Cryptanalysis and Improvement of a Proxy-protected Signature Scheme

Recently, Liu et al. (Liu YC, Wen HA, Lin CL, Hwang T. Proxy-protected signature secure against the undelegated proxy signature attack. Computers and Electrical Engineering 2007; 33: 177-85) proposed one proxy signature scheme that is the improvement of Zhou et al.´s (Zhou Y, Cao Z, Lu R. Provably secure proxy-protected signature schemes based on factoring. Appl Math Comput2005; 164(1):83-98) proxy signature scheme based on the hard problem of integer factorization. In this paper, however,we show that Liu et al.´s scheme and Zhou et al.´s scheme are insecure against the replaced warrant attack: the original signer can prepare a new warrant and then remove the original warrant from the proxy signature to forge a new and valid proxy signature with the new warrant. To avoid the problem, an improved proxy signature scheme is proposed and its security is analyzed.