Contract Signing Protocol Research Articles

Multiplex Contract Signing Protocol

利用Cha-Cheon 的基于身份的签名方案提出了一个可证安全的基于身份的可验证加密签名(verifiablyencrypted signature,简称VES)方案,并利用该方案和基于身份的代理可验证加密签名(proxy verifiably encryptedsignature,简称PVES)方案提出了一个新颖的多元合同签署协议.信息交换过程中,原始签名者或代理签名者分别利用VES 或PVES 实现承诺消息的交换与认证,并未使用复杂的零知识证明系统,从而有效避免了大量运算.当争议发生时,可信第三方从VES 或PVES 中恢复出有效的合同签名,以保证签署者的公平性.安全性分析结果表明,协议满足不可否认性、时效性以及公平性.;Utilizing the Cha-Cheon’s identity-based signature scheme, a provably secure identity-based verifiably encrypted signature (VES) scheme is proposed. Utilizing the proposed scheme and identity-based proxy verifiably encrypted signature (PVES) scheme, a novel multiplex contract signing protocol is also proposed. The original signer or proxy signer uses VES or PVES to realize the interaction and certification of the commitment message in the information exchange process. The proposed scheme does not need the zero-knowledge proof and excessive computation. An optimized trusted third party who participates in the protocol extracts the formal signature from the VES or PVES only when problem occurs. The performance analysis results show that the scheme satisfies non-repudiation, timeliness and fairness.

Optimistic Fair Contract Signing Protocol Based on Hybrid Verifiably Encrypted Signature Scheme

Optimistic Fair Contract Signing Protocol Based on Hybrid Verifiably Encrypted Signature Scheme

Game-based verification of contract signing protocols with minimal messages

A multi-party contract signing (MPCS) protocol is used for a group of signers to sign a digital contract over a network. We analyse the protocols of Mauw, Radomirović and Torabi Dashti (MRT), using the finite-state model checker Mocha. Mocha allows for the specification of properties in alternating-time temporal logic (ATL) with game semantics, and the model checking problem for ATL requires the computation of winning strategies. This gives us an intuitive interpretation of the verification problem of crucial properties of MPCS protocols. MRT protocols can be generated from minimal message sequences, depending on the number of signers. We discover an attack on fairness in a published MRT protocol with three signers and a general attack on abuse-freeness for all MRT protocols. For both attacks, we present solutions. The abuse-freeness attack leads us to a revision of the methodology to construct an MRT protocol. Following this revised methodology, we design a number of MRT protocols using minimal message sequences for three and four signers, all of whom have been successfully model checked in Mocha.

Simple and Efficient Contract Signing Protocol

In this paper, a new contract signing protocol is proposed based on the RSA signature scheme. The protocol will allow two parties to sign the same contract and then exchange their digital signatures. The protocol ensures fairness in that it offers parties greater security: either both parties receive each other's signatures or neither does. The protocol is based on offline Trusted Third Party (TTP) that will be brought into play only if one party fails to sign the contract. Otherwise, the TTP remains inactive. The protocol consists of only three messages that are exchanged between the two parties.

LTL model-checking for security protocols

This thesis is about the application of automated reasoning techniques to the formal analysis of security protocols. More in detail, it proposes a general model-checking framework for security protocols based on a set-rewriting formalism that, coupled with the use of Linear Temporal Logic, allows for the specification of assumptions on principals and communication channels as well as complex security properties that are normally not handled by state-of-the-art protocol analyzers. The approach successfully combines encoding techniques originally developed for planning with bounded model-checking techniques. The effectiveness of the approach proposed is assessed against the formal analysis of relevant security protocols, with the detection of a severe security flaw in Google's SAML-based SSO for Google Apps and a previously unknown attack on a patched version of the ASW contract-signing protocol.

Deciding strategy properties of contract-signing protocols

Research on the automatic analysis of cryptographic protocols has so far concentrated on reachability properties, such as secrecy and authentication. In this article, we prove that certain game-theoretic security properties, including balance for contract-signing protocols, can be decided in a Dolev-Yao style model with a bounded number of sessions. The decision algorithm that we develop is based on standard constraint-solving procedures, which, in the past, have successfully been employed in tools for reachability properties. Our result thus paves the way for extending these tools to deal with game-theoretic security properties.

An Abuse-Free Fair Contract-Signing Protocol Based on the RSA Signature

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 a fair way, 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.

