摘要
在信息技术和通信产业如此重要的今天,面对越来越多样化的攻击手段,如何保证通信数据的机密性、完整性、可认证性、可授权及不可抵赖性正成为计算机和信息安全领域越发重要的研究课题。作为目前核心技术之一的可认证密钥交换(Authenticated Key Exchange, AKE)为通信的双方提供了两种服务。一种是AKE为通信双方建立会话密钥(Session Key),即只有彼此所知的对称密钥,该密钥用来实现接下来传输过程中的数据机密性和数据完整性。另一种是AKE能使通信双方确认彼此的身份。根据认证要素不同,AKE可分为基于口令(Password)、基于对称密钥(Symmetric Key)、基于公钥(Public Key)和基于混合因素等不同种类。本文研究了基于口令的可认证密钥交换系统(PAKE)和无线网络中AKE系统的安全性和复杂性,主要研究工作创新之处如下:
(1)完全基于口令的双服务器可认证密钥交换系统
本文设计了一个新的完全基于口令的双服务器可认证密钥交换系统(Password-only Two-server AKE, PTAKE).该系统同时克服了传统单服务器系统的单点失败和多服务器系统高昂的系统开销的不足。新设计的系统不但满足此类系统的最强安全需求,即在主动攻击者(Acitve Adversary)攻破两台服务器中的任何一台时系统仍能抵御离线字典攻击,而且总共只需要6次消息传输。与目前唯一一个具有相同安全水平的系统比较,新系统的通信复杂度降低了40%而且计算复杂度没有增加。本文还利用同态单向函数和同态加密机制首次提出了一个泛型设计的PTAKE (Generic PTAKE),证明了即使在主动型攻击者控制两台服务器中的前台服务器或者被动攻击者(Passive Adversary)控制后台服务器时,该系统仍能抵御离线字典攻击。
(2)对双重指数模运算快速算法的复杂度研究
双重指数模运算广泛应用于包括上述PTAKE在内的诸多基于数论的密码系统中,是这些系统中开销最大的运算,它的执行效率将直接影响到整个系统的性能。本文对当前此运算的主流快速算法进行了比较与分析,特别是利用马尔科夫概率模型对此前认为最快的WLLC算法进行了复杂度分析。理论分析和实验数据表明,WLLC算法需要的平均模乘次数为1.556k(k表示指数的长度),修正了原有的分析结果1.306k。这表明目前基于标准符号数位码的双重指数模运算算法复杂度仍然无法降低到1.5k次模乘运算以下。
(3)匿名无线安全漫游协议
匿名无线安全漫游协议(Secure Wireless Roaming, SWR)在帮助漫游用户和外地服务器之间建立安全信道的同时能够保护该用户的隐私,其核心是用户以匿名的方式与外地服务器进行可认证密钥交换。本文在已有工作的基础上提出了更加完善的匿名SWR安全需求,包括用户身份可认证、服务器身份可认证、安全会话密钥的建立、向前安全性、用户匿名性和不可追踪性等。我们采用CK模型下的模块化构造法设计了一个完全基于对称密钥的匿名SWR协议。该协议仅需要4次消息传输,是目前同类协议中通信代价最低的。由于该协议仅需要对称密钥加密和消息认证码而不涉及公钥基础结构(Public Key Infrastructure, PKI),所以计算复杂度也是已知协议中最低的。
(4)向前安全可撤回群签名机制
作为重要的密码学基础工具,可撤回群签名机制可以用来构造一种新的AKE以实现本地化漫游(Localized Anonymous Roaming)。虽然此举可以降低服务器间通信负载,但是,群签名机制的使用会大幅度增加计算复杂度且导致用户撤回机制过于复杂。为了降低该类协议的复杂度,本文设计了一种高效的向前安全可撤回群签名机制。该机制的签名、验证和用户撤回计算复杂度以及群公钥长度、用户私钥长度和签名长度均为常量级,不依赖于群中用户数量或者已撤回用户数,是第一个常量级的可撤回群签名机制。
Nowadays information technologies and communication industries play important roles in human community. With the rapid development of various attacks through network, providing confidentiality, integrity, authentication, authorization and non-reputation of data becomes a very critical topic in computer science and information security. As a key technology, an Authenticated Key Exchange (AKE) protocol provides two kinds of services for two communication parties. First, it allows the two parties to establish a session key which is pure symmetric known by each other only. The established key is used for realizing the data confidentiality and data integrity in the coming data transmission. Second, it provides a mechanism for two parties to be convinced that it is communicating with the intended party. According to the different authentication factors, there are several kinds of AKE, such as Password-only AKE (PAKE), symmetric key based AKE, public key based AKE and hybrid AKE. This dissertation focuses on the security and complexity issues of PAKE and AKE for wireless network. The main work and contribution are shown as follows:
(1) Password-only Two-server Authenticated Key Exchange (PTAKE)
In this dissertation, we propose a novel PTAKE. It overcomes the disadvantage of the conventional single-server scheme that the single point of failure as well as the disadvantage of the multi-server scheme that the expensive system costs. It not only satisfies the strongest security requirement for PTAKE that the system is secure against offline dictionary attacks even if any one of the two servers is corrupted by an active adversary, but also it requires six communication rounds only. Namely, our scheme reduces the number of communication rounds by 40% when compared with other most efficient scheme while maintaining about the same degree of computational complexity. Furthermore, we propose a generic PTAKE with satisfying the lower security level for PTAKE that the system is secure against offline dictionary attacks even if the front one of the two servers is corrupted by an active adversary or the backend server is corrupted by a passive adversary.
