CryptoDB

Miyako Ohkubo

Publications

Year
Venue
Title
2018
PKC
Structure Preserving Signatures (SPS) allow the signatures and the messages signed to be further encrypted while retaining the ability to be proven valid under zero-knowledge. In particular, SPS are tailored to have structure suitable for Groth-Sahai NIZK proofs. More precisely, the messages, signatures, and verification keys are required to be elements of groups that support efficient bilinear-pairings (bilinear groups), and the signature verification consists of just evaluating one or more bilinear-pairing product equations. Since Groth-Sahai NIZK proofs can (with zero-knowledge) prove the validity of such pairing product equations, it leads to interesting applications such as blind signatures, group signatures, traceable signatures, group encryption, and delegatable credential systems.In this paper, we further improve on the SPS scheme of Abe, Hofheinz, Nishimaki, Ohkubo and Pan (CRYPTO 2017) while maintaining only an $O(\lambda )$ O(λ)-factor security reduction loss to the SXDH assumption. In particular, we compress the size of the signatures by almost 40%, and reduce the number of pairing-product equations in the verifier from fifteen to seven. Recall that structure preserving signatures are used in applications by encrypting the messages and/or the signatures, and hence these optimizations are further amplified as proving pairing-product equations in Groth-Sahai NIZK system is not frugal. While our scheme uses an important novel technique introduced by Hofheinz (EuroCrypt 2017), i.e. structure-preserving adaptive partitioning, our approach to building the signature scheme is different and this leads to the optimizations mentioned. Thus we make progress towards an open problem stated by Abe et al. (CRYPTO 2017) to design more compact SPS-es with smaller number of group elements.
2018
ASIACRYPT
We construct the first (almost) tightly-secure unbounded-simulation-sound quasi-adaptive non-interactive zero-knowledge arguments (USS-QA-NIZK) for linear-subspace languages with compact (number of group elements independent of the security parameter) common reference string (CRS) and compact proofs under standard assumptions in bilinear-pairings groups. In particular, under the SXDH assumption, the USS-QA-NIZK proof size is only seventeen group elements with a factor $O(\log {Q})$ loss in security reduction to SXDH. The USS-QA-NIZK primitive has many applications, including structure-preserving signatures (SPS), CCA2-secure publicly-verifiable public-key encryption (PKE), which in turn have applications to CCA-anonymous group signatures, blind signatures and unbounded simulation-sound Groth-Sahai NIZK proofs. We show that the almost tight security of our USS-QA-NIZK translates into constructions of all of the above applications with (almost) tight-security to standard assumptions such as SXDH and, more generally, $\mathcal{D}_k$-MDDH. Thus, we get the first publicly-verifiable (almost) tightly-secure multi-user/multi-challenge CCA2-secure PKE with practical efficiency under standard bilinear assumptions. Our (almost) tight SPS construction is also improved in the signature size over previously known constructions.
2017
CRYPTO
2016
CRYPTO
2016
JOFC
2016
JOFC
2015
EPRINT
2015
EPRINT
2015
EUROCRYPT
2014
CRYPTO
2014
CRYPTO
2014
TCC
2014
EPRINT
2014
EPRINT
2013
PKC
2012
EUROCRYPT
2012
ASIACRYPT
2011
CRYPTO
2011
ASIACRYPT
2010
EPRINT
This paper addresses the construction of signature schemes whose verification keys, messages, and signatures are group elements and the verification predicate is a conjunction of pairing product equations. We answer to the open problem of constructing constant-size signatures by presenting an efficient scheme. The security is proven in the standard model based on a novel non-interactive assumption called Simultaneous Flexible Pairing Assumption that can be justified and has an optimal bound in the generic bilinear group model. We also present efficient schemes with advanced properties including signing unbounded number of group elements, allowing simulation in the common reference string model, signing messages from mixed groups in the asymmetric bilinear group setting, and strong unforgeability. Among many applications, we show two examples; an adaptively secure round optimal blind signature scheme and a group signature scheme with efficient concurrent join. As a bi-product, several homomorphic trapdoor commitment schemes and one-time signature schemes are presented, too. In combination with the Groth-Sahai proof system, these schemes contribute to an efficient instantiation of modular constructions of cryptographic protocols.
2010
EPRINT
The proloferation of RFID tags enhances everyday activities, such as by letting us reference the price, origin and circulation route of specific goods. On the other hand, this lecel of traceability gives rise to new privacy issues and the topic of developing cryptographic protocols for RFID- tags is garnering much attention. A large amount of research has been conducted in this area. In this paper, we reconsider the security model of RFID- authentication with a man-in-the-middle adversary and communication fault. We define model and security proofs via a game-based approach makes our security models compatible with formal security analysis tools. We show that an RFID authentication protocol is robust against the above attacks, and then provide game-based (hand-written) proofs and their erification by using CryptoVerif.
2010
CRYPTO
2009
ASIACRYPT
2009
PKC
2002
ASIACRYPT
2001
ASIACRYPT
2000
ASIACRYPT

Eurocrypt 2017
Asiacrypt 2017
Eurocrypt 2013