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IAGO: So will I turn her virtue into pitch,
And out of her own goodness make the net
That shall enmesh them all.
—The Tragedy of Othello, II, iii, 361–363.
Cryptographic systems are sensitive to environment. Using cryptosystems over a network introduces many problems. This chapter presents examples of these problems and discusses techniques for dealing with them. First comes a description of stream and block ciphers, followed by a review of the organization of the network layers. We then present several network protocols to show how these techniques are used in practice.
The key point of this chapter is that the strength of a cryptosystem depends in part on how it is used. A mathematically strong cryptosystem is vulnerable when used incorrectly.
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