Efficient Convertible Undeniable Signature Schemes
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 extension to threshold signatures, where the capability of signing (and of verifying signatures) is shared among n parties using a t out of n threshold scheme.
[Mic97] Michels M. and Stadler M.. Efficient Convertible Undeniable Signature Schemes. In Proc. 4th. Workshop on Selected Areas in Cryptography (SAC'97), pages 231-244, Ottawa, Canada, 1997.