Group signature schemes are fundamental cryptographic tools that enable unlinkably anonymous authentication, in the same fashion that digital signatures provide the basis for strong authentication protocols. In this paper we present the first group signature scheme with constantsize parameters that does not employ any trapdoor function. This novel type of group signature scheme allows public parameters to be shared among organizations. Such sharing represents a highly desirable simpli cation over existing schemes, which require each organization to maintain a separate cryptographic domain.

Undeniable signatures are digital signatures which are not universally verifiable but can only be checked with the signer's help. However, the signer cannot deny the validity of a correct signature. An extended concept, convertible undeniable signatures, allows the signer to convert single undeniable signatures or even the whole scheme into universally verifiable signatures or into an ordinary digital signature scheme, respectively. In this paper we propose a new convertible undeniable signature scheme and provide proofs for all relevant security properties. The scheme is based on Schnorr's signature scheme and it is efficient. Unlike previous efficient solutions, this new scheme can be used as a basis for an efficient...