Original Signer's Forgery Attacks on Discrete Logarithm Based Proxy Signature Schemes

A proxy signature scheme enables a proxy signer to sign messages on behalf of the original signer. In this paper, we demonstrate that a number of discrete logarithm based proxy signature schemes are vulnerable to an original signer’s forgery attack. In this attack, a malicious original signer can impersonate a proxy signer and produce a forged proxy signature on a message. A third party will incorrectly believe that the proxy signer was responsible for generating the proxy signature. This contradicts the strong unforgeability property that is required of proxy signatures schemes. We show six proxy signature schemes vulnerable to this attack including Lu et al.’s proxy blind multi-signature scheme, Xue and Cao’s proxy blind signature scheme, Fu et al. and Gu et al.’s anonymous proxy signature schemes, Dai et al. and Huang et al.’s nominative proxy signature schemes are all insecure against the original signer’s forgery.