A protocol is a series of steps, involving two or more entities, designed to accomplish a task. If mistrusting entities interact in non-face-to-face way over an open network environment, such as the Internet, various security requirements can occur, and they can be satisfied by using cryptographic technologies. A crytographic protocol is a protocol that uses cryptography to achieve the task and to satisfy security requirements together.
Recently, with the advance of the Internet, many traditional off-line services, such as voting, cash, and auction, are moving to online services over the Internet.Because these real world applications are familiar with us, various and complicated security requirements are already existent. To satisfy them, proper cryptographic primitive technologies should be used in well-designed way. Efficiency or performance issue is also very important in the real world. To improve efficiency, some application protocols are designed assuming the existence of trusted third party(TTP)who provides special service. Note that TTP is very familiar experience in our real world. But, in non-face-to-face interaction over an open network environment, trusting somebody over the network is difficult , therefore reducing the trustedness on TTP is an important research issue.
In this thesis ,we try to review recent active researches on real world cryptographic protocols, such as electronic voting, electronic auction, mobile signature on a message and a condition together. By imposing a signer-chosen condition which describes expected action of a specified receiver, conditional signature can be used as a private negotiation statement in two-party communication. We model negotiation problem using conditional signature and then construct a fair exchange protocol. We show that matching negotiation and real exchange give a fair exchange of digital signatures.