International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Fuchun Guo

Publications

Year
Venue
Title
2022
EUROCRYPT
Optimal Tightness for Chain-Based Unique Signatures
Fuchun Guo Willy Susilo
Unique signatures are digital signatures with exactly one unique and valid signature for each message. The security reduction for most unique signatures has a natural reduction loss (in the existentially unforgeable against chosen-message attacks, namely EUF-CMA, security model under a non-interactive hardness assumption). In Crypto 2017, Guo {\it et al.} proposed a particular chain-based unique signature scheme where each unique signature is composed of $n$ BLS signatures computed sequentially like a blockchain. Under the computational Diffie-Hellman assumption, their reduction loss is $n\cdot q_H^{1/n}$ for $q_H$ hash queries and it is logarithmically tight when $n=\log{q_H}$. However, it is currently unknown whether a better reduction than logarithmical tightness for the chain-based unique signatures exists. We show that the proposed chain-based unique signature scheme by Guo {\it et al.} must have the reduction loss $q^{1/n}$ for $q$ signature queries when each unique signature consists of $n$ BLS signatures. We use a meta reduction to prove this lower bound in the EUF-CMA security model under any non-interactive hardness assumption, and the meta-reduction is also applicable in the random oracle model. We also give a security reduction with reduction loss $4\cdot q^{1/n}$ for the chain-based unique signature scheme (in the EUF-CMA security model under the CDH assumption). This improves significantly on previous reduction loss $n\cdot q_H^{1/n}$ that is logarithmically tight at most. The core of our reduction idea is a {\em non-uniform} simulation that is specially invented for the chain-based unique signature construction.
2022
CRYPTO
Multimodal Private Signatures
We introduce Multimodal Private Signature (MPS) - an anonymous signature system that offers a novel accountability feature: it allows a designated opening authority to learn \emph{some partial information}~$\ms{op}$ about the signer's identity $\ms{id}$, and nothing beyond. Such partial information can flexibly be defined as $\ms{op} = \ms{id}$ (as in group signatures), or as $\ms{op} = \mb{0}$ (like in ring signatures), or more generally, as $\ms{op} = G_j(\ms{id})$, where $G_j(\cdot)$ is certain disclosing function. Importantly, the value of $op$ is known in advanced by the signer, and hence, the latter can decide whether she/he wants to disclose that piece of information. The concept of MPS significantly generalizes the notion of tracing in traditional anonymity-oriented signature primitives, and can enable various new and appealing privacy-preserving applications. We formalize the definitions and security requirements for MPS. We next present a generic construction to demonstrate the feasibility of designing MPS in a modular manner and from commonly used cryptographic building blocks (ordinary signatures, public-key encryption and NIZKs). We also provide an efficient construction in the standard model based on pairings, and a lattice-based construction in the random oracle model.
2017
CRYPTO
2016
ASIACRYPT
2016
ASIACRYPT