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IEEE Symposium on Security and Privacy (S&P)

Efficient and practical fair exchange protocols with off-line TTP

Feng Bao, Robert Huijie Deng, Wenbo Mao

Published Nov 27, 2002• 295 citations

Abstract

We present protocols for fair exchange of electronic data (digital signatures, payment and confidential data) between two parties A and B. Novel properties of the proposed protocols include: 1) offline trusted third party (TTP), i.e., TTP does not take part in the exchange unless one of the parties behaves improperly; 2) only three message exchanges are required in the normal situation; 3) true fair exchange, i.e., either A and B obtain each other's data or no party receives anything useful; no loss can be incurred to a party no matter how maliciously the other party behaves during the exchange. This last property is in contrast to previously proposed protocols with offline TTP ([1] and [21]), where a misbehaving party may get another party's data while refusing to send his document to the other party, and the TTP can provide affidavits attesting to what happened during the exchange. To our knowledge, the protocols presented here are the first exchange protocols which use offline TTP and at the same time guarantee true fair exchange of digital messages. We introduce a novel cryptographic primitive, called the Certificate of Encrypted Message Being a Signature (CEMBS), as the basic building block of the fair exchange protocols. It is used to prove that an encrypted message is a certain party's signature on a public file, without revealing the signature. We also give two examples to show in detail how the certificate can be constructed.

DOI: 10.1109/secpri.1998.674825

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We present protocols for fair exchange of electronic data (digital signatures, payment and confidential data) between two parties A and B. Novel properties of the proposed protocols include: 1) offline trusted third party (TTP), i.e., TTP does not take part in the exchange unless one of the parties behaves improperly; 2) only three message exchanges are required in the normal situation; 3) true fair exchange, i.e., either A and B obtain each other's data or no party receives anything useful; no loss can be incurred to a party no matter how maliciously the other party behaves during the exchange. This last property is in contrast to previously proposed protocols with offline TTP ([1] and [21]), where a misbehaving party may get another party's data while refusing to send his document to the other party, and the TTP can provide affidavits attesting to what happened during the exchange. To our knowledge, the protocols presented here are the first exchange protocols which use offline TTP and at the same time guarantee true fair exchange of digital messages. We introduce a novel cryptographic primitive, called the Certificate of Encrypted Message Being a Signature (CEMBS), as the basic building block of the fair exchange protocols. It is used to prove that an encrypted message is a certain party's signature on a public file, without revealing the signature. We also give two examples to show in detail how the certificate can be constructed.

One of the essential security services needed to safeguard online transactions is fair exchange. In fair exchange protocols two parties can exchange their signatures in a fair manner, so that either each party gain the other's signature or no one obtain anything useful. This paper examines security solutions for achieving fair exchange. It proposes new security protocols based on the "Certified Encrypted Message Being Signature" (CEMBS) by using RSA signature scheme. This protocol relies on the help of an "off-line Semi-Trusted Third Party" (STTP) to achieve fairness. They provide with confidential protection from the STTP for the exchanged items by limiting the role and power of the STTP. Three different protocols have been proposed. In the first protocol, the two main parties exchange their signatures on a common message. In the second protocol, the signatures are exchanged on two different messages. While in the third one, the exchange is between confidential data and signature.

we propose a new efficient protocol. which allows a pair of potentially mistrusting parties to exchange digital signatures over the Internet in a fair way. The protocol relies on an off-line Trusted Third Party (TTP). which does not take part in the exchange unless any of the parties behaves improperly or other faults occur. Efficiency of the protocol is achieved by using a cryptographic primitive, called designated confirmer signatures. The confirmer commitment schemes and ordinary signatures are combines to form the confirmer signatures. The security of the protocol is analyzed and the fairness of it is guaranteed.

A non-repudiation protocol enables the fair exchange of an electronic message and an irrefutable digital receipt between two mistrusting parties over the Internet. That is, at the end of any execution instance of such a protocol, either both parties obtain their expected items or neither party does . In this paper, we first argue that it is really meaningful in practice to exploit generic fair non-repudiation protocols with transparent off-line trusted third party (TTP). Namely, in those protocols, each involved party could use any secure digital signature algorithm to produce non-repudiation evidences; and the issued evidences are the same regardless of whether the TTP is involved or not. Then, we present such a fair non-repudiation protocol to overcome some limitations and shortcomings in previous schemes. Technical discussions are provided to show that our protocol is not only secure but also the most efficient solution, compared with existing non-repudiation protocols. In addition, some potential extensions are also pointed out.

We propose a new ecient protocol, which allows a pair of potentially mistrusting parties to exchange digital signatures over the Internet in a fair way, such that after the protocol is running, either each party obtains the other's signature, or neither of them does. The pro- tocol relies on an o-line Trusted Third Party (TTP), which does not take part in the exchange unless any of the parties behaves improperly or other faults occur. Eciency of the protocol is achieved by using a cryptographic primitive, called conrmable signatures (or designated con- rmer signatures in its original proposal (9)). We recommend using a new ecient conrmable signature scheme in the proposed fair exchange pro- tocol. This scheme combines the family of discrete logarithm (DL) based signature algorithms and a zero-knowledge (ZK) proof on the equality of two DLs. The protocol has a practical level of performance: only a moderate number of communication rounds and ordinary signatures are required. The security of the protocol can be established from that of the underlying signature algorithms and that of the ZK proof used.

In this paper, a new fair certified email protocol is proposed. The proposed protocol uses offline trusted third party (TTP) that will only be involved if a party misbehaves, otherwise the TTP is inactive. The protocol is based on the verifiable and recoverable encryption of keys. In this regard, the recipient of an email will be able to verify if the encrypted key used to encrypt the email is correct. The protocol comprises of only three messages in the exchange sub-protocol and three messages in the recovery sub-protocol. The proposed protocol is more efficient compared with related protocols.