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Perfect Ambiguous Optimistic Fair Exchange.

Yan Wang, Man Ho Au, Willy Susilo

Published Jan 1, 2012• 5 citations
OpenAlex

Abstract

Abstract. Protocol for fair exchange of digital signatures is essential in many applications including contract signing, electronic commerce, or even peer-to-peer file sharing. In such a protocol, two parties, Alice and Bob, would like to exchange digital signatures on some messages in a fair way. It is known that a trusted arbitrator is necessary in the realization of such a protocol. We identify that in some scenarios, it is required that prior to the completion of the protocol, no observer should be able to tell whether Alice and Bob are conducting such an exchange. Consider the following scenario in which Apple engages Intel in an exchange protocol to sign a contract that terminates their OEM agreement. The information would be of value to a third party (such as the stock broker, or other OEM companies). If the protocol transcript can serve as an evidence that such a communication is in progress, any observer of this communication, including the employees of both companies, would be tempted to capture the transcript and sell it to outsiders. We introduce a new notion called perfect ambiguous optimistic fair exchange (PAOFE), which is particularly suitable to the above scenario. PAOFE fulfils all traditional requirements of cryptographic fair exchange of digital signatures and, in addition, guarantees that the communication transcript cannot be used as a proof to convince others that the protocol is in progress. Specifically, we formalize the notion of PAOFE and present a rigorous security model in the multi-user setting under the chosen-key attack. We also present a generic construction of PAOFE from existing cryptographic primitives and prove that our proposal is secure with respect to our definition in the standard model. 1

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Contributions on fair exchange of digital signatures

Yang Wang

Fair exchange protocols aim to allow two parties to exchange digital items in a fair manner. Optimistic fair exchange (OFE) is a kind of protocols that solves the fair exchange problem with the help of a trusted third party (TTP), usually referred to as an `arbitrator'. The participation of the arbitrator is only required when there is a dispute between the exchanging parties. In the literature, the highest level of security of optimistic fair exchange is the multi-user security in the chosen-key model, proposed by Huang, Yang, Wong and Susilo in CT-RSA 2008. They showed that an efficient optimistic fair exchange scheme, secure in this sense, can be constructed generically from a conventional signature and a ring signature. In particular, the underlying ring signature is required to be unforgeable under an adaptive attack, against a static adversary in the 2-user setting.\nConcurrent signatures, introduced by Chen, Kudla and Paterson in Eurocrypt 2004, allow two parties to produce two ambiguous signatures until an extra piece of information, the keystone, is released by one of the parties. Upon the release of the keystone, the ambiguity will be revoked and the signatures bind to their true signers concurrently. Concurrent signatures, however, are known to fall just short of fully solving the long standing fair exchange of signatures problem. The price for not requiring any TTP is that the party who holds the keystone always has an advantage over the other party in controlling when and whether the protocol completes or not.\nIn this thesis, the fair exchange of digital signatures problem is studied. In the first part of this thesis, the OFE security is strengthened and a more practical model which is called enhanced chosen-key model is proposed. Unlike the existing multiuser setting and chosen-key model, an adversary in the enhanced chosen-key model is further provided with the signing oracle that outputs full signatures generated by the signer, and may even be allowed to have access to the arbitrator's secret key. The necessity for the new model is demonstrated. It is shown that two existing methodologies for constructing optimistic fair exchange schemes remain valid in the enhanced chosen-key model.\nNext, the efficiency of OFE constructions is improved. A model for 2-user ring signatures called `unforgeability against restricted adaptive attacks' is proposed and it is shown that the new ring model is strictly weaker than the model of unforgeability under an adaptive attack, against a static adversary in the 2-user setting. Based on the finding that 2-user ring signatures secure in this weaker model will suffice to guarantee the security of the resulting OFE schemes in the enhanced chosenkey model, the most efficient OFE scheme whose security does not rely on random oracles is proposed.\nNew situations in OFE are also investigated, and the applications of OFE are extended. The scenarios where the digital items to be exchanged are threshold signatures are considered, and the notion of threshold-oriented OFE (TOFE) together with a security model and an efficient concrete scheme is proposed. It is identified that in some scenarios, it is required that prior to the completion of the protocol, no information about the actual exchange should be revealed to an outsider. To achieve this purpose, the notion of perfect ambiguous optimistic fair exchange (PAOFE) is introduced, the security model for PAOFE is proposed and a generic construction based on existing primitives is offered. OFE in the attribute-based setting is considered and the notion of attribute-based optimistic fair exchange (ABOFE) is proposed. The security model and a construction for ABOFE are presented as well.\nFinally, concurrent signatures is revisited. It is showed that theoretically the party who controls the keystone in a concurrent signature scheme has more advantages than previously thought. In particular, the notion is examined and the advantages are classified into three levels.It is demonstrated that concurrent signatures may be abused in practical applications if one fails to take these advantages into account.