A New Fair Non-repudiation Protocol for Secure Negotiation and Contract Signing

A New Fair Non-repudiation Protocol for Secure Negotiation and Contract Signing

Deductive verification of alternating systems

Abstract Alternating systems are models of computer programs whose behavior is governed by the actions of multiple agents with, potentially, different goals. Examples include control systems, resource schedulers, security protocols, auctions and election mechanisms. Proving properties about such systems has emerged as an important new area of study in formal verification, with the development of logical frameworks such as the alternating temporal logic ATL *. Techniques for model checking ATL * over finite-state systems have been well studied, but many important systems are infinite-state and thus their verification requires, either explicitly or implicitly, some form of deductive reasoning. This paper presents a theoretical framework for the analysis of alternating infinite-state systems. It describes models of computation, of various degrees of generality, and alternating-time logics such as ATL * and its variations. It then develops a proof system that allows to prove arbitrary ATL * properties over these infinite-state models. The proof system is shown to be complete relative to validities in the weakest possible assertion language. The paper then derives auxiliary proof rules and verification diagrams techniques and applies them to security protocols, deriving a new formal proof of fairness of a multi-party contract signing protocol where the model of the protocol and of the properties contains both game-theoretic and infinite-state (parameterized) aspects.

ON THE POWER OF QUANTUM TAMPER-PROOF DEVICES

We show how quantum tamper-proof devices (QTPD's) can be used to attack and to develop security protocols. On one hand, we prove that it is possible to transfer proofs of zero-knowledge protocols using QTPD's. This attack can be extended to other security schemes where privacy is important. On the other hand, we present a fair contract signing protocol using QTPD's where there is no communication with Judge during the exchange phase (which is impossible classically). In the latter case, we make use of decoherence in the quantum state of the QTPD to implement a global clock over the asynchronous network. QTPD's seem to be possible to implement with existing quantum hardware, due to the fact that it is hard to isolate quantum memory from interference. These theoretical results contribute to justify the implementation of QTPD's.

Fair multi-party contract signing using private contract signatures

A multi-party contract signing protocol allows a set of participants to exchange messages with each other with a view to arriving in a state in which each of them has a pre-agreed contract text signed by all the others. Garay and Mackenzie (GM) proposed such protocol based on private contract signatures, but it was later shown to be flawed by Chadha, Kremer and Scedrov (CKS); the authors CKS also provided a fix to the GM protocol by revising one of its sub-protocols. We show an attack on the revised GM protocol for any number ( n > 4) of signers. Furthermore, we argue that our attack shows that the message exchange structure of GM’s main protocol is flawed: whatever the trusted party does will result in unfairness for some signer. This means that it is impossible to define a trusted party protocol for Garay and MacKenzie’s main protocol; we call this “resolve-impossibility”. We propose a new optimistic multi-party contract signing protocol, also based on private contract signatures. We present a proof that our protocol satisfies fairness as well as its formal analysis in NuSMV model checker for the case of five signers. The protocol requires n ( n - 1 ) ( ⌈ n / 2 ⌉ + 1 ) messages to be sent in the optimistic execution, which is about half the number of messages required by the state-of-the-art Baum-Waidner and Waidner protocol, and in contrast with Baum-Waidner and Waidner, it does not use a non-standard notion of a signed contract.

Exogenous Probabilistic Computation Tree Logic

We define a logic EpCTL for reasoning about the evolution of probabilistic systems. System states correspond to probability distributions over classical states and the system evolution is modelled by probabilistic Kripke structures that capture both stochastic and non–deterministic transitions. The proposed logic is a temporal enrichment of Exogenous Probabilistic Propositional Logic (EPPL). The model-checking problem for EpCTL is analysed and the logic is compared with PCTL; the semantics of the former is defined in terms of probability distributions over sets of propositional symbols, whereas the latter is designed for reasoning about distributions over paths of possible behaviour. The intended application of the logic is as a specification formalism for properties of communication protocols, and security protocols in particular; to demonstrate this, we specify relevant security properties for a classical contract signing protocol and for the so–called quantum one–time pad.

Analysis of probabilistic contract signing1

We present three case studies, investigating the use of probabilistic model checking to automatically analyse properties of probabilistic contract signing protocols. We use the probabilistic model checker PRISM to analyse three protocols: Rabin's probabilistic protocol for fair commitment exchange; the probabilistic contract signing protocol of Ben-Or, Goldreich, Micali, and Rivest; and a randomised protocol for signing contracts of Even, Goldreich, and Lempel. These case studies illustrate the general methodology for applying probabilistic model checking to formal verification of probabilistic security protocols. For the Ben-Or et al. protocol, we demonstrate the difficulty of combining fairness with timeliness. If, as required by timeliness, the judge responds to participants' messages immediately upon receiving them, then there exists a strategy for a misbehaving participant that brings the protocol to an unfair state with arbitrarily high probability, unless unusually strong assumptions are made about the quality of the communication channels between the judge and honest participants. We quantify the tradeoffs involved in the attack strategy, and discuss possible modifications of the protocol that ensure both fairness and timeliness. For the Even et al. protocol, we demonstrate that the responder enjoys a distinct advantage. With probability 1, the protocol reaches a state in which the responder possesses the initiator's commitment, but the initiator does not possess the responder's commitment. We then analyse several variants of the protocol, exploring the tradeoff between fairness and the number of messages that must be exchanged between participants.

