计算机网络文献翻译报告

英文原文：

CHAPTER 8 Security in Computer Networks

Way back in Section 1.6 we described some of the more prevalent and damaging classes of Internet attacks, including malware attacks, denial of service, sniffing, source masquerading, and message modification and deletion. Although we have since learned a tremendous amount about computer networks, we still haven’t examined how to secure networks from those attacks. Equipped with our newly acquired expertise in computer networking and Internet protocols, we’ll now study in-depth secure communication and, in particular, how computer networks can be defended from those nasty bad guys.

Let us introduce Alice and Bob, two people who want to communicate and wish to do so “securely.” This being a networking text, we should remark that Alice and Bob could be two routers that want to exchange routing tables securely, a client and server that want to establish a secure transport connection, or two e-mail appli- cations that want to exchange secure e-mail —all case studies that we will consider later in this chapter. Alice and Bob are well-known fixtures in the security commu- nity, perhaps because their names are more fun than a generic entity named “A”that wants to communicate securely with a generic entity named “B.” Love affairs, wartime communication, and business transactions are the commonly cited human needs for secure communications; preferring the first to the latter two, we’re happy to use Alice and Bob as our sender and receiver, and imagine them in this first scenario.

We said that Alice and Bob want to communicate and wish to do so “securely ”but what precisely does this mean? As we will see, security (like love) is a many- splendored thing; that is, there are many facets to security. Certainly, Alice and Bob would like for the contents of their communication to remain secret from an eavesdropper. They probably would also like to make sure that when they are communicating, they are indeed communicating with each other, and that if their communication is tampered with by an eavesdropper, that this tampering is detected. In the first part of this chapter, we’ll cover the fundamental cryptography techniques that allow for encrypting communication, authenticating the party with whom one is communicating, and ensuring message integrity.

In the second part of this chapter, we’ll examine how the fundamental

crypto- graphy principles can be used to create secure networking protocols. Once again taking a top-down approach, we’ll examine secure protocols in each of the (top four) layers, beginning with the application layer. We’ll examine how to secure e- mail, how to secure a TCP connection, how to provide blanket security at the net- work layer, and how to secure a wireless LAN. In the third part of this chapter we’ll consider operational security, which is about protecting organizational networks from attacks. In particular, we’ll take a careful look at how firewalls and intrusion detection systems can enhance the security of an organizational network.

What Is Network Security?

Let’s begin our study of network security by returning to our lovers, Alice and Bob, who want to communicate “securely.” What precisely does this mean? Certainly, Alice wants only Bob to be able to understand a message that she has sent, even though they are communicating over an insecure medium where an intruder (Trudy, the intruder) may intercept whatever is transmitted from Alice to Bob. Bob also wants to be sure that the message he receives from Alice was indeed sent by Alice, and Alice wants to make sure that the person with whom she is communicat- ing is indeed Bob. Alice and Bob also want to make sure that the contents of their messages have not been altered in transit. They also want to be assured that they can communicate in the first place (i.e., that no one denies them access to the resources needed to communicate). Given these considerations, we can identify the following desirable properties of secure communication.

●Confidentiality. Only the sender and intended receiver should be able

to under- stand the contents of the transmitted message. Because eavesdroppers may inter- cept the message, this necessarily requires that the message be somehow encrypted so that an intercepted message cannot be understood by an intercep- tor. This aspect of confidentiality is probably the most commonly perceived meaning of the term secure communication. We’ll study cryptographic tech- niques for encrypting and decrypting data in Section 8.2.

●Message integrity. Alice and Bob want to ensure that the content of

their com- munication is not altered, either maliciously or by

accident, in transit. Extensions to the checksumming techniques that

we encountered in reliable transport and data link protocols can be

used to provide such message integrity. We will study message integrity

in Section 8.3.

