Forgery Attacks on Two Pairing-Based Proxy Signature Schemes

As a kind of digital signatures, proxy signature has generally been paid more attentions. It is useful when one needs to delegate his signing capability to other party. Recently, Sun et al. proposed a proxy multi-signature scheme without random oracles and Liu et al. proposed a multi-proxy signature scheme with proxy revocation. Both of the two schemes are proved to be existentially unforgeable in the standard model. In this paper, we show that Sun et al.´s scheme is vulnerable to insider attack. That is, a dishonest proxy signer can reconstruct a new warrant and generate forged proxy signatures on any illegal messages which are not delegated by the original signers. In addition, we disprove Liu et al.´s security claims and show that a successful universal forgery is feasible of the scheme.