INVITATION TO CYBERSECURITY 146 to undo the scrambling and recover the original information. In computer cryptography, the scrambling process is called encryption and the unscrambling process is called decryption. Encryption turns the original, readable information, plaintext, into unreadable ciphertext. Encryption and decryption algorithms are parameterized with a key. The key is the secret knowledge that is needed to recover the original data (see Figure 7.1). Figure 7.1 Alice encrypts a message to Bob using their shared key. Cybersecurity and cryptography are inextricable, just like locks and keys are synonymous with security in physical space. Cryptography is the bedrock of cybersecurity because it is the main way that confidentiality and integrity are preserved in cyberspace—two of three main CIA triad pillars covered in Chapter 4. This chapter provides an overview of cryptography starting with its history and then exploring how cryptography is implemented in cyberspace. 7.1 Classic Cryptography “Few persons can be made to believe that it is not quite an easy thing to invent a method of secret writing which shall baffle investigation. Yet it may be roundly asserted that human ingenuity cannot concoct a cipher which human ingenuity cannot resolve.” - “A Few Words on Secret Writing” by Edgar Allan Poe Many cryptographic schemes have been invented throughout human history. This section covers only a few of the major developments. The techniques learned in classical cryptography are directly applicable to computer cryptography in the modern era. In this section we will examine some implementation details of classic cryptography to drive home the fundamental techniques that are used. 7.1.1 Letter Substitution: Ciphers “E however predominates so remarkably that an individual sentence of any length is rarely seen, in which it is not the prevailing character.” - “The Gold Bug” by Edgar Allan Poe One of the most famous early examples of cryptography is from Julius Caesar. He encoded his military orders using a shift of the Greek alphabet—substituting the original letter
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