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# Elementary Cryptography

Cryptographysecret writingis the strongest tool for controlling against many kinds of security threats.

Chapter 2

Elementary Cryptography

In this chapter

Concepts of encryption

Cryptanalysis: how encryption systems are "broken"

Symmetric (secret key) encryption and the DES and AES algorithms

Asymmetric (public key) encryption and the RSA algorithm

Key exchange protocols and certificates

Digital signatures

Cryptographic hash functions

Cryptographysecret writingis the strongest tool for controlling against many kinds of security threats. Well-disguised data cannot be read, modified, or fabricated easily. Cryptography is rooted in higher mathematics: group and field theory, computational complexity, and even real analysis, not to mention probability and statistics. Fortunately, it is not necessary to understand the underlying mathematics to be able to use cryptography.

We begin this chapter by examining what encryption does and how it works. We introduce the basic principles of encryption with two simple encryption methods: substitution and transposition. Next, we explore how they can be expanded and improved to create stronger, more sophisticated protection. Because weak or flawed encryption provides only the illusion of protection, we also look at how encryption can fail. We analyze techniques used to break through the protective scheme and reveal the original text. Three very popular algorithms are in use today: DES, AES, and RSA. We look at them in some detail to see how these and other algorithms can be used as building blocks with protocols and structures to perform other computing tasks, such as signing documents, detecting modification, and exchanging sensitive data.

Chapter 12 offers a deeper analysis of encryption techniques and algorithms, including their mathematical bases, the mechanisms that make them work, and their limitations. Most users of cryptography will never invent their own algorithms, just as most users of electricity do not build their own power generators. Still, deeper knowledge of how cryptography works can help you use it effectively, just as deeper knowledge of energy issues helps you understand the environmental and cost trade- offs among different energy sources. This chapter offers you a rudimentary understanding of what cryptography is; but we encourage you to study the details in Chapter 12 to better understand the implications of each choice of cryptographic algorithm.

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