•Jan 1, 2014•0 citations

Revisiting Optimistic Fair Exchange Based on Ring Signatures

Yang Wang, Man Ho Au, Willy Susilo

Optimistic fair exchange (OFE) is a kind of protocol that solves the fair exchange problem with the help of a trusted third party, usually referred to as an arbitrator. Participation of the arbitrator is required only when there is a dispute among the exchanging parties. Thus, the majority of the executions of the exchange does not involve the arbitrator and hence the term optimistic. The passive nature of the arbitrator makes optimistic fair exchange a desirable tool in applications, such as contract signing and electronic commerce. The highest level of security of optimistic fair exchange in the literature is the multiuser security in the chosen-key model, proposed by Huang, Yang, Wong, and Susilo in CT-RSA 2008. They showed that an efficient optimistic fair exchange scheme secure in this sense can be constructed generically from a conventional digital signature and a two-party ring signature. In particular, the underlying ring signature is required to be unforgeable under an adaptive attack, against a static adversary in the 2-user setting. In this paper, we propose a new security model for two-party ring signatures called unforgeability against restricted adaptive attacks and demonstrate that our new model is strictly weaker than the model of unforgeable under an adaptive attack, against a static adversary in the 2-user setting. We make an observation that two-party ring signatures secure in this weaker model will suffice to guarantee the security of the resulting OFE scheme following the aforementioned generic construction. Based on this observation, more efficient OFE schemes secure in the standard model can be constructed. Specifically, we prove that the well-known Bender, Katz, and Morselli's 2-user ring signature is secure in our weakened model. Based on this two-party ring signature, we construct an OFE secure in the chosen-key model offering multiuser security in the standard model under the computational Diffie-Hellman assumption. The assumption is arguably weaker than those used in all existing constructions, which rely on the random oracle model, decisional assumptions, or the strong Diffie-Hellman assumptions. It is also worth noting that our scheme is the most efficient one in the standard model, and offers comparable efficiency against those secure under the random oracle model.

IEEE Transactions on Information Forensics and Security•Sep 17, 2014•1 citation

Group-Oriented Fair Exchange of Signatures.

Qiong Huang, Duncan S. Wong, Willy Susilo

In an Optimistic Fair Exchange (OFE) for digital signatures, two parties exchange their signatures fairly without requiring any online trusted third party. The third party is only involved when a dispute occurs. In all the previous work, OFE has been considered only in a setting where both of the communicating parties are individuals. There is little work discussing about the fair exchange between two groups of users, though we can see that this is actually a common scenario in actual OFE applications. In this paper, we introduce a new variant of OFE, called Group-Oriented Optimistic Fair Exchange (GOFE). A GOFE allows two users from two different groups to exchange signatures on behalf of their groups in a fair and anonymous manner. Although GOFE may be considered as a fair exchange for group signatures, it might be inefficient if it is constructed generically from a group signature scheme. Instead, we show that GOFE is backward compatible to the Ambiguous OFE (AOFE). Also, we propose an efficient and concrete construction of GOFE, and prove its security under the security models we propose in this model. The security of the scheme relies on the decision linear assumption and strong Diffie-Hellman assumption under the random

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Optimistic fair exchange of distributed signatures