●End-point authentication. Both the sender and receiver should be

able to confirm the identity of the other party involved in the

communication—to confirm that the other party is indeed who

or what they claim to be. Face-to-face human communication solves

this problem easily by visual recognition. When communicating

entities exchange messages over a medium where they cannot see

the other party, authentication is not so simple. When a user wants

to access an inbox, how does the mail server ver- ify that the user

is the person he or she claims to be? We study end-point

authentication in Section 8.4.

●Operational security. Almost all organizations (companies,

universities, and so on) today have networks that are attached to

the public Internet. These net- works therefore can potentially be compromised. Attackers can attempt to deposit worms into the

hosts in the network, obtain corporate secrets, map the internal

network configurations, and launch DoS attacks. We’ll see in

Section 8.9 that operational devices such as firewalls and intrusion

detection systems are used to counter attacks against an

organization’s network. A firewall sits between the organization

’s network and the public network, controlling packet access to

and from the network. An intrusion detection sys- tem performs “

deep packet inspection,” alerting the network administrators about

suspicious activity.

Having established what we mean by network security, let’s next consider exactly what information an intruder may have access to, and what actions can be taken by the intruder. Figure 8.1 illustrates the scenario. Alice, the sender, wants to send data to Bob, the receiver. In order to exchange data securely, while meeting the requirements of confidentiality, end-point authentication, and message integrity, Alice and Bob will exchange control messages and data messages (in much the same way that TCP senders and receivers exchange control segments and data seg- ments). All or some of these

messages will typically be encrypted. As discussed in Section 1.6, an intruder can potentially perform

● eavesdropping —sniffing and recording control and data messages

on the channel.

● modification, insertion, or deletion of messages or message content.

Figure 8.1 Sender, receiver, and intruder (Alice, Bob, and Trudy)

As we ’ll see, unless appropriate countermeasures are taken, these capabilities allow an intruder to mount a wide variety of security attacks: snooping on commu- nication (possibly stealing passwords and data), impersonating another entitity, hijacking an ongoing session, denying service to legitimate network users by over- loading system resources, and so on. A summary of reported attacks is maintained at the CERT Coordination Center [CERT 2012].

Having established that there are indeed real threats loose in the Internet, what are the Internet equivalents of Alice and Bob, our friends who need to com- municate securely? Certainly, Bob and Alice might be human users at two end systems, for example, a real Alice and a real Bob who really do want to exchange secure e-mail. They might also be participants in an electronic commerce transac- tion. For example, a real Bob might want to transfer his credit card number securely to a Web server to purchase an item online. Similarly, a real Alice might want to interact with her bank online. The parties needing secure communication might themselves also be part of the network infrastructure. Recall that the domain name system (DNS, see Section 2.5) or routing daemons that exchange routing information (see Section 4.6) require secure communication between two parties. The same is true for network management applications, a topic we exam- ine in Chapter

9. An intruder that could actively interfere with DNS lookups (as discussed in Section 2.5), routing computations [RFC 4272], or network manage- ment functions [RFC 3414] could wreak havoc in the Internet.

Secure sender Secure receiver

Data Data

Control, data messages Channel Alice Bob

Having now established the framework, a few of the most important defi- nitions, and the need for network security, let us next delve into cryptography. While the use of cryptography in providing confidentiality is self-evident, we’ll see shortly that it is also central to providing end-point authentication and message integrity—making cryptography a cornerstone of network security.

Principles of Cryptography

Although cryptography has a long history dating back at least as far as Julius Caesar, modern cryptographic techniques, including many of those used in the Internet, are based on advances made in the past 30 years. Kahn’s book, The Codebreakers [Kahn 1967], and Singh’s book, The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography [Singh 1999], provide a fascinating look at the long history of cryptography. A complete discussion of cryptography itself requires a complete book [Kaufman 1995; Schneier 1995] and so we only touch on the essential aspects of cryptography, particularly as they are practiced on the Internet. We also note that while our focus in this section will be on the use of cryptography for confidentiality, we’ll see shortly that cryptographic techniques are inextricably woven into authentication, message integrity, nonrepudiation, and more.

