 2019) by producing proofs that are shorter by a factor of $7.5$.
At the core lies a technique that utilizes the module-homomorphic BDLOP commitment scheme (SCN 2018) over the fully splitting cyclotomic ring $\mathbb{Z}_q[X]/(X^d + 1)$ to prove scalar products with the NTT vector of a secret polynomial.

2019

PKC

On Tightly Secure Primitives in the Multi-instance Setting
Abstract

We initiate the study of general tight reductions in cryptography. There already exist a variety of works that offer tight reductions for a number of cryptographic tasks, ranging from encryption and signature schemes to proof systems. However, our work is the first to provide a universal definition of a tight reduction (for arbitrary primitives), along with several observations and results concerning primitives for which tight reductions have not been known.Technically, we start from the general notion of reductions due to Reingold, Trevisan, and Vadhan (TCC 2004), and equip it with a quantification of the respective reduction loss, and a canonical multi-instance extension to primitives. We then revisit several standard reductions whose tight security has not yet been considered. For instance, we revisit a generic construction of signature schemes from one-way functions, and show how to tighten the corresponding reduction by assuming collision-resistance from the used one-way function. We also obtain tightly secure pseudorandom generators (by using suitable rerandomisable hard-core predicates), and tightly secure lossy trapdoor functions.

2019

ASIACRYPT

On the Non-existence of Short Vectors in Random Module Lattices
Abstract

Recently, Lyubashevsky & Seiler (Eurocrypt 2018) showed that small polynomials in the cyclotomic ring $$\mathbb {Z}_q[X]/(X^n+1)$$, where n is a power of two, are invertible under special congruence conditions on prime modulus q. This result has been used to prove certain security properties of lattice-based constructions against unbounded adversaries. Unfortunately, due to the special conditions, working over the corresponding cyclotomic ring does not allow for efficient use of the Number Theoretic Transform (NTT) algorithm for fast multiplication of polynomials and hence, the schemes become less practical.In this paper, we present how to overcome this limitation by analysing zeroes in the Chinese Remainder (or NTT) representation of small polynomials. As a result, we provide upper bounds on the probabilities related to the (non)-existence of a short vector in a random module lattice with no assumptions on the prime modulus. We apply our results, along with the generic framework by Kiltz et al. (Eurocrypt 2018), to a number of lattice-based Fiat-Shamir signatures so they can both enjoy tight security in the quantum random oracle model and support fast multiplication algorithms (at the cost of slightly larger public keys and signatures), such as the Bai-Galbraith signature scheme (CT-RSA 2014), $$\mathsf {Dilithium\text {-}QROM}$$ (Kiltz et al., Eurocrypt 2018) and $$\mathsf {qTESLA}$$ (Alkim et al., PQCrypto 2017). Our techniques can also be applied to prove that recent commitment schemes by Baum et al. (SCN 2018) are statistically binding with no additional assumptions on q.

#### Coauthors

- Jonathan Bootle (1)
- Muhammed F. Esgin (1)
- Eduard Hauck (1)
- Dennis Hofheinz (1)
- Eike Kiltz (1)
- Julian Loss (1)
- Vadim Lyubashevsky (4)
- Maxime Plançon (1)
- Gregor Seiler (5)