Yujue Wang, Bo Qin, Qianhong Wu +3 more

Consider a scenario in which one party A consisting of a number of manufactories would like to sign a contract with another party B who is a selling company so that they can unify prices to avoid over competition. This may be done via Internet to reduce travelling cost. To cater for such applications, we propose the concept of optimistic fair exchange of distributed signatures (OFEDS) which allows an authorized set of signers to fairly exchange signatures with a verifier, and a trusted arbitrator will intervene in the protocol only when there are disputes between two parties. We consider the security requirements for OFEDS, in which besides the standard security requirements for existing optimistic fair exchange protocols, robustness is introduced to ensure that OFEDS can be successfully performed even when there exist some dishonest signers. We instantiate a non-interactive construction of OFEDS based on the Computational Diffie-Hellman (CDH) assumption in the random oracle model. Our work shows that for any general monotone access structure, there exists a CDH-based OFEDS protocol if all the involved signers are honest.

Communications Security Conference (CSC)•Jan 1, 2014•3 citations

Generic Construction of Privacy-Preserving Optimistic Fair Exchange Protocols.

Qingwen Guo, Yuzhao Cui, Xiaomeng Zou +1 more

Privacy-preserving optimistic fair exchange (P2OFE) is a kind of protocols which aim to solve the fairness problem in the exchange of digital signatures between two parties and in the meanwhile protect their privacy. In P2OFE, no one else including the semi-trusted third party in charge of arbitration can confirm an exchange even after resolving a dispute. In this paper we present a black-box construction of P2OFE based on a tag-based public key encryption scheme and a standard digital signature scheme. Our construction follows the ‘sign-then-encrypt’ paradigm, and is secure in the standard model. Our construction is generic and admits more instantiations of P2OFE.

Journal of Internet Services and Information Security•Jan 1, 2017•3 citations

A traceable optimistic fair exchange protocol

Ramin Ganjavi, Maryam Rajabzadeh Asaar, Mahmoud Salmasizadeh

An Optimistic fair exchange (OFE) protocol is a good way for two parties to exchange their digital items in a fair way such that at the end of the protocol, both of them receive their items or none of them receives anything. In 2008, Huang et al. presented an efficient OFE protocol, but there is a security problem in their scheme. If the trusted third party (TTP) is dishonest and colludes with the verifier, the TTP can complete the transaction without getting signer's agreement. To solve this problem, we propose an OFE protocol in which there is a tracing algorithm to detect the party who completes the transaction. Furthermore, we obtain the accountability property introduced by Huang et al. in 2011, more efficiently and in the standard model while their protocol is in the random oracle model. We prove the security of our scheme under the chosen-key model and multi-user setting in the standard model.

