How to Protect a Signature from Being Shown to a Third Party

Many attempts to controlling who and under which circumstances can verify our signatures have been made so far. For this purpose one can use undeniable signatures, designated confirmer signatures or designated verifier signatures. We introduce a model of new kind of signatures, called dedicated digital signatures (or dds for short). The core idea is that a designated verifier can present a standard signature of the signer derived from dds to a third party, but at the price of revealing the private key of the designated verifier or at the price of revealing the designated verifier’s signature of a particular message. Therefore the verifier will show the signature only in very special situations. We present a construction of a dds based on ElGamal signatures and its modifications that allow to obtain additional important features.