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.