Podpisy niezaprzeczalne - podpisy z dodatkową funkcjonalnością

Today, the role of digital signatures becoming more and more significant. The signature provides authentication and integrity of a message but quite often other features like additional functionality is required from the signature. Because of that the algorithms used in ordinary signing are not sufficient, they need to be special algorithms with some additional characteristics. Undeniable signatures are a form of digital signature that have two distinctive features: the verification process is interactive (the signatures cannot be verified without signer's cooperation), a disavowal protocol allows to determine whether a given signature is a forgery. These signatures can be used, for example, to protect a software against unauthorized persons. The safety of these signatures is guaranteed by techniques of modern cryptography which is based on advanced mathematical tools and computer technology. The purpose of the article is to present selected schemes (protocols) of undeniable signatures (based on discrete log systems) such as Chaum-van Antwerpen protocol, Chaum-van Antwerpen protocol protecting against disavowal and a convertible undeniable signature protocol. Main definitions and theorems are presented and all described protocols are illustrated with examples.