A group signature, proposed by Chaum and van Heyst, allows a group member to sign messages anonymously on behalf of the group. A group signature has a feature of tracing, that is, the identity of a signer can be revealed by a designated entity in case of dispute. The function of tracing spoils the property of anonymity to the designated entity because he can identify the signer of a group signature by using the tracing algorithm. To enhance the property of anonymity to one entity, a tracing manager is often introduced independently to the group manager, but this means that the tracing manager spoils the property of anonymity. In case of group signature with a CRL-based revocation scheme, the revocation function also spoils the property of anonymity to a designated entity. To sum up, there has been a single entity who can break the anonymity for any group signature scheme. In this paper, we present a privacy-enhanced group signature scheme with CRL-based revocation by introducing an idea of the threshold cryptosystems. We divide one group manager into two equal and independent managers. Our scheme satisfies the property of anonymity to any single group manager unless both are collude.
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