Cryptographic techniques allow a sender to disguise data so that an intruder can gain no information from the intercepted data. The receiver, of course, must be able to recover the original data from the disguised data. Figure 8.2 illustrates some of the important terminology.

Suppose now that Alice wants to send a message to Bob. Alice’s message in its original form (for example, “Bob, I love you. Alice”) is known as plaintext, or cleartext. Alice encrypts her plaintext message using an encryption algorithm so that the encrypted message, known as ciphertext, looks unintelligible to any intruder. Interestingly, in many modern cryptographic systems, including those used in the Internet, the encryption technique itself is known—published, stan- dardized, and available to everyone (for example, [RFC 1321; RFC 3447; RFC 2420; NIST 2001]), even a potential intruder! Clearly, if everyone knows the method for encoding data, then there must be some secret information that prevents an intruder

from decrypting the transmitted data. This is where keys come in.

Key:

Figure 8.2 Cryptographic components In Figure 8.2, Alice provides a key, K A , a string of numbers or characters, as input to the encryption algorithm. The encryption algorithm takes the key and the

plaintext message, m, as input and produces ciphertext as output. The notation K A (m ) refers to the ciphertext form (encrypted using the key K A ) of the plaintext message, m . The actual encryption algorithm that uses key K A will be evident from the context. Similarly, Bob will provide a key, K B , to the decryption algorithm that takes the ciphertext and Bob ’s key as input and produces the original plain- text as output. That is, if Bob receives an encrypted message K A (m ), he decrypts it by computing K B (K A (m )) = m. In symmetric key systems, Alice ’s and Bob ’s keys are identical and are secret. In public key systems, a pair of keys is used. One of the keys is known to both Bob and Alice (indeed, it is known to the whole world). The other key is known only by either Bob or Alice (but not both).

Encryption algorithm

Decryption algorithm Plaintext Plaintext Ciphertext Channel K A K B Alice Bob

译文：

第八章计算机网络中的安全

早在1.6节就阐述了一些流行的和危险的网络攻击，包括恶意的软件攻击、拒绝服务、嗅探、源伪装以及报文修改和删除。虽然我们学习过有关计算机网络的相关知识，但却没有考察如何使用网络安全，使计算机远离攻击的威胁。在获得了新的计算机网络和网络协议的专业知识后，现在我们将深入地学习安全通信，尤其是计算机网络可以防御那些讨厌的坏家伙的原理。

首先，我们介绍一下Alice和Bob，他们两个人要进行通信，并希望这个过程是安全的。因为本书是网络教科书，所以Alice和Bob可以是两台需要安全地交换路由选择表的路由器，也可以是希望建立一个安全传输链接的客户和服务器，或者是两个交换安全电子邮件的电子邮件程序，所有这些案例都是在本站后面要考虑的。总之，Alice和Bob是安全领域中我们熟知的两个设备，也许因为使用Alice和Bob更有趣，这和命名A的普通实体需要安全地与名为B的普通实体进行通信的作用是一样的。需要安全通信的例子通常包括不正当的情人关系、战时通信和商务往来；我们更喜欢用第一个例子而不是后两个，并乐于使用Alice和Bob作为我们的发送方和接收方，并以第一种例子作为背景来讨论接下来的问题。

我们说过，Alice和Bob要进行通信并希望做到安全，那么此处的安全的确切意义是什么呢？正如我们将会看到的那样，安全性（像爱一样）是多姿多彩的；换句话说，安全性有很多方面。毫无疑问，Alice和Bob希望他们之间的通信内容是保密的。他们可能也想希望他们需要通信时确实是在和对方在通信，还希望如果他们之间的通信被窃听者篡改时，他们能够知道该通信已被这种篡改破坏。在本章的第一部分，我们将会讨论能够加密通信的密码技术，鉴别正在与他通信的对方并确保报文完整性。

