Dolev-Yao攻击者模型的形式化描述

模型检测技术能够实现安全协议的自动化分析,是一种高效的形式化分析方法。然而,对于攻击者的建模却一直缺乏通用的方法,这导致了模型检测方法的自动化程度降低。本文为安全协议分析中,应用最为广泛的Dolev-Yao攻击者模型建立了一套形式化描述方法。遵循这一方法,可以使用任何建模语言机械地建立Dolev-Yao攻击者模型,从而大大地减少了人工分析的成份。同时,本文还使用该方法验证了两个目的完全不同的协议,证明了它的通用性。     

一种基于规划理论的密码协议形式模型

将规划理论引入到密码协议形式化分析领域,结合密码协议在实际网络环境中的运行特点和规律,提出了密码协议攻击规划理论;建立了一种对密码协议进行安全性验证的形式化模型,即密码协议攻击规划问题模型;给出了模型的一阶语法、形式定义及相关运算语义.同时,分析了Dolev-Yao模型的不足之处,基于基本消息元素策略对其进行了改进;并通过增强应用语义来保证改进模型的可行性,从而避免了"状态空间爆炸"问题的发生,提高了密码协议攻击规划问题模型的完备性;并给出了基于该模型的NS公钥协议分析实例.提出的密码协议形式模型是证伪的,目的在于对密码协议进行验证,并查找协议中可能存在的漏洞,既可以方便地进行手工推导证明,也非常易于自动化实现.     

基于攻击者行为能力的SVO协议分析

SVO原逻辑不适用于证明基于证书的认证与密钥交换协议安全性问题。为此,提出2个SVO逻辑相关公理,对证书真实性与会话密钥安全性进行判断及验证,并结合Dolev-Yao安全模型,从攻击者行为能力的角度评估密码协议的安全性。经分析证明Mangipudi协议未能提供前向安全且存在假冒攻击的安全缺陷,因此给出一个在Dolev-Yao安全模型下可证明安全的解决方案。     

基于状态转移系统的安全协议形式模型

提出一种基于状态转移系统的安全协议模型,以Dolev-Yao攻击者模型为前提假设,以状态转移系统为框架,用语义编码的方式定义消息和事件,用重写关系定义协议规则,用事件的集合来描述协议的安全属性,并给出安全属性的检验策略。该模型能够对安全协议进行精确的形式化描述,且便于实现自动化分析。     

基于图元的事件图生成算法

与单轮运行情形不同,多轮并发运行的密码协议存在更为复杂的安全性问题。并发运行密码协议的形式化分析对象包括密码协议的多轮并发运行和多个密码协议的并发运行两种情形,且二者具有统一的形式化模型。基于扩展的串空间模型和Spi演算理论,提出用于并发运行密码协议安全属性验证的事件图模型。图元是事件图的构造单元,它满足消息事件之间的通信关系和前驱关系约束以及消息语句的新鲜性约束。定义消息事件之间、图元之间以及消息事件和图元之间的前缀、组合和选择运算,并给出事件图生成算法。     

密码协议验证中的Petri网方法

如何验证密码协议的安全性是一个复杂的问题,只有形式化的验证方法才能证明密码协议的绝对正确.利用Petri网给出了一种用于密码协议验证的形式化方法.在合理假设的基础上,区分合法用户与攻击者在执行协议时的前提条件,列出执行协议后的结果,在此基础上建立了攻击者的Petri网模型.最后,用这种方法对NSPK协议进行了验证,证明了最初的NSPK协议中存在一个安全问题,而改进的NSPK协议则消除了这个问题.证明了这种方法的有效性.     

一种基于问题求解理论的密码协议形式模型

提出了一种基于问题求解理论的密码协议模型，给出了模型的基本语法以及基于ρ演算的形式语义，明确了模型推理过程中涉及到的一些关键性的概念和命题。该模型具有以下特点：能够对密码协议进行精确的形式化描述；具有合理可靠的可证明语义；对密码协议安全性的定义精确合理；便于实现自动化推理。所有这些均确保了基于该模型的密码协议安全性分析的合理性和有效性，为正确的分析密码协议的安全性提供了可靠依据。     

Pw/GAKE:基于口令的高效组群密钥生成协议

针对组群通讯环境中的身份认证式密钥交换问题,基于Diffie-Hellman判定性问题的难解性假设构造了一个基于口令的身份认证组群密钥交换协议Pw/GAKE。该协议具有很高的计算效率,所有组群成员仅需参与一轮消息传输和一次广播、仅需进行幂指数运算和散列运算而无需借助任何复杂的公钥密码方案,因此特别适合于无线/移动自组网络环境中的中小规模组群。在随机oracle模型下证明了该协议的安全性。该证明将协议Pw/GAKE的安全性质归结为Abdella-Pointcheval所建立的2-方密钥交换协议SPAKE的安全性质,显示出Pw/GAKE的构造具有很强的递归特征。     

Neuman-Stubblebine协议的串空间模型及分析

串空间模型是一种新兴的密码协议形式化分析工具,其理论中理想和诚实概念的提出大大减少了协议的证明步骤.首次利用串空间理论从机密性和认证性两个方面对Neuman-Stubblebine协议进行了分析.分析结果证明该协议是安全的,而且理想对公开密钥算法和对称密钥算法产生的协议的分析都是有效的.     

计算可靠且高效的群组密钥协商协议符号化分析方法

符号化分析方法将密码原语视为黑盒,且未研究其需要满足的具体安全属性,因而计算可靠性一直备受质疑.而且,这类方法在分析参与者数目较多的群组密钥协商协议时不具有高效性.针对上述两个问题,文中提出了一种计算可靠且高效的群组密钥协商协议符号化分析方法.该方法通过为符号化表达式设计一个Pattern函数,定义了模幂运算需要满足的具体安全属性,并基于此证明了符号化分析Burmester-Desmedt协议在通用可组合安全框架下是计算可靠的.进而通过数学归纳法,证明了Burmester-Desmedt协议的安全性与参与者数目无关,避免了参与者数目对符号化分析造成的影响,提高了符号化分析方法的效率.     

