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International journal of network security

Cryptanalysis of Syverson's Rational Exchange Protocol

Almudena Alcaide, Juan Tapiador, Julio Cesar Hernandez-Castro, Arturo Ribagorda

Published Sep 1, 2008• 2 citations
OpenAlex

Abstract

The notion of rational exchange introduced by Syverson in 1998 is a particularly interesting alternative when an e?cient scheme for fair exchange is required but the existence of a trusted third party simply cannot be assumed. A rational exchange protocol cannot provide true fairness, but it ensures that rational –i.e. self-interested– parties would have no reason to deviate from the protocol. In this paper, we identify some weaknesses in Syverson’s rational exchange protocol which were neither detected by the original author nor by subsequent analysis. After presenting some attacks, we indicate how the scheme should be modi?ed to overcome these vulnerabilities. We also provide a formal analysis of our enhancement using BAN logic.

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The notion of rational exchange introduced by Syverson in 1998 is a particularly interesting alternative when an e?cient scheme for fair exchange is required but the existence of a trusted third party simply cannot be assumed. A rational exchange protocol cannot provide true fairness, but it ensures that rational –i.e. self-interested– parties would have no reason to deviate from the protocol. In this paper, we identify some weaknesses in Syverson’s rational exchange protocol which were neither detected by the original author nor by subsequent analysis. After presenting some attacks, we indicate how the scheme should be modi?ed to overcome these vulnerabilities. We also provide a formal analysis of our enhancement using BAN logic.
In the setting of ( , ) mn rational secret sharing, n rational players wish to share a secret s , arbitrarily m players can reconstruct it, they always choose the strategies which can bring them more utilities.The security requirement includes privacy, correctness and fairness.Fairness is a central objective of the rational secret sharing scheme, complete fair means either all players get the secret s or none of them get it, but most existing schemes do not meet this nature.In this work, a rational secret sharing protocol is proposed, which neither need particular communication channel, nor assume the existence of honest minority, in addition, the scheme can resist the collusion attack with () k k m  players, and can achieve sequential equilibrium.Theoretical analysis shows that the proposed protocol is complete fair.
The techniques of verifiable secret sharing and verifiable encryption are used in the construction of two party fair exchange protocols.The novelty of the construction lies in the introduction of the distributed semi-trusted third party.Compared to the previous fair exchange protocols with a single semi-trusted third party,the newly constructed fair exchange protocol can further guarantee the security and fairness of the transaction due to the existence of the distributed semi-trusted third party.
This thesis is concerned with two security mechanisms: authenticated key transport and rational exchange protocols. These mechanisms are potential building blocks in the security architecture of a range of different services. Authenticated key transport protocols are used to build secure channels between entities, which protect their communications against eaves-dropping and alteration by an outside attacker. In contrast, rational exchange protocols can be used to protect the entities involved in an exchange transaction from each other. This is important, because often the entities do not trust each other, and both fear that the other will gain an advantage by misbehaving. Rational exchange protocols alleviate this problem by ensuring that a misbehaving party cannot gain any advantages. This means that misbehavior becomes uninteresting and it should happen only rarely. The thesis is focused on the construction of formal models for authenticated key transport and rational exchange protocols. In the first part of the thesis, we propose a formal model for key transport protocols, which is based on a logic of belief. Building on this model, we also propose an original systematic protocol construction approach. The main idea is that we reverse some implications that can be derived from the axioms of the logic, and turn them into synthesis rules. The synthesis rules can be used to construct a protocol and to derive a set of assumptions starting from a set of goals. The main advantage is that the resulting protocol is guaranteed to be correct in the sense that all the specified goals can be derived from the protocol and the assumptions using the underlying logic. Another important advantage is that all the assumptions upon which the correctness of the protocol depends are made explicit. The protocol obtained in the synthesis process is an abstract protocol, in which idealized messages that contain logical formulae are sent on channels with various access properties. The abstract protocol can then be implemented in several ways by replacing the idealized messages and the channels with appropriate bit strings and cryptographic primitives, respectively. We illustrate the usage of the logic and the synthesis rules through an example: We analyze an authenticated key transport protocol proposed in the literature, identify several weaknesses, show how these can be exploited by various attacks, and finally, we redesign the protocol using the proposed systematic approach. We obtain a protocol that resists against the presented attacks, and in addition, it is simpler than the original one. In the second part of the thesis, we propose an original formal model for exchange protocols, which is based on game theory. In this model, an exchange protocol is represented as a set of strategies in a game played by the protocol parties and the network that they use to communicate with each other. We give formal definitions for various properties of exchange protocols in this model, including rationality and fairness. Most importantly, rationality is defined in terms of a Nash equilibrium in the protocol game. The model and the formal definitions allow us to rigorously study the relationship between rational exchange and fair exchange, and to prove that fairness implies rationality (given that the protocol satisfies some further usual properties), but the reverse is not true in general. We illustrate how the formal model can be used for rigorous verification of existing protocols by analyzing two exchange protocols, and formally proving that they satisfy the definition of rational exchange. We also present an original application of rational exchange: We show how the concept of rationality can be used to improve a family of micropayment schemes with respect to fairness without substantial loss in efficiency. Finally, in the third part of the thesis, we extend the concept of rational exchange, and describe how similar ideas can be used to stimulate the nodes of a self-organizing ad hoc network for cooperation. More precisely, we propose an original approach to stimulate the nodes for packet forwarding. Like in rational exchange protocols, our design does not guarantee that a node cannot deny packet forwarding, but it ensures that it cannot gain any advantages by doing so. We analyze the proposed solution analytically and by means of simulation.
Recently developed cryptographic techniques make it possible to construct fair exchange protocols with an off-line trusted third party (TTP). The technique is referred to as a verifiable encryption scheme (VES) and proves that a ciphertext is the result of the encryption of a specified value without revealing this value. In this paper we apply the technique to multi-party fair exchange (MPFE) protocols which leads to the trusted neutral party (TNP, it is not the third party in multi-party protocols) involved in the protocol to be off-line. Multi-party fair exchange has been studied by Asokan et al. (1996) and Franklin and Tsudik (1998) where the TNP is on-line. For off-line TNP-based MPFE, the issues are different from that of on-line TNP MPFE. We give a definition of fairness of the MPFE with off-line TNP. We then present our MPFE protocol and prove its fairness.