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Contributions on fair exchange of digital signatures

Yang Wang

Published Jan 1, 2014• 0 citations
OpenAlex

Abstract

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.

Related papers

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

Collusion-Resistance in Optimistic Fair Exchange

Yang Wang, Willy Susilo, Man Ho Au +1 more

Optimistic fair exchange (OFE) is a type of cryptographic protocols aimed at solving the fair exchange problem over open networks with the help of a third party to settle disputes between exchanging parties. It is well known that a third party is necessary in the realization of a fair exchange protocol. However, a fully trusted third party may not be available over open networks. In this paper, the security of most of the proposed OFE protocols depends on the assumption that the third party is semitrusted in the sense that it may misbehave on its own but does not conspire with either of the main parties. The existing security models of OFE have not taken into account the case where the potentially dishonest third party may collude with a signer in the sense of sharing its secret key with the signer. In this paper, to reduce the trust level of the arbitrator and increase the security of OFE, we propose an enhanced security model that, for the first time, captures this scenario. We also show a separation between the existing model and our enhanced model with a concrete counter example. Finally, we revisit two popular approaches in the construction of OFE protocols, which are based on verifiably encrypted signature and conventional signature plus ring signature, respectively. Our result shows that the conventional signature plus ring signature approach approach remains valid in our enhanced model. However, for schemes based on verifiably encrypted signature, slight modifications are needed to guarantee the security.

IEEE Transactions on Information Forensics and Security•May 21, 2014•2 citations

Revisiting Security Against the Arbitrator in Optimistic Fair Exchange

Yang Wang, Willy Susilo, Man Ho Au

The problem to allow two parties (say Alice and Bob) to sign an electronic contract online fairly is an interesting problem and it has been studied for a long time. Optimistic fair exchange (OFE) is an efficient remedy to this problem, which incorporates a semi-trusted third party called arbitrator, who will be called in to resolve a dispute during an exchange between Alice and Bob. It should be noted that the arbitrator is not required to be fully trusted, or else such an entity may not be available in practice. To reduce the trust level of the arbitrator, the requirement that the arbitrator should not be able to produce a full signature without the signer's content has been proposed in literature. Nevertheless, we observe that the existing OFE models do not capture the realistic situation that the arbitrator itself should not be able to generate a partial signature on a new message. This requirement is essential since we only put a partial trust towards the arbitrator, but on the other hand, we also assume that the arbitrator will not do such a forgery. Therefore, to reflect this situation, we propose an enhanced model of OFE that explicitly captures this requirement. We demonstrate the difference between our enhanced model and the existing chosen-key model through a concrete OFE scheme that serves as a counterexample. Since our model is strictly stronger than the existing model, we investigate the security of the existing schemes in our enhanced model. Interestingly, we show that OFE schemes based on verifiably encrypted signature and those based on ring signature can remain secure in our enhanced model given slight modifications to the primitives.

The Computer Journal•Feb 12, 2015•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

•Jan 1, 2009•0 citations

An optimistic fair exchange protocol for proxy signatures

Kosar Ghorbani, Maryam Rajabzadeh Asaar, Mahmoud Salmasizadeh

In this paper we present an optimistic fair exchange (OFE) protocol for proxy signatures. This protocol is useful in scenarios that a signer cannot participate in an OFE protocol himself and he needs to delegate his signing power to another one. We give a new security model for this type of protocols because the best and the most complete model until now does not consider all security aspects of this protocol. We construct a generic scheme and give a paradigm for it. Our scheme is setup-free and multi-user secure in the random oracle and chosen-key model.

International Symposium on Telecommunications•Sep 1, 2014•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
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.
Optimistic fair exchange (OFE) is a type of cryptographic protocols aimed at solving the fair exchange problem over open networks with the help of a third party to settle disputes between exchanging parties. It is well known that a third party is necessary in the realization of a fair exchange protocol. However, a fully trusted third party may not be available over open networks. In this paper, the security of most of the proposed OFE protocols depends on the assumption that the third party is semitrusted in the sense that it may misbehave on its own but does not conspire with either of the main parties. The existing security models of OFE have not taken into account the case where the potentially dishonest third party may collude with a signer in the sense of sharing its secret key with the signer. In this paper, to reduce the trust level of the arbitrator and increase the security of OFE, we propose an enhanced security model that, for the first time, captures this scenario. We also show a separation between the existing model and our enhanced model with a concrete counter example. Finally, we revisit two popular approaches in the construction of OFE protocols, which are based on verifiably encrypted signature and conventional signature plus ring signature, respectively. Our result shows that the conventional signature plus ring signature approach approach remains valid in our enhanced model. However, for schemes based on verifiably encrypted signature, slight modifications are needed to guarantee the security.
The problem to allow two parties (say Alice and Bob) to sign an electronic contract online fairly is an interesting problem and it has been studied for a long time. Optimistic fair exchange (OFE) is an efficient remedy to this problem, which incorporates a semi-trusted third party called arbitrator, who will be called in to resolve a dispute during an exchange between Alice and Bob. It should be noted that the arbitrator is not required to be fully trusted, or else such an entity may not be available in practice. To reduce the trust level of the arbitrator, the requirement that the arbitrator should not be able to produce a full signature without the signer's content has been proposed in literature. Nevertheless, we observe that the existing OFE models do not capture the realistic situation that the arbitrator itself should not be able to generate a partial signature on a new message. This requirement is essential since we only put a partial trust towards the arbitrator, but on the other hand, we also assume that the arbitrator will not do such a forgery. Therefore, to reflect this situation, we propose an enhanced model of OFE that explicitly captures this requirement. We demonstrate the difference between our enhanced model and the existing chosen-key model through a concrete OFE scheme that serves as a counterexample. Since our model is strictly stronger than the existing model, we investigate the security of the existing schemes in our enhanced model. Interestingly, we show that OFE schemes based on verifiably encrypted signature and those based on ring signature can remain secure in our enhanced model given slight modifications to the primitives.
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
In this paper we present an optimistic fair exchange (OFE) protocol for proxy signatures. This protocol is useful in scenarios that a signer cannot participate in an OFE protocol himself and he needs to delegate his signing power to another one. We give a new security model for this type of protocols because the best and the most complete model until now does not consider all security aspects of this protocol. We construct a generic scheme and give a paradigm for it. Our scheme is setup-free and multi-user secure in the random oracle and chosen-key model.
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.