## CryptoDB

### Khoa Nguyen

#### Publications

Year
Venue
Title
2019
PKC
Zero-knowledge elementary databases (ZK-EDBs) are cryptographic schemes that allow a prover to commit to a set $\mathsf {D}$ of key-value pairs so as to be able to prove statements such as “x belongs to the support of $\mathsf {D}$ and $\mathsf {D}(x)=y$” or “x is not in the support of $\mathsf {D}$”. Importantly, proofs should leak no information beyond the proven statement and even the size of $\mathsf {D}$ should remain private. Chase et al. (Eurocrypt’05) showed that ZK-EDBs are implied by a special flavor of non-interactive commitment, called mercurial commitment, which enables efficient instantiations based on standard number theoretic assumptions. On the other hand, the resulting ZK-EDBs are only known to support proofs for simple statements like (non-)membership and value assignments. In this paper, we show that mercurial commitments actually enable significantly richer queries. We show that, modulo an additional security property met by all known efficient constructions, they actually enable range queries over keys and values – even for ranges of super-polynomial size – as well as membership/non-membership queries over the space of values. Beyond that, we exploit the range queries to realize richer queries such as $k$-nearest neighbors and revealing the $k$ smallest or largest records within a given range. In addition, we provide a new realization of trapdoor mercurial commitment from standard lattice assumptions, thus obtaining the most expressive quantum-safe ZK-EDB construction so far.
2019
ASIACRYPT
Code-based cryptography has a long history but did suffer from periods of slow development. The field has recently attracted a lot of attention as one of the major branches of post-quantum cryptography. However, its subfield of privacy-preserving cryptographic constructions is still rather underdeveloped, e.g., important building blocks such as zero-knowledge range proofs and set membership proofs, and even proofs of knowledge of a hash preimage, have not been known under code-based assumptions. Moreover, almost no substantial technical development has been introduced in the last several years.This work introduces several new code-based privacy-preserving cryptographic constructions that considerably advance the state-of-the-art in code-based cryptography. Specifically, we present 3 major contributions, each of which potentially yields various other applications. Our first contribution is a code-based statistically hiding and computationally binding commitment scheme with companion zero-knowledge (ZK) argument of knowledge of a valid opening that can be easily extended to prove that the committed bits satisfy other relations. Our second contribution is the first code-based zero-knowledge range argument for committed values, with communication cost logarithmic in the size of the range. A special feature of our range argument is that, while previous works on range proofs/arguments (in all branches of cryptography) only address ranges of non-negative integers, our protocol can handle signed fractional numbers, and hence, can potentially find a larger scope of applications. Our third contribution is the first code-based Merkle-tree accumulator supported by ZK argument of membership, which has been known to enable various interesting applications. In particular, it allows us to obtain the first code-based ring signatures and group signatures with logarithmic signature sizes.
2018
CRYPTO
We provide lattice-based protocols allowing to prove relations among committed integers. While the most general zero-knowledge proof techniques can handle arithmetic circuits in the lattice setting, adapting them to prove statements over the integers is non-trivial, at least if we want to handle exponentially large integers while working with a polynomial-size modulus q. For a polynomial L, we provide zero-knowledge arguments allowing a prover to convince a verifier that committed L-bit bitstrings x, y and z are the binary representations of integers X, Y and Z satisfying $Z=X+Y$ over $\mathbb {Z}$. The complexity of our arguments is only linear in L. Using them, we construct arguments allowing to prove inequalities $X<Z$ among committed integers, as well as arguments showing that a committed X belongs to a public interval $[\alpha ,\beta ]$, where $\alpha$ and $\beta$ can be arbitrarily large. Our range arguments have logarithmic cost (i.e., linear in L) in the maximal range magnitude. Using these tools, we obtain zero-knowledge arguments showing that a committed element X does not belong to a public set S using $\widetilde{\mathcal {O}}(n \cdot \log |S|)$ bits of communication, where n is the security parameter. We finally give a protocol allowing to argue that committed L-bit integers X, Y and Z satisfy multiplicative relations $Z=XY$ over the integers, with communication cost subquadratic in L. To this end, we use our protocol for integer addition to prove the correct recursive execution of Karatsuba’s multiplication algorithm. The security of our protocols relies on standard lattice assumptions with polynomial modulus and polynomial approximation factor.
2018
PKC
Lattice-based group signature is an active research topic in recent years. Since the pioneering work by Gordon, Katz and Vaikuntanathan (Asiacrypt 2010), ten other schemes have been proposed, providing various improvements in terms of security, efficiency and functionality. However, in all known constructions, one has to fix the number N of group users in the setup stage, and as a consequence, the signature sizes are dependent on N.In this work, we introduce the first constant-size group signature from lattices, which means that the size of signatures produced by the scheme is independent of N and only depends on the security parameter $\lambda$λ. More precisely, in our scheme, the sizes of signatures, public key and users’ secret keys are all of order $\widetilde{\mathcal {O}}(\lambda )$O~(λ). The scheme supports dynamic enrollment of users and is proven secure in the random oracle model under the Ring Short Integer Solution (RSIS) and Ring Learning With Errors (RLWE) assumptions. At the heart of our design is a zero-knowledge argument of knowledge of a valid message-signature pair for the Ducas-Micciancio signature scheme (Crypto 2014), that may be of independent interest.
2017
ASIACRYPT
2017
ASIACRYPT
2016
EUROCRYPT
2016
ASIACRYPT
2016
ASIACRYPT
2015
EPRINT
2015
EPRINT
2015
PKC
2015
ASIACRYPT
2014
PKC
2014
EPRINT
2013
PKC

Asiacrypt 2019
Asiacrypt 2018
Asiacrypt 2017