Proxy Confirmation Signatures

The undeniable signature, introduced by Chaum et al. in 1989, provides a nice property that the signer has an additional control over who will benefit from being convinced by the signature. However, a conspicuous drawback of undeniable signature is that the signer may be unavailable or refuse to cooperate. Chaum in 1994 proposed a designated confirmer signature scheme to protect the recipient’s right. There exists a confirmer, who can always help the recipient prove the validity of the signature to others. Unfortunately, Chaum’s paper did not consider that a malicious confirmer proves the validity of the signature to any persons as his will or even leaks the sensitive information to the signer’s enemies. This paper proposes a new signature scheme called proxy confirmation signature where the proxy confirmer can only acquire a temporary proxy confirmation capability instead of a perpetual one from the signer. That is, the signer not only can delegate the confirmation capability to the proxy confirmer, but also can revoke the proxy confirmer’s capability for avoiding the abuse. Moreover, our scheme also provides a technique to properly restrict the proxy confirmer to convincing only some specified verifiers that the signature is valid.