How to Convert any Digital Signature Scheme into a Group Signature Scheme

Group signatures, introduced by Chaum and van Heijst, allow members of a group to sign messages anonymously on behalf of the group. In this paper we show, how any digital signature scheme can be converted into a group signature scheme. This solves an open problem posed by Chaum and van Heijst. To obtain this result, the encrypted identity of the signer is signed together with the message. The correct-ness of the encryption is ensured by using the technique of an indirect discourse proof, introduced in FrTY96]. It proves that the group center has the future capability to decrypt the signers identity. Furthermore, an eecient extension to the general case of a (t; n) threshold group signature scheme with 2 t < n anonymous signers is described. Finally, we discuss some extensions of both schemes, like selective and total conversion into ordinary digital signatures. Abstract. Group signatures, introduced by Chaum and van Heijst, allow members of a group to sign messages anonymously on behalf of the group. In this paper we show, how any digital signature scheme can be converted into a group signature scheme. This solves an open problem posed by Chaum and van Heijst. To obtain this result, the encrypted identity of the signer is signed together with the message. The correct-ness of the encryption is ensured by using the technique of an indirect discourse proof, introduced in FrTY96]. It proves that the group center has the future capability to decrypt the signers identity. Furthermore, an eecient extension to the general case of a (t; n) threshold group signature scheme with 2 t < n anonymous signers is described. Finally, we discuss some extensions of both schemes, like selective and total conversion into ordinary digital signatures.