On the Security of Some Nonrepudiable Threshold Proxy Signature Schemes

A (t,n) threshold proxy signature scheme enables an original signer or a group of original signers to delegate the signature authority to a proxy group of n members such that not less than t proxy signers can cooperatively sign messages on behalf of the original signer or the original signer group. In the paper, we show that Sun's and Yang et al.'s threshold proxy signature schemes are insecure against the original signer's forgery, and that Tzeng et al.'s threshold multi-proxy multi-signature scheme is vulnerable against the actual original signer group's forgery. We also show that Hsu et al.'s threshold proxy signature scheme suffers from the conspiracy of the original signer and the secret share dealer SA, and that Hwang et al.'s threshold proxy signature scheme is universally forgeable. In other words, none of the above-mentioned schemes holds the unforgeability and provides non-repudiation.