ABSTRACT Recently, Tsenget al. proposed a novel key agreement protocol based on chaotic maps. They claimed that their protocol achieved the session key agreement between server and users with user's anonymity and security. Although Niu and Wang proposed a new improvement, the presented scheme involved an additional participant, the trusted party, in such a way that the system cost raised a lot. To inherit the superiority of Tsenget al. but remove the security weaknesses, it is worthwhile to point out the kernel of drawback in the scheme of Tsenget al. and, further, propose a security‐enhanced scheme by overcoming the drawback with slight modifications. Copyright © 2012 John Wiley & Sons, Ltd.

A (t,n) threshold signature scheme allows n members of a group to share a secret key such that any t members can create a valid group signature. Of course, any t members can compromise this system. However designing a (t,n) threshold signature scheme based on the Digital Signature Standard (DSS) is difficult if there is no trusted party to distribute the shares of the secret. Langford (1995) proposed a (t/sup 2/-t+1)-out-of-n threshold DSS signature scheme without a trusted party. It requires t/sup 2/-t+1 members to sign a document, but t members can conspire to destroy this scheme. Gennaro et al. (1996) improved that result by proposing a robust threshold DSS signature scheme which requires 2t+1 members in the signing procedure. This paper proposes a new method to reduce the number of signers to t+1. We also extend our scheme to a generalized case.