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Design of a Traceable Optimistic Fair Exchange Protocol

Ganjavi, Ramin | 2014

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 46627 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Salmasizadeh, Mahmoud
  7. Abstract:
  8. Fair exchange of digital items through computer network is an important research topic of modern cryptography. Generallty, a fair exchange protocol is a way which helps two parties to exchange their digital items fairly, so that at the end of the protocol execution, both parties recieve their desirable items or none of them recieves anything. In a practical applications, fair exchange protocol is widely used in different but related fields like contract signing protocols,non-repudiation protocols,e-ayment system and certified e-mails. In a fair exchange protocol, a trusted third party (TTP) acts as an arbitrator between two parties. To reduce the traffic load of the arbitrator, the concept of optimistic fair exchange (OFE) protocols proposed in which the arbitrator’s involvement in the protocol is off-line. In these protocols, the arbitrator is semi-trusted between the signer and the verifier and interferes just when a dispute occurs. In previous studies, in order to make OFE protocols more efficient, prevent verifier from abusing and to protect the anonymity of a signer in a group, several OFE protocols presented that have some security problems in situations like colluding between the verifier and the arbitrator or between the signer in a group and the verifier.In this thesis, to prevent the collusion between the verifier and the arbitrator,we propose a traceable OFE protocol. First, we define the concept of traceable OFE and present a security model that is multi-user setting and chosen-key model. Then, we propose a generic scheme and prove its security in the multi-user setting and chosen-key model under the standard model.We also present a concrete scheme in order to realize our generic scheme.All schemes proposed so far have either the partial signature ambiguity or the full signature ambiguity, and without any of these properties there would be security problems. We propose the firs OFE protocol that has the ambiguity in both the partial and the full signature along with the traceability property. We define this protocol as well and present a security model that is multi-user setting and chosen-key model. Then, we propose a generic scheme and prove its security in the multi-user setting and chosen-key model under the standard model. We also present a concrete scheme in order to realize our generic scheme.Since both presented protocol have the ability to trace the signer, it is not possible for the signer in a group to colludes with the verifier. Also due to the possibility to trace the identity of the signer, these protocols have the accountability property
  9. Keywords:
  10. Provable Security ; Optimistic Fair Exchange ; Traceability

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