In 1996, a new category of signature scheme called a proxy signature was proposed by Mambo, Usuda, and Okamoto. The proxy signature scheme allows a designated person, called a proxy signer, to sign on behalf of an original signer. In Mambo's paper, one kind of cryptanalysis on their scheme was considered. In this paper, it will be shown that the reported attack in Mambo's paper is not practical and a simple counter-measure can be easily developed. The proxy signature scheme plays the role in many practical applications and receives great attention after it was proposed. In 1999, Sung and Hsieh developed an enhanced version of proxy signature scheme. However, it will be proven that the Sung-Hsieh scheme is not secure. A simple modified version will be suggested.
Proceedings of the 2000 ICS:Workshop on Cryptology and Information Security，頁54-59