On Delegatability of Some Strong Designated Verifier Signature Schemes

A strong designated verifier signature scheme makes it possible for a signer to convince a designated verifier that she has signed a message in such a way that the designated verifier cannot transfer the signature to a third party, and no third party can even verify the validity of a designated verifier signature. In 2005, Lipmaa, Wang, and Bao identified a new essential security property, non delegatability, of designated verifier signature schemes. Briefly, in a non delegatability designated verifier signature scheme, neither a signer nor a designated verifier can delegate the signing rights to any third party without revealing their secret keys. However, this paper shows that four recently proposed strong designated verifier signature schemes are delegatable. These schemes do not satisfy non delegatability secure requirement of strong designated verifier signature schemes.