14 1. HISTORICAL INTRODUCTION Figure 1.4. An embedded digital image that says “Boss says we should blow up the bridge”. used exclusively to describe processes that conceal the presence of a secret message, which may or may not be additionally protected by a cipher or code. The content of the message is not altered through the process of disguising it. The use of wax tablets discussed in §1.1 is an example of ancient steganography. Modern steganography, discussed in Chapter 10, not only conceals the content of messages, but hides them in plain sight in digital images, music, and other digitized media. The computer has provided a modern day invisible ink as these messages are not discernable by the naked eye or ear (see Figure 1.4). Quantum computing has made quantum cryptography possible. Quantum cryptography uses quantum mechanical effects, in particular in quantum communication and computation, to perform encryption and de- cryption tasks. One of the earliest and best known uses of quantum cryp- tography is in the exchange of a key, called quantum key distribution. Earlier cryptology used mathematical theorems to protect the keys to mes- sages from possible eavesdroppers, such as the RSA key encryption system discussed in Chapter 8. The advantage of quantum cryptography is that it allows fast completion of various tasks that are seemingly impractical using only classical methods, and it holds forth the possibility of algorithms to do the seemingly impossible, though so far such algorithms have not been found. Chapter 10 includes a longer discussion of quantum cryptography and the mathematics and physics behind it. 1.4. Summary In this chapter we encountered many of the issues and key ideas of the subject (see [12] for an entertaining history of the subject). The first are various reasons requiring information protection. The case of Mary Stuart, Queen of Scots, and Anthony Babington show the grave consequences when ciphers are broken. While the effects here are confined to individuals, in

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