Deciding the Security of Protocols with Commuting Public Key Encryption

Many cryptographic protocols and attacks on these protocols make use of the fact that the order in which encryption is performed does not affect the result of the encryption, i.e., encryption is commutative. However, most models for the automatic analysis of cryptographic protocols can not handle such encryption functions since in these models the message space is considered a free term algebra. In this paper, we present an NP decision procedure for the insecurity of protocols that employ RSA encryption, which is one of the most important instances of commuting public key encryption.     

On the decidability of cryptographic protocols with open-ended data structures

Formal analysis of cryptographic protocols has concentrated mainly on protocols with closed-ended data structures, i.e., protocols where the messages exchanged between principals have fixed and finite format. In many protocols, however, the data structures used are open-ended, i.e., messages have an unbounded number of data fields. In this paper, decidability issues for such protocols are studied. We propose a protocol model in which principals are described by transducers, i.e., finite automata with output, and show that in this model security is decidable and PSPACE-hard in presence of the standard Dolev-Yao intruder.     

One Picture Is Worth a Dozen Connectives: A Fault-Tree Representation of NPATRL Security Requirements

In this paper, we show how we can increase the ease of reading and writing security requirements for cryptographic protocols at the Dolev-Yao level of abstraction by developing a visual language based on fault trees. We develop such semantics for a subset of NRL protocol analyzer temporal requirements language (NPATRL), a temporal language used for expressing safety requirements for cryptographic protocols, and show that the subset is sound and complete with respect to the semantics. We also show how the fault trees can be used to improve the presentation of some specifications that we developed in our analysis of the group domain of interpretation (GDOI) protocol. Other examples involve a property of Kerberos 5 and a visual account of the requirements in Lowe's authentication hierarchy.     

Towards a Quantitative Analysis of Security Protocols

This paper contributes to further closing the gap between formal analysis and concrete implementations of security protocols by introducing a quantitative extension of the usual Dolev-Yao intruder model. This extended model provides a basis for considering protocol attacks that are possible when the intruder has a reasonable amount of computational power, in particular when he is able, with a certain probability, to guess encryption keys or other particular kind of data such as the body of a hashed message. We also show that these extensions do not augment the computational complexity of the protocol insecurity problem in the case of a finite number of interleaved protocol sessions.     

Relating symbolic and cryptographic secrecy

We investigate the relation between symbolic and cryptographic secrecy properties for cryptographic protocols. Symbolic secrecy of payload messages or exchanged keys is arguably the most important notion of secrecy shown with automated proof tools. It means that an adversary restricted to symbolic operations on terms can never get the entire considered object into its knowledge set. Cryptographic secrecy essentially means computational indistinguishability between the real object and a random one, given the view of a much more general adversary. In spite of recent advances in linking symbolic and computational models of cryptography, no relation for secrecy under active attacks is known yet. For exchanged keys, we show that a certain strict symbolic secrecy definition over a specific Dolev-Yao-style cryptographic library implies cryptographic key secrecy for a real implementation of this cryptographic library. For payload messages, we present the first general cryptographic secrecy definition for a reactive scenario. The main challenge is to separate secrecy violations by the protocol under consideration from secrecy violations by the protocol users in a general way. For this definition, we show a general secrecy preservation theorem under reactive simulatability, the cryptographic notion of secure implementation. This theorem is of independent cryptographic interest. We then show that symbolic secrecy implies cryptographic payload secrecy for the same cryptographic library as used in key secrecy. Our results thus enable formal proof techniques to establish cryptographically sound proofs of secrecy for payload messages and exchanged keys.     

A Computationally Sound Mechanized Prover for Security Protocols

We present a new mechanized prover for secrecy properties of security protocols. In contrast to most previous provers, our tool does not rely on the Dolev-Yao model, but on the computational model. It produces proofs presented as sequences of games; these games are formalized in a probabilistic polynomial-time process calculus. Our tool provides a generic method for specifying security properties of the cryptographic primitives, which can handle shared-key and public-key encryption, signatures, message authentication codes, and hash functions. Our tool produces proofs valid for a number of sessions polynomial in the security parameter, in the presence of an active adversary. We have implemented our tool and tested it on a number of examples of protocols from the literature.     

Multi-Attacker Protocol Validation

Security protocols have been analysed focusing on a variety of properties to withstand the Dolev-Yao attacker. The Multi-Attacker treat model allows each protocol participant to behave maliciously intercepting and forging messages. Each principal may then behave as a Dolev-Yao attacker while neither colluding nor sharing knowledge with anyone else. This feature rules out the applicability of existing equivalence results in the Dolev-Yao model. The analysis of security protocols under the Multi-Attacker threat model brings forward yet more insights, such as retaliation attacks and anticipation attacks, which formalise currently realistic scenarios of principals competing each other for personal profit. They are variously demonstrated on a classical protocol, Needham-Schroeder??s, and on a modern deployed protocol, Google??s SAML-based single sign-on protocol. The general threat model for security protocols based on set-rewriting that was adopted in AVISPA (Armando et al. 2005) is extended to formalise the Multi-Attacker. The state-of-the-art model checker SATMC (Armando and Compagna, Int J Inf Secur 6(1):3?C32, 2007) is then used to automatically validate the protocols under the new threats, so that retaliation and anticipation attacks can automatically be found. The tool support scales up to the Multi-Attacker threat model at a reasonable price both in terms of human interaction effort and of computational time.