Improvement of Yang et al.'s threshold proxy signature scheme

Since the first (t, n) threshold proxy signature scheme was proposed, the threshold proxy signature has enjoyed a considerable amount of interest from the cryptographic research community. In 2001, Hsu et al. proposed a non-repudiable threshold proxy signature scheme with known signer, but the efficiency is rather low and a system authority (SA) is also required in this scheme. Recently, to overcome these shortcomings, Yang et al. proposed an improvement of Hsu et al.'s scheme that is very efficient and without employing a SA. However, in this paper, we shows that Yang et al.'s scheme is not secure against the warrant attack. That is, the adversary can replace the warrant of the proxy signature. To resist this attack, based on Yang et al.'s scheme, we propose a new and more efficient scheme without a secure channel.