在本章的第二部分，我们将研究基本的密码学原则怎样被应用于生成安全的网络协议。我们再次采用自顶向下方法，从应用层开始，将逐层（上面四层）研究安全协议。我们将研究如何加密电子邮件，如何加密一条TCP连接，如何在网络层提供覆盖式安全性，以及如何使无线LAN安全。在本章的第三部分，我们将考虑运行的安全性，这与保护机构网络免受攻击有关。特别的是，我们将仔细观察防火墙和入侵检测系统是怎样加强机构网络的安全性的。

网络安全是什么？

我们还是以要进行安全通信的情人Alice和Bob为例，开始我们的网络安全研究。这意味着什么呢？显然，Alice希望即使他们在一个不安全的媒体上进行通信，也只有Bob能够明白她发送的报文，其中入侵者（设入侵者叫Trudy）能够在这个媒体上截获这个报文。Bob也需要确认从Alice接收的报文确实是Alice发送的，并且Alice 也要确认和她进行通信的人就是Bob。Alice和Bob还要确认通信过程中报文没有被修改。他们首先也要确认他们能够通信（即无人能够拒绝他们进行通信所接入的资源）。考虑了这些问题后，我们能够指出安全通信具有下列所需要的特性。

●保密性。仅由发送方和其希望的接收方能够理解传输的报文的内容。因

为窃听者可以截获报文，这就要求报文必须在一定程度上进行加密，是

被截取的报文无法被窃听者理解。保密性的这个方面大概就是通常意义

上对于术语安全通信的理解。我们将会在8.2节学习数据加密和解密的密

码学技术。

●报文完整性。Alice和Bob希望确保其通信内容在传输过程中没有改变—

—或者恶意篡改或意外的改动。我们在可靠传输和数据链路协议中遇到

的检验和技术在扩展后能够用于提供这种报文完整性，我们将在8.3节中

研究这个主题。

●端点鉴定。发送方和接收方都应该能够证实通信时涉及的另一方，以确

定通信的另一方确实具有他们所声称的身份。人们的面对面通信可以通

过视觉轻易的解决这个问题。当通信实体在不能看到对方的媒体上交换

报文时，坚定就不是那么简单的了。当某用户要访问一个邮箱，邮件服

务器如何证实该用户就是他或她所声称的那个人呢？我们将在8.4节学

习端点鉴定技术。

●运行安全性。几乎所有的机构（公司、大学等等）今天都有了与公共因

特网相连的网络。这些网络都因此潜在地能够被危及安全。攻击者能试

图在网络主机中安放蠕虫，获取公司秘密，勘察内部网络配置并发起DoS

攻击。我们将在8.9节看到诸如防火墙和入侵检测系统等运行设备正被运

用于反制对机构网络的攻击。防火墙位于机构网络和公共网络之间，控

制接入和来自网络的分组。入侵检测系统执行深度分组检查任务，向网

络管理员发起有关可疑活动的警告。

明确了我们所指的网络安全的具体含义后，接下来要考虑的是入侵者可能要访问的是哪些信息，以及入侵者可能会采取哪些行动。图8-1阐述了一种情况。Alice（发送方）想要发送数据给Bob（接收方）。为了安全的交换数据，即在满足保密性、端点鉴定和报文完整性要求下，Alice和Bob将交换控制报文和数据报文（以非常类似于TCP发送和接受双方交换控制报文和数据报文的方式进行）。通常将这些报文全部或部分加密。如在1.6节所讨论的那样，入侵者能够潜在地执行下列行为：

数据数据

控制、数据报文

安全发送方安全接收方

信道

图8-1 发送方、接收方和入侵者（Alice、Bob和Trudy）

●窃听——坚挺并记录信道上传输的控制报文和数据报文。

●修改、插入、或删除报文或报文内容。

如我们将要看到的那样，除非采取适当的措施，否则上述能力使入侵者可以用各种各样的方式发动多种攻击：窃听通信内容（可能窃取口令和数据），假冒另一个实体，“劫持”一个正在进行的会话，通过使系统资源过载拒绝合法用户的服务请求等等。CERT协调中心对已报道的攻击进行了总结【CERT 2012】。