Mechanical proofs about BW multi-party contract signing protocol

We report on the verification of a multi-party contract signing protocol described by Baum-Waidner and Waidner (BW). Based on Paulson's inductive approach, we give the protocol model that includes infinitely many signatories and contract texts signing simultaneously. We consider composite attacks of the dishonest signatory and the external intruder, formalize cryptographic primitives and protocol arithmetic including attack model, show formal description of key distribution, and prove signature key secrecy theorems and fairness property theorems of the BW protocol using the interactive theorem, prover Isabelle/HOL.

Compositional analysis of contract-signing protocols

We develop a general method for proving properties of contract-signing protocols using a specialized protocol logic. The method is applied to the Asokan–Shoup–Waidner and the Garay–Jacobson–MacKenzie protocols. Our method offers certain advantages over previous analysis techniques. First, it is compositional: the security guarantees are proved by combining the independent proofs for the three subprotocols of each protocol. Second, the formal proofs are carried out in a “template” form, which gives us a reusable proof that may be instantiated for the two example protocols, as well as for other protocols with the same arrangement of messages. Third, the proofs follow the design intuition. In particular, in proving game-theoretic properties like fairness, we demonstrate success for the specific strategy that the protocol designer had in mind, instead of non-constructively proving that a strategy exists. Finally, our logical proofs demonstrate security when an unbounded number of sessions are executed in parallel.

Multiparty Contract Signing Over a Reliable Network

Most contract signing protocols make use of a trusted third party (TTP) to ensure fairness. It has been shown that in the crash network model, this is inevitable. However, for stronger networks, where misbehavior is allowed but failure excluded, the necessity of a TTP has not yet been debated. We consider a strong network model, that includes reliable broadcast, bounded delays and timestamps and use it to describe a simple multiparty contract signing protocol that does not rely on a TTP. This shows that by strengthening the assumptions on the network, the transfer of trust from one dedicated server to the network is feasible. The result is commented in a more general setting of multiparty protocols and problems. The correctness of the proposed protocol for any number of participants is proven using process algebra techniques.

Formal Analysis of Multiparty Contract Signing

We analyze the multiparty contract-signing protocols of Garay and MacKenzie (GM) and of Baum and Waidner (BW). We use a finite-state tool, Mocha, which allows specification of protocol properties in a branching-time temporal logic with game semantics. While our analysis does not reveal any errors in the BW protocol, in the GM protocol we discover serious problems with fairness for four signers and an oversight regarding abuse-freeness for three signers. We propose a complete revision of the GM subprotocols in order to restore fairness.

Practical Implementations of a Non-disclosure Fair Contract Signing Protocol

Contract signing is a practical application of the fair exchange of digital signatures. This application used to be realized by directly adopting the results of the fair exchange of signatures, which do not completely meet the requirements of the signing of a secret contract. The assistance of a trusted third party (TTP) and some cryptographic technology are required to allow two parties to exchange their signatures through the network in a fair manner because these two parties potentially may be dishonest or mistrust each other. This paper presents a subtle method of preventing the off-line TTP from gaining the exchanged signature and the corresponding message when a dispute occurs between the two parties wherein the TTP is required to take part in the exchange procedure. An advanced concept, the non-disclosure property, is proposed in order to prevent a party from misusing evidence left during the exchange process. Two approaches, namely the secret divide method and the convertible signature are demonstrated. To satisfy the properties of the traditional paper-based contract signing, the technique of multi-signature scheme is used in the proposed protocols.

An efficient contract signing protocol using the aggregate signature scheme to protect signers' privacy and promote reliability

Contract signing conducted over the Internet arouses concerns of fairness. Two signing parties exchange their signatures in a fair manner, so that no party can gain an advantage over the other. This paper will discuss the security issues of signers' privacy and reliability of the system for the contract signing protocol. The trusted third party (TTP), responsible for maintaining the fairness in the protocol, cannot get useful information about the exchanged signatures by solving the dispute between two parties. The concept of semi-trusted third party (STTP) adopted from Franklin and Reiter's paper (in 1997) can be used to reach our goal. However, Franklin and Reiter's protocol uses an on-line third party and is inapplicable to the exchange of signatures. Hence, we propose an efficient contract signing protocol with off-line STTP using the aggregate signature proposed by Boneh and Gentry. Furthermore, the use of aggregate signature and bilinear pairings will effectually promote system reliability.

The ASW Protocol Revisited: A Unified View

We revisit the analysis of the ASW contract signing protocol and use a unified view of the protocol as a whole as a basis to reason about the protocol and its objectives. This line of reasoning yields a simpler and clearer model of agents and protocol objectives which is within the scope of standard security analysis methods, as it does not require fairness constraints and uses only standard authentication and secrecy properties. We also analyse this model for finitely and infinitely many sessions of the protocol using the automated analysis tools OFMC and its extension OFMC-FP.