Step-out group signatures

Group signature schemes enable to create digital signatures such that the signers are hidden in a group of potential signers. However, in a case of need it is possible to reveal the actual signer either by a group administrator or collectively by the group members. We design a new kind of signatures that we call step-out group signature where the situation is reversed: any member of the group except the signer may prove that he or she was not the signer. This is a dual solution that is useful in certain scenarios: in many cases it is unnecessary to find the signer, it suffices to eliminate some potential signers (e.g. during prosecutions and court procedures). Our solution is more convenient for implementing personal data protection rules: since the signer is not revealed, there is no need to protect this information. On the other hand, the traditional scheme may lead to serious legal problems: if the legal case is to find out whether Bob has created group signature s , it might be illegal to reveal that Alice has created s .