已经知道在因特网中某个角落存在真实的威胁，则Alice、Bob（我们的两个需要安全通信的朋友）在因特网上的对应的实体是什么呢？当然，Alice、Bob可以是两个位于端系统的人类用户，例如，真实的Alice和真实的Bob真的需要交换安全电子邮件。他们也可以参与电子商务事务。例如，一个真实的Bob希望安全的向一台Web 服务器传输他的信用卡号码，以在线购买商品。类似的，真实的Alice要与银行在线交互。需要安全通信的各方自身也可能是网络基础设施的一部分。前面讲过，域名系统（DNS，参见2.5节）或交换路由选择信息的路由选择守护程序（参见4.6节）需要在双方之间安全通信。对于网络管理应用也有相同的情况，网络管理是第9章学习的主题。能主动干扰DNS查找和更新（如在2.5节讨论的那样）、路由选择计算【RFC 4272】或网络管理功能【RFC 3414】的入侵者能够给因特网造成不可估量的破坏。

建立了上述框架，明确了一些重要意义以及网络安全需求之后，我们将深入学习密码学。应用密码学来提供机密性是不言而喻的，同时我们将很快看到他对于提供端点鉴定、报文完整性也起到了核心作用，这使得密码学成为网络安全的基石。

密码学的原则

尽管密码学的漫长历史可以追溯到朱利叶斯·凯撒，但现代密码技术（包括正在今天的因特网中应用的许多技术）基于的是过去30年所取得进展。Kahn的著作《破译者》回顾了引人入胜的密码学的悠久的历史。对密码学全面的讨论需要一本完整的书，所以我们只能初步了解密码学的基本方面，特别是这些东西正在今天的因特网上发挥作用。我们也注意到尽管本节的重点是密码学在保密性方面的应用，但我们将很快看到密码学技术与鉴别、报文完整性和不可否认性等是紧密相关的。

密码技术使得发送方可以伪装数据，使入侵者不能从截取到的信息中获得任何信息。当然，接收方必须能够从伪装的数据中恢复出原始数据。图8-2说明了一些重要的术语。

现在假设Alice要向Bob发送一个报文。Alice报文的最初形式被称为明文。Alice 使用加密算法加密其明文报文，生成的加密报文称为密文，该密文在任何入侵者看来是不明白的。有趣的是在许多现代密码系统中，包括因特网上所使用的那些，加密技术本身是已知的，即公开发行的、标准化的和任何人都可使用的，即对潜在的入侵者也是如此！显然，如果任何人都知道数据编码的方法，则一定有一些秘密信息可以组

织入侵者解密被传输的数据。这些秘密信息就是密钥。

图8-2 密码学组成部分

在图8-2中，Alice 提供了一个密钥Ka ，它是一串数字或字符，作为加密算法的输入。加密算法以密钥和明文报文m 为输入，生成的密文作为输出。用符号Ka （m ）表示明文报文m 的密文形式。使用密钥Ka 的实际加密算法显然与上下文有关。类似的Bob 将为解密算法提供密钥Kb ，将密文和Bob 的密钥作为输入，输出起始明文。也就是说，如果Bob 接收到一个加密的报文Ka （m ），他可通过计算Kb （Ka （m ））=m 进行解密。在对称密钥系统中，Alice 和Bob 的密钥是相同的和秘密的。在公开密钥系统中，使用一对密钥：一个密钥为Bob 和Alice 两人所知（实际上全世界都知道），另一个密钥只有Bob 或Alice 知道（不是双方都知道）。

加密算法

解密算法 明文

明文

信道 密文 图例：

密钥