International ISC Conference on Information Security and Cryptology (ISCISC)•Sep 1, 2014•2 citations
Abstract. Protocol for fair exchange of digital signatures is essential in many applications including contract signing, electronic commerce, or even peer-to-peer file sharing. In such a protocol, two parties, Alice and Bob, would like to exchange digital signatures on some messages in a fair way. It is known that a trusted arbitrator is necessary in the realization of such a protocol. We identify that in some scenarios, it is required that prior to the completion of the protocol, no observer should be able to tell whether Alice and Bob are conducting such an exchange. Consider the following scenario in which Apple engages Intel in an exchange protocol to sign a contract that terminates their OEM agreement. The information would be of value to a third party (such as the stock broker, or other OEM companies). If the protocol transcript can serve as an evidence that such a communication is in progress, any observer of this communication, including the employees of both companies, would be tempted to capture the transcript and sell it to outsiders. We introduce a new notion called perfect ambiguous optimistic fair exchange (PAOFE), which is particularly suitable to the above scenario. PAOFE fulfils all traditional requirements of cryptographic fair exchange of digital signatures and, in addition, guarantees that the communication transcript cannot be used as a proof to convince others that the protocol is in progress. Specifically, we formalize the notion of PAOFE and present a rigorous security model in the multi-user setting under the chosen-key attack. We also present a generic construction of PAOFE from existing cryptographic primitives and prove that our proposal is secure with respect to our definition in the standard model. 1
Fair exchange protocols aim to allow two parties to exchange digital items in a fair manner. Optimistic fair exchange (OFE) is a kind of protocols that solves the fair exchange problem with the help of a trusted third party (TTP), usually referred to as an `arbitrator'. The participation of the arbitrator is only required when there is a dispute between the exchanging parties. In the literature, the highest level of security of optimistic fair exchange is the multi-user security in the chosen-key model, proposed by Huang, Yang, Wong and Susilo in CT-RSA 2008. They showed that an efficient optimistic fair exchange scheme, secure in this sense, can be constructed generically from a conventional signature and a ring signature. In particular, the underlying ring signature is required to be unforgeable under an adaptive attack, against a static adversary in the 2-user setting.\nConcurrent signatures, introduced by Chen, Kudla and Paterson in Eurocrypt 2004, allow two parties to produce two ambiguous signatures until an extra piece of information, the keystone, is released by one of the parties. Upon the release of the keystone, the ambiguity will be revoked and the signatures bind to their true signers concurrently. Concurrent signatures, however, are known to fall just short of fully solving the long standing fair exchange of signatures problem. The price for not requiring any TTP is that the party who holds the keystone always has an advantage over the other party in controlling when and whether the protocol completes or not.\nIn this thesis, the fair exchange of digital signatures problem is studied. In the first part of this thesis, the OFE security is strengthened and a more practical model which is called enhanced chosen-key model is proposed. Unlike the existing multiuser setting and chosen-key model, an adversary in the enhanced chosen-key model is further provided with the signing oracle that outputs full signatures generated by the signer, and may even be allowed to have access to the arbitrator's secret key. The necessity for the new model is demonstrated. It is shown that two existing methodologies for constructing optimistic fair exchange schemes remain valid in the enhanced chosen-key model.\nNext, the efficiency of OFE constructions is improved. A model for 2-user ring signatures called `unforgeability against restricted adaptive attacks' is proposed and it is shown that the new ring model is strictly weaker than the model of unforgeability under an adaptive attack, against a static adversary in the 2-user setting. Based on the finding that 2-user ring signatures secure in this weaker model will suffice to guarantee the security of the resulting OFE schemes in the enhanced chosenkey model, the most efficient OFE scheme whose security does not rely on random oracles is proposed.\nNew situations in OFE are also investigated, and the applications of OFE are extended. The scenarios where the digital items to be exchanged are threshold signatures are considered, and the notion of threshold-oriented OFE (TOFE) together with a security model and an efficient concrete scheme is proposed. It is identified that in some scenarios, it is required that prior to the completion of the protocol, no information about the actual exchange should be revealed to an outsider. To achieve this purpose, the notion of perfect ambiguous optimistic fair exchange (PAOFE) is introduced, the security model for PAOFE is proposed and a generic construction based on existing primitives is offered. OFE in the attribute-based setting is considered and the notion of attribute-based optimistic fair exchange (ABOFE) is proposed. The security model and a construction for ABOFE are presented as well.\nFinally, concurrent signatures is revisited. It is showed that theoretically the party who controls the keystone in a concurrent signature scheme has more advantages than previously thought. In particular, the notion is examined and the advantages are classified into three levels.It is demonstrated that concurrent signatures may be abused in practical applications if one fails to take these advantages into account.
