## CryptoDB

### Kwangjo Kim

#### Publications

Year
Venue
Title
2009
EPRINT
Chameleon signatures are based on well established hash-and-sign paradigm, where a \emph{chameleon hash function} is used to compute the cryptographic message digest. Chameleon signatures simultaneously provide the properties of non-repudiation and non-transferability for the signed message. However, the initial constructions of chameleon signatures suffer from the problem of key exposure: the signature forgery results in the signer recovering the recipient's trapdoor information, $i.e.,$ the private key. This creates a strong disincentive for the recipient to forge signatures, partially undermining the concept of non-transferability. Recently, some specific constructions of key-exposure free chameleon hashing are presented, based on RSA or pairings, using the idea of Customized Identities". In this paper, we propose the first key-exposure free chameleon hash scheme based on discrete logarithm systems, without using the gap Diffile-Hellman groups. Moreover, one distinguished advantage of the resulting chameleon signature scheme is that the property of message hiding" or message recovery" can be achieved freely by the signer. Another main contribution in this paper is that we propose the first identity-based chameleon hash scheme without key exposure, which gives a positive answer for the open problem introduced by Ateniese and de Mederious in 2004.
2008
EPRINT
Ring signature was proposed to keep signer's anonymity when it signs messages on behalf of a ring" of possible signers. In this paper, we propose a novel notion of ring signature which is called attribute-based ring signature. In this kind of signature, it allows the signer to sign message with its attributes from attribute center. All users that possess of these attributes form a ring. The identity of signer is kept anonymous in this ring. Furthermore, anyone out of this ring could not forge the signature on behalf of the ring. Two constructions of attribute-based ring signature are also presented in this paper. The first scheme is proved to be secure in the random oracle model, with large universal attributes. We also present another scheme in order to avoid the random oracle model. It does not rely on non-standard hardness assumption or random oracle model. Both schemes in this paper are based on standard computational Diffie-Hellman assumption.
2004
EPRINT
Chameleon signatures are based on well established hash-and-sign paradigm, where a \emph{chameleon hash function} is used to compute the cryptographic message digest. Chameleon signatures simultaneously provide the properties of non-repudiation and non-transferability for the signed message, $i.e.,$ the designated recipient is capable of verifying the validity of the signature, but cannot disclose the contents of the signed information to convince any third party without the signer's consent. One disadvantage of the initial chameleon signature scheme is that signature forgery results in the signer recovering the recipient's trapdoor information, $i.e.,$ private key. Therefore, the signer can use this information to deny \emph{other} signatures given to the recipient. This creates a strong disincentive for the recipient to forge signatures, partially undermining the concept of non-transferability. In this paper, we firstly propose a chameleon hashing scheme in the gap Diffie-Hellman group to solve the problem of key exposure. We can prove that the recipient's trapdoor information will never be compromised under the assumption of Computation Diffie-Hellman Problem (CDHP) is intractable. Moreover, we use the proposed chameleon hashing scheme to design a chameleon signature scheme.
2003
EPRINT
Key authentication is very important in secret communications and data security. Recently, Lee, Hwang and Li proposed a new public key authentication scheme for cryptosystems with a trusty server. However, in this paper, we will show that Lee-Hwang-Li's key authentication scheme is not secure, from the obtained public information, any one can get the private key of the user. And then, we propose an improved scheme. We conclude that our new key authentication scheme not only resolves the problems appeared but also is secure.
2003
EPRINT
We argue that traditional ID-based systems from pairings seem unsuitable for designing group signature schemes due to the problem of key escrow. In this paper we propose new ID-based public key systems without trustful KGC from bilinear pairings. In our new ID-based systems, if dishonest KGC impersonates an honest user to communicate with others, the user can provide a proof of treachery of the KGC afterwards, which is similar to CA-based systems. Furthermore, we propose a group signature scheme under the new systems, the security and performance of which rely on the new systems. The size of the group public key and the length of the signature are independent on the numbers of the group.
2002
ASIACRYPT
2002
EPRINT
With positive applications of Weil pairing (Tate pairing) to cryptography, ID-based encryption schemes, digital signature schemes, blind signature scheme, two-party authenticated key agreement schemes, and tripartite key agreement scheme were proposed recently, all of them using bilinear pairing (Weil or Tate pairing). In this paper, we propose an ID-based one round authenticated tripartite key agreement protocol. The authenticity of the protocol is assured by a special signature scheme, so that messages carrying the information of two ephemeral keys can be broadcasted authentically by an entity. Consequently, one instance of our protocol results in eight session keys for the three entities. Security attributes of our protocol are presented, and the computational overhead and bandwidth of the broadcast messages are analyzed as well.
2002
EPRINT
Group signature and blind signature are very important primitives in cryptography. A group signature scheme allows a group member to sign messages anonymously on behalf of the group and a blind signature scheme can ensure anonymity of the sender of a message. Recently, S. Xia and J. You proposed a group signature scheme with strong separability in which the revocation manager can work without the involvement of the membership manager and J.J-R. Chen and A.P. Chen proposed a blind signature scheme based on dual complexities (which combines factorization and discrete logarithm problem). In this paper, we give a universal forgery attack on Xia-You's group signature scheme which any one (not necessarily a group member) can produce a valid group signature on an arbitrary message, and it is untraceable by the group revocation manager. For Chen-Chen's blind signature scheme, we show that it could not meet the untraceability property of a blind signature, $i.e.$, it could not ensure anonymity of the user.
2002
EPRINT
In ISC 2002, J. Zheng proposed a new public key cryptosystem whose security is based upon the algebraic problem of reducing a high degree matrix to its canonical form by similarity transformations. In this paper, we show that factoring a polynomial over a finite field can be used to break down Zheng's public key cryptosystem. The complexity of our attack is polynomial time. In other word, the underlying problem of Zheng's public key cryptosystem is not a hard'' problem.
1994
ASIACRYPT
1991
ASIACRYPT
1990
CRYPTO

#### Program Committees

Eurocrypt 2009
PKC 2007
Asiacrypt 2006
PKC 2005
PKC 2004
Asiacrypt 2003
Eurocrypt 2003
Asiacrypt 2002
Crypto 2002
PKC 2002
PKC 2001 (Program chair)
PKC 2000
Asiacrypt 1999
PKC 1999
Asiacrypt 1998
Asiacrypt 1996
Asiacrypt 1996 (Program chair)

#### Coauthors

Seongtaek Chee (1)
Xiaofeng Chen (3)
Hideki Imai (1)
Sangjin Lee (1)
Jin Li (1)
Shengli Liu (2)
Tsutomu Matsumoto (1)
Haibo Tian (1)
Baodian Wei (1)
Fangguo Zhang (8)