Computer networks are typically a shared resource used by many applications representing different interests. The Internet is particularly widely shared, being used by competing businesses, mutually antagonistic governments, and opportunistic criminals. Unless security measures are taken, a network conversation or a distributed application may be compromised by an adversary.
Consider some threats to secure use of, for example, the World Wide Web. Suppose you are a customer using a credit card to order an item from a website. An obvious threat is that an adversary would eavesdrop on your network communication, ...