Optimistic fair exchange (OFE) is a kind of protocol that solves the fair exchange problem with the help of a trusted third party, usually referred to as an arbitrator. Participation of the arbitrator is required only when there is a dispute among the exchanging parties. Thus, the majority of the executions of the exchange does not involve the arbitrator and hence the term optimistic. The passive nature of the arbitrator makes optimistic fair exchange a desirable tool in applications, such as contract signing and electronic commerce. The highest level of security of optimistic fair exchange in the literature is the multiuser security in the chosen-key model, proposed by Huang, Yang, Wong, and Susilo in CT-RSA 2008. They showed that an efficient optimistic fair exchange scheme secure in this sense can be constructed generically from a conventional digital signature and a two-party ring signature. In particular, the underlying ring signature is required to be unforgeable under an adaptive attack, against a static adversary in the 2-user setting. In this paper, we propose a new security model for two-party ring signatures called unforgeability against restricted adaptive attacks and demonstrate that our new model is strictly weaker than the model of unforgeable under an adaptive attack, against a static adversary in the 2-user setting. We make an observation that two-party ring signatures secure in this weaker model will suffice to guarantee the security of the resulting OFE scheme following the aforementioned generic construction. Based on this observation, more efficient OFE schemes secure in the standard model can be constructed. Specifically, we prove that the well-known Bender, Katz, and Morselli's 2-user ring signature is secure in our weakened model. Based on this two-party ring signature, we construct an OFE secure in the chosen-key model offering multiuser security in the standard model under the computational Diffie-Hellman assumption. The assumption is arguably weaker than those used in all existing constructions, which rely on the random oracle model, decisional assumptions, or the strong Diffie-Hellman assumptions. It is also worth noting that our scheme is the most efficient one in the standard model, and offers comparable efficiency against those secure under the random oracle model.
In an Optimistic Fair Exchange (OFE) for digital signatures, two parties exchange their signatures fairly without requiring any online trusted third party. The third party is only involved when a dispute occurs. In all the previous work, OFE has been considered only in a setting where both of the communicating parties are individuals. There is little work discussing about the fair exchange between two groups of users, though we can see that this is actually a common scenario in actual OFE applications. In this paper, we introduce a new variant of OFE, called Group-Oriented Optimistic Fair Exchange (GOFE). A GOFE allows two users from two different groups to exchange signatures on behalf of their groups in a fair and anonymous manner. Although GOFE may be considered as a fair exchange for group signatures, it might be inefficient if it is constructed generically from a group signature scheme. Instead, we show that GOFE is backward compatible to the Ambiguous OFE (AOFE). Also, we propose an efficient and concrete construction of GOFE, and prove its security under the security models we propose in this model. The security of the scheme relies on the decision linear assumption and strong Diffie-Hellman assumption under the random
Consider a scenario in which one party A consisting of a number of manufactories would like to sign a contract with another party B who is a selling company so that they can unify prices to avoid over competition. This may be done via Internet to reduce travelling cost. To cater for such applications, we propose the concept of optimistic fair exchange of distributed signatures (OFEDS) which allows an authorized set of signers to fairly exchange signatures with a verifier, and a trusted arbitrator will intervene in the protocol only when there are disputes between two parties. We consider the security requirements for OFEDS, in which besides the standard security requirements for existing optimistic fair exchange protocols, robustness is introduced to ensure that OFEDS can be successfully performed even when there exist some dishonest signers. We instantiate a non-interactive construction of OFEDS based on the Computational Diffie-Hellman (CDH) assumption in the random oracle model. Our work shows that for any general monotone access structure, there exists a CDH-based OFEDS protocol if all the involved signers are honest.
Privacy-preserving optimistic fair exchange (P2OFE) is a kind of protocols which aim to solve the fairness problem in the exchange of digital signatures between two parties and in the meanwhile protect their privacy. In P2OFE, no one else including the semi-trusted third party in charge of arbitration can confirm an exchange even after resolving a dispute. In this paper we present a black-box construction of P2OFE based on a tag-based public key encryption scheme and a standard digital signature scheme. Our construction follows the ‘sign-then-encrypt’ paradigm, and is secure in the standard model. Our construction is generic and admits more instantiations of P2OFE.
An Optimistic fair exchange (OFE) protocol is a good way for two parties to exchange their digital items in a fair way such that at the end of the protocol, both of them receive their items or none of them receives anything. In 2008, Huang et al. presented an efficient OFE protocol, but there is a security problem in their scheme. If the trusted third party (TTP) is dishonest and colludes with the verifier, the TTP can complete the transaction without getting signer's agreement. To solve this problem, we propose an OFE protocol in which there is a tracing algorithm to detect the party who completes the transaction. Furthermore, we obtain the accountability property introduced by Huang et al. in 2011, more efficiently and in the standard model while their protocol is in the random oracle model. We prove the security of our scheme under the chosen-key model and multi-user setting in the standard model.