【作者】 苏马婧；

【导师】 方滨兴；

【作者基本信息】 哈尔滨工业大学 ， 信息安全， 2013， 博士

【摘要】 作为过去十几年里最具影响力的互联网技术之一，P2P技术引起互联网应用系统结构和用户行为模式的巨大改变，从原来中心式共享的C/S结构变为用户直接共享的分布式P2P结构，这增强了系统服务能力和可扩展性，并催生了多种基于P2P技术的互联网应用，吸引了规模庞大的用户群体。P2P技术的不断进步使现有P2P应用系统不断优化改进，性能大幅提升，然而，这些改进也使现有系统结构更加复杂，对其进行全面认识和评估也变得更加困难。此外，由于P2P系统本身的匿名性、开放性、动态性等特点，一些性能上的改进更可能会带来新的安全上的隐患，P2P系统中的盗版问题也成为制约P2P发展和广泛应用的重要因素。本文以BitTorrent这一应用最广泛的P2P文件共享系统为例，通过测量的方法进一步加深了对现行P2P系统的理解和认识，发现系统存在的安全问题，并提出了相应的解决方案。本文的主要工作和贡献包括：设计主被动相结合的测量系统，对BitTorrent网络进行了快速全面的测量，并分析了资源分布情况和用户的行为特征，测量结果显示：BitTorrent网络中资源分布呈现明显的不均衡性和“热点”现象，共享资源大部分为未授权的音视频内容。本文在不同时间尺度对Swarm演化规律进行建模，改进了现有节点到达模型和离开模型，发现用户兴趣和日周期性是影响BT系统Swarm演化的主要因素。这些测量结果也为后续分析和设计模拟器以对改进方法进行评估提供了基础。对BitTorrent网络拓扑及相关的性能特征进行了测量，分析了节点的度特征、小世界特性、节点下载速度以及下载完成度等，并从协议设计和客户端实现的角度对测量结果进行了分析，发现BT网络具有较强的健壮性，节点距离较短，稳定阶段BT网络拓扑不是全连通的，BT网络更接近于随机网络而不是一个Scale-free网络，且不具有小世界特征。此外，本文还发现节点连接数和节点下载速度之间没有显著的相关性。针对测量过程中发现的利用BitTorrent系统节点来源交换协议(Peer Ex-change，PEX)进行DDoS攻击的可能性，从协议设计和用户行为角度分析了PEX协议的脆弱性，并在可控环境中证实该漏洞可被利用发动持续的连接消耗型DDoS攻击。对此，本文章提出了基于评分的信誉机制(ReputationExchange)来增强系统的安全性，在节点评价时考虑了资源情况、数据有效性和传输效率、推荐信誉和历史信誉等因素，并通过PEX消息实现高效地信誉分发，理论分析和实验结果均证明REX能够有效对抗DDoS攻击。该机制也可应用于其他P2P文件共享系统中。为能够利用P2P系统高效地分发版权内容，对抗盗版问题，本文提出了一种基于随机加密的P2P版权文件分发机制。针对现有版权保护系统存在的问题，本文通过加密来防止未授权用户获取明文内容，利用P2P系统的随机性增加了密钥空间对抗共谋攻击，并对文件块校验机制进行改进防止内容污染攻击。理论分析和模拟实验证明了该机制是安全的，系统开销可接受且易于部署。更多还原

【Abstract】 During the last decades, as one of the most influential Internet technologies, P2PTechnology causes a revolution in the system architecture and user behavior of Internetapplications: changing from pure centralized C/S architecture to various distributed P2Parchitectures, from sharing by servers to sharing directly among users. This increases theservice capability of the system and brings good scalability, hence a variety of P2P-basedapplications have been designed, attracting enormous users. The rapid development ofP2P technologies bring lots of promotions to P2P systems and improve the performanceHowever, these improvements also make the system more complex, and it is more difcultto get a comprehensive understanding and evaluation to the P2P systems. Moreover, dueto characteristics such as anonymous, openness and dynamic, new features and extensionsused to improve performance of P2P systems may also lead to new security problems.The privacy issue has also become an important factor in restricting the development andwidely use of P2P technologies. This dissertation takes the BitTorrent, the most widelyused P2P file sharing system, as an example, gives more understandings to current P2Psystems by the measurement approach, discovers security issues and presents appropriatesolutions. The main work and contributions in this dissertation include:We perform a rapid and comprehensive measurement on the BitTorrent system bydesigning a new measurement system combing active and passive approaches. By themeasurement results, we analyze the resource distributions and user behaviors. We findthat the resources in BitTorrent system appear obvious unbalanced distribution and hot-spot phenomenon. Most of the shared files are unauthorized video and audio content.Wealso analyze the BT swarm evolution in diferent time scales, and improve existing peerarrival model and peer leaving model. We find user interest and diurnal periodicity are themain factors influencing swarm evolution. These measurement results provide a founda-tion for subsequent analysis and designing simulator to evaluate our proposed methods.We study the BitTorrent network topology and its performance-related character-istics (peer degree, peer distance, clustering coefcient, peer download speed and peerdownload percentage, etc.) by measurement, and analyze the results from the protocoldesign and client software implementation perspective. We find the BT network is robustto the peer departure and has short peer distances. The network in the steady stage is not fully connected. BT network is closed to a random network other than a scale-freenetwork, and it does not exhibit the small-work character. There is no strong correlationbetween peer connection degree and download speed.Considering the possibility of exploiting the BitTorrent Peer Exchange (PEX) proto-col to launch DDoS attacks, we analyze the vulnerabilities of PEX from protocol designand user behavior, and confirm that these vulnerabilities can be used to launch a persis-tent connection-exhaustive DDoS attack by controlled-experiments. To enhance the sys-tem security, we proposed a score-based reputation mechanism, referred to as ReputationExchange, REX. We take resource completeness, data validation, transmission efciency,recommend reputation and historical reputation as main factors, and use the PEX mes-sage to efectively distribute reputation score. Theoretical analysis and experiment resultsshow REX can efectively mitigate the efect of the attack. This mechanism can be usedin other P2P file-sharing systems.In order to take the advantage of P2P to efectively distribute copyrighted contentand resist privacy, we propose a stochastic-encryption-based copyrighted content deliv-ery scheme for P2P networks. By analyzing existing copyright protection system, in thispaper, we use encryption to avoid accessing plaintext content without authorization, ap-ply the random character of P2P system to enlarge key space to resist collusion attacks,and modify existing piece hash scheme to prevent content poisoning attacks. Theoreticalanalysis and simulation experiments show the security of the scheme. It is easy to deployand its overhead is acceptable.更多还原