Journal:Informatica
Volume 18, Issue 4 (2007), pp. 535–546
Abstract
In 2004, Abe et al. proposed a threshold signer-ambiguous signature scheme from variety of keys. Their scheme is a generalized case of the ring signature scheme, and it allows the key types to be based on the trapdoor one-way permutations (TOWP) or sigma-protocols including Schnorr's signature scheme. However, the signed message is public for all, which may result in disputes. In this paper, we present a novel threshold signer-ambiguous signature scheme, having the signed message concealed and keeping who the receivers are secret from variety of keys.
Journal:Informatica
Volume 18, Issue 1 (2007), pp. 55–60
Abstract
Recently, Chen, Chung, and Huang proposed a traceable proxy multisignature scheme based on the elliptic curve cryptosystem. However, this paper shows that the original signers can produce a valid signature as the proxy signer does in the proxy protected scheme. Therefore, Chen et al.'s proxy-protected scheme cannot protect the proxy signer from being forged by the original signers. We further find that the early work of Chen et al. in 2003 suffers the same attack. To overcome this problem, an improved scheme will be presented.
Journal:Informatica
Volume 17, Issue 3 (2006), pp. 347–362
Abstract
This paper introduces a new concept of convertible user designating confirmer partially blind signature, in which only the designated confirmer (designated by the user) and the user can verify and confirm the validity of given signatures and convert given signatures into publicly verifiable ones. We give a formal definition for it and propose a concrete provably secure scheme with a proof of security and a brief analysis of efficiency. Assuming the intractabilities of the Discrete Logarithm Problem and the ROS-Problem, the proposed scheme is unforgeable under adaptive chosen-message attack.
Journal:Informatica
Volume 14, Issue 3 (2003), pp. 393–402
Abstract
In 2001, Hsu et al. proposed a non‐repudiable threshold proxy signature with known signers. In their scheme, the proxy group cannot deny having signed the proxy signature if they did. However, Hsu et al.'s scheme is vulnerable to some attacks. A malicious original signer or malicious proxy signer can impersonate some other proxy signers to generate proxy signatures. In this article, we shall present our cryptanalysis of the Hsu et al.'s scheme. After that, we shall propose a new threshold proxy signature that can overcome the weaknesses.
Journal:Informatica
Volume 14, Issue 2 (2003), pp. 205–212
Abstract
Sun's nonrepudiation threshold proxy signature scheme is not secure against the collusion attack. In order to guard against the attack, Hwang et al. proposed another threshold proxy signature scheme. However, a new attack is proposed to work on both Hwang et al.'s and Sun's schemes. By executing this attack, one proxy signer and the original signer can forge any valid proxy signature. Therefore, both Hwang et al.'s scheme and Sun's scheme were insecure.
Journal:Informatica
Volume 11, Issue 1 (2000), pp. 15–18
Abstract
Recently, Harn proposed an efficient scheme that can batch verification multiple RSA digital signatures. His scheme can reduce signature verification time. However, there is a weakness in his scheme. In this study, we present two methods to against his scheme.