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arXiv:0912.3980

Fair Exchange of Digital Signatures using RSA-based CEMBS and Offline STTP

Jamal Ali Hussein, Mumtaz A. AlMukhtar

Published Dec 20, 2009• 0 citations
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Abstract

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.

DOI
10.48550/arxiv.0912.3980

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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 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.
This work presents a novel protocol for fair exchange of DSA-based signatures to support an e-commerce activity of digital contract signing. The protocol employs an off-line semitrusted third party (STTP), which participates in the protocol execution only in exceptional circumstances and which involvement in the protocol is transparent. The protocol can tolerate the STTP's own misbehaviour (i.e. if it does not collude with other participants of the protocol) and it prevents the STTP from gaining the exchanged signatures or the signed document. The protocol is based on the principle of verifiable and recoverable signature encryption, and in comparison with related protocols is more efficient.
The difference between the fair exchange protocols and the non-repudiation protocols is proposed. Based on the publicly verifiable secret sharing scheme and group encryption scheme, a new multi-party fair exchagne protocol with an off-line semi-Trusted Third Party(off-STTP) is presented. The off-STTP who need not be completely trusted intervenes into the exchange in case of trouble. That the off-STTP can only decipher a sub-secret key makes sure the confidentiality of exchange data and the true fairness of the barter. The participants can barter with others at their will and the exchange topology is unknown to others including the off-STTP.
Fairness is the basic requirement of E-Commerce protocols. RSA is one of the most widely used cryptosystems. A fair-exchange protocol allows two parties to exchange items in a fair way so that either each party gets the other's item, or neither party does. In this paper construction and architecture of the existing fair exchange protocols are analyzed. Both practicality and efficiency problems of these protocols are also presented. Based on this analysis, an optimistic fair exchange protocol totally based on RSA signature scheme is proposed. The novel scheme employs verifiably encrypted RSA signatures in the extended integer ring that is elaborately constructed. The security and efficiency of the newly devised scheme are also proved and examined. It is showed that the proposed scheme is secure and efficient.
Based on the verifiably encrypted signature scheme without random oracles recently proposed by Gorantla et al.,a new scheme of optimistic fair exchange of digital signatures is proposed in this paper.The two players firstly exchange their verifiably encrypted signature on agreed-upon messages then their ordinary signature after the verification of each other's verifiably encrypted signature.If one of them executes the protocol dishonestly,the other can resort to a trusted third party so as to get the purpose of fair exchange of signatures.The proposed scheme has properties of short signature,low computation and verification cost.It can realize exchange of digital signatures fairly and efficiently.
A fair contract-signing protocol allows two potentially mistrusted parities to exchange their commitments (i.e., digital signatures) to an agreed contract over the Internet in afairway, so that either each of them obtains the other's signature, or neither party does. Based on the RSA signature scheme, a new digital contract-signing protocol is proposed in this paper. Like the existing RSA-based solutions for the same problem, our protocol is not only fair, but also optimistic, since the trusted third party is involved only in the situations where one party is cheating or the communication channel is interrupted. Furthermore, the proposed protocol satisfies a new property-abuse-freeness. That is, if the protocol is executed unsuccessfully, none of the two parties can show the validity of intermediate results to others. Technical details are provided to analyze the security and performance of the proposed protocol. In summary, we present the first abuse-free fair contract-signing protocol based on the RSA signature, and show that it is both secure and efficient.