(2) Complexity Analysis of An Fast Modular Duplex-exponentiation Algorithm
The PTAKE schemes mentioned above and many other existing crypto systems require efficient modular duplex-exponentiation operations in order to make the systems fast in practice as it is the most expensive operations for them. In this dissertation, we target to examine the computational complexity of the famous fast algorithms. Particularly, we provide a formal complexity analysis for WLLC algorithm under Markov probabilistic model, which was claimed to be the fastest algorithm. The complexity analysis and the experimental results show that the actual computational complexity of WLLC algorithm should be 1.556k:rather than 1.306k, where k is the larger bit length of the two exponents. It implies that the best modular duplex-exponentiations algorithm based on canonical-sighed-digit technique is still not able to overcome the 1.5k barrier.
(3) Anonymous Secure Wireless Roaming(Anonymous SWR)
In order to build a secure channel between the roaming user and the service provider with providing user privacy (i.e., user anonymity and user untraceability), the Anonymous Secure Wireless Roaming protocol has been proposed, the core function of which is to provide AKE between the two parties. In this dissertation, we focus on the proposal of the all-round security requirements for Anonymous SWR which captures the following security properties including mutual authentication between roaming user and foreign server, key establishment and key privacy against backend server, forward secrecy, user anonymity and user untraceability. And we propose a pure symmetric key based Anonymous SWR protocol using the CK modular approach. To best of our knowledge, it seems to be the first pure symmetric key based anonymous SWR. Compared with other existing Anonymous SWR protocols, both of the computation complexity and communication complexity of our protocol are lowest, since it involves only 4 message flows and no PKI (Public Key Infrastructure) but only highly efficient cryptographic operations are needed which include Message Authentication Code (MAC) and symmetric key encryption.
(4) Group Signature with Forward Secure Revocation.
As an important cryptographic tool, group signature has been widely employed by various crypto systems, especially it is employed to construct a localized anonymous roaming protocol as a core building block. Although for this roaming protocol, the communication burden of the servers will be alleviated much, the computational complexity and user revocation complexity will increase quickly due to the usage of group signature. In order to overcome this disadvantage, we propose an efficient group signature with forward secure revocation with satisfying constant signing and verifying complexity as well as constant size in signature public key and signing key.
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