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Security Analysis and their Improvement of Two Signature Schemes

Recently, Wang et. al presented a short group signature scheme without random oracles, and Kang et al. gave an efficient ID-based strong designated verifier signature scheme. And security proof of the two signature schemes were provided. Unfortunately, in this work we show that Wang et al.'s group signature is universally forgeable. Namely, any one can forge a group signature on arbitrary message, then an improved vision is presented; And we also show that Kang et al.'s scheme is forgeable, namely, a third party can produce a signature on arbitrary message m by a given signature information. Finally, we give the corresponding attack on the two signature schemes.