Information Security. Mark Stamp

give rise to security flaws, these problems are created unintentionally by well‐meaning developers. On the other hand, some software is written with the intent of doing evil. Examples of such malicious software, or malware, includes the all‐too‐familiar computer viruses and worms that plague the Internet today. How do these nasty beasts do what they do, and what can Alice's Online Bank do to limit their damage? What can Trudy do to increase the nastiness of such pests? We'll consider these and related questions.

Of course, Bob has many software concerns, too. For example, when Bob enters his password on his computer, how does he know that his password has not been captured and sent to Trudy? If Bob conducts a transaction at www.alicesonlinebank.com , how does he know that the transaction he sees on his screen is the same transaction that actually goes to the bank? That is, how can Bob be confident that his software (not to mention the network) is behaving as it should, instead of as Trudy would like it to behave? We'll consider these sorts of questions as well.

1.3 About This Book

      Lampson [69] believes that real‐world security boils down to the following:

       Specification/policy — What is the system supposed to do?

       Implementation/mechanism — How does it do it?

       Correctness/assurance — Does it really work?

Your humble author would humbly³ add a fourth category:

       Human nature — Can the system survive “clever” users?

      The focus of this book is primarily on the implementation/mechanism front. Your self‐assured author assures you that this is appropriate, nay essential, for an introductory course, since the strengths, weaknesses, and inherent limitations of the mechanisms directly affect all other aspects of security. In other words, without a reasonable understanding of the mechanisms, it is not possible to have an informed discussion of other relevant security issues.

The material in this book is divided into four major parts. The first part deals with cryptography, while the next part covers access control. Part III shifts the focus to network security, where the emphasis is on security protocols. The final major part of the book deals with the vast and relatively ill‐defined topic of software. Hopefully, the previous discussion of AOB⁴ has convinced you that these major themes are all relevant to real‐world information security.

      In the remainder of this chapter, we'll give a quick preview of each of these four major themes. The chapter concludes with a summary, followed by several not‐to‐be‐missed homework problems.

      1.3.1 Cryptography

      Cryptography is a fundamental tool in information security. Cryptography has many uses, including providing confidentiality and integrity, among other vital information security functions. We'll discuss cryptography in detail, as a working knowledge of crypto basics is essential background for any informed discussion of information security.

We'll begin our coverage of cryptography with a look at a handful of classic cipher systems. In addition to their obvious historical and entertainment value, these classic ciphers illustrate the fundamental principles that are employed in modern digital cipher systems, but in a more user‐friendly format.

      With this background, we'll be prepared to study modern cryptography. Symmetric key cryptography and public key cryptography are the two major branches of cryptography, and each plays a prominent role in information security. We'll spend an entire chapter on symmetric ciphers, and another chapter on public key systems. We then turn our attention to cryptographic hash functions, which are another fundamental security tool. Hash functions are used in many different contexts, some of which are surprising, or even bordering on the counterintuitive (e.g., blockchain).

      Then we'll briefly consider a few special topics that are related to cryptography. For example, we'll discuss steganography, where the goal is, essentially, to hide information in plain sight.

      1.3.2 Access Control

As mentioned above, access control deals with authentication and authorization. In the area of authentication, we'll consider many issues related to passwords. Passwords are the most oft‐used form of authentication today, but this is primarily because passwords are cheap, and definitely not because they are the most secure option.⁵

      We'll consider how to securely store passwords. Then we'll delve into the issues surrounding secure password selection and related issues. In real world systems, passwords often represent a major security vulnerability.

      The alternatives to passwords include biometrics and various physical devices, such as smartcards. We'll consider some of the security benefits of these alternate forms of authentication. In particular, we'll discuss several biometric authentication techniques.

Recall that authorization deals with restrictions placed on authenticated users. The two classic methods for enforcing such restrictions are so‐called access control lists⁶ and capabilities. We'll look at the plusses and minuses of each of these methods.

      Authorization leads naturally to a few relatively specialized topics. We'll discuss multilevel security, which leads us into the rarified air of security modeling. We also discuss covert channels and inference control, which are challenging issues to deal with in practical systems.

      1.3.3 Network Security

Our third major topic is network security, where our emphasis is on security protocols. First, we provide a general introduction to networking, with special attention to the security issues that arise. This includes a discussion of firewalls, for example.

      Then we consider the problems that arise when authenticating over a network. Many examples are provided, each of which illustrates a particular security pitfall. For example, replay attacks are a critical issue, and hence we consider effective ways to prevent such attacks.

      Cryptography is an essential ingredient in authentication protocols. We'll give examples of protocols that use symmetric cryptography, as well as examples that rely on public key cryptography. Hash functions also have an important role to play in security protocols.

      Our study of simplified authentication protocols will illustrate some of the many subtleties that can arise in this field—a seemingly insignificant change can completely change the security of a protocol. We'll also highlight a variety of specific techniques that are commonly used in real‐world security protocols.

      Then we'll move on to study several real‐world security protocols. First, we look at the so‐called Secure Shell, or SSH, which is a relatively simple example. Next, we consider the Secure Sockets Layer, or SSL, which is used extensively to secure e‐commerce on the Internet. The SSL protocol is elegant and efficient, and it is well designed for its specific purpose.

      We also discuss IPsec, which is another Internet security protocol. Conceptually, SSL and IPsec share many similarities, but the implementations differ greatly. In contrast to SSL, IPsec is complex—it's often said to be over‐engineered. Due to its complexity, some fairly significant security issues are present in IPsec. The contrast between SSL and IPsec illustrates some of the inherent challenges in designing security protocols.

      Another real‐world protocol that we'll consider is Kerberos, which is an authentication system based on symmetric cryptography. Kerberos follows a much different approach than either SSL or IPsec.

      We'll also discuss two wireless security protocols, WEP and GSM. Both of these protocols have