International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Wakaha Ogata

Affiliation: Tokyo Institute of Technology

Publications

Year
Venue
Title
2008
EPRINT
Blind HIBE and its Applications to Identity-Based Blind Signature and Blind Decryption
Le Trieu Phong Wakaha Ogata
We explicitly describe and analyse \textit{blind} hierachical identity-based encryption (\textit{blind} HIBE) schemes, which are natural generalizations of blind IBE schemes \cite{gh07}. We then uses the blind HIBE schemes to construct: (1) An identity-based blind signature scheme secure in the standard model, under the computational Diffie-Hellman (CDH) assumption, and with much shorter signature size and lesser communication cost, compared to existing proposals. (2) A new mechanism supporting a user to buy digital information over the Internet without revealing what he/she has bought, while protecting the providers from cheating users.
2006
ASIACRYPT
2006
EPRINT
On a Variation of Kurosawa-Desmedt Encryption Scheme
Le Trieu Phong Wakaha Ogata
Kurosawa-Desmedt encryption scheme is a variation of Cramer-Shoup encryption schemes, which are the first practical schemes secure against adaptive chosen ciphertext attack in standard model. We introduce a variant of Kurosawa-Desmedt encryption scheme, which is not only secure against adaptive chosen ciphertext attack but also slightly more efficient than the original version.
2005
PKC
2004
EPRINT
The Security of the FDH Variant of Chaum's Undeniable Signature Scheme
In this paper, we first introduce a new kind of adversarial goal called {\em forge-and-impersonate} in undeniable signature schemes. Note that forgeability does not necessarily imply impersonation ability. We then classify the security of the FDH variant of Chaum's undeniable signature scheme according to three dimensions, the goal of adversaries, the attacks and the ZK level of confirmation and disavowal protocols. We finally relate each security to some well-known computational problem. In particular, we prove that the security of the FDH variant of Chaum's scheme with NIZK confirmation and disavowal protocols is equivalent to the CDH problem, as opposed to the GDH problem as claimed by Okamoto and Pointcheval.
2003
EPRINT
Divisible Voting Scheme
Electronic voting is a prime application of cryptographic tools. Many researches are addressing election or confidence voting in this area. We address a new type of voting scheme ``Divisible Voting Scheme,'' in which each voter has multiple ballots where the number of ballots can be different among the voters. This type of voting is popular, however there is no secure protocol which achieves this type of voting. We first define the divisible voting scheme and show naive protocols based on existing voting schemes. Then we propose two efficient divisible voting schemes. The first scheme uses multisets, the second scheme uses $L$-adic representation of number of ballots. The total cost for a voter is $O(M^2 \log (N))$ in the first scheme and $O(M \log(N))$ in the second scheme where $M$ is the number of candidates to vote for and $N$ is the number of ballots for a voter.
2002
ASIACRYPT
2002
EPRINT
Oblivious Keyword Search
Wakaha Ogata Kaoru Kurosawa
In this paper, we introduce a notion of Oblivious Keyword Search ($OKS$). Let $W$ be the set of possible keywords. In the commit phase, a database supplier $T$ commits $n$ data. In each transfer subphase, a user $U$ can choose a keyword $w \in W$ adaptively and find $Search(w)$ without revealing $w$ to $T$, where $Search(w)$ is the set of all data which includes $w$ as a keyword. We then show two efficient protocols such that the size of the commitments is only $(nB)$ regardless of the size of $W$, where $B$ is the size of each data. It is formally proved that $U$ learns nothing more and $T$ gains no information on the keywords which $U$ searched. We further present a more efficient adaptive $OT_k^n$ protocol than the previous one as an application of our first $OKS$ protocol.
2002
EPRINT
Bit-Slice Auction Circuit
Kaoru Kurosawa Wakaha Ogata
In this paper, we introduce a bit-slice approach for auctions and present a more efficient circuit than the normal approach for the highest-price auction. Our circuit can be combined with any auction protocol based on general circuit evaluation. Especially, if we combine with the mix and match technique, then we can obtain a highest-price auction protocol which is at least seven times faster. A second-price auction protocol is also easily constructed from our circuit.
2001
PKC
2000
ASIACRYPT
1999
JOFC
1996
EUROCRYPT
1995
CRYPTO
1993
EUROCRYPT
1992
AUSCRYPT
1991
ASIACRYPT
1990
EUROCRYPT