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Applied Cryptography: Protocols, Algorithms and Source Code in C, 20th Anniversary Edition by Bruce Schneier

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CHAPTER 2

Protocol Building Blocks

2.1 INTRODUCTION TO PROTOCOLS

The whole point of cryptography is to solve problems. (Actually, that's the whole point of computers—something many people tend to forget.) Cryptography solves problems that involve secrecy, authentication, integrity, and dishonest people. You can learn all about cryptographic algorithms and techniques, but these are academic unless they can solve a problem. This is why we are going to look at protocols first.

A protocol is a series of steps, involving two or more parties, designed to accomplish a task. This is an important definition. A “series of steps” means that the protocol has a sequence, from start to finish. Every step must be executed in turn, and no step can be taken before the previous step is finished. “Involving two or more parties” means that at least two people are required to complete the protocol; one person alone does not make a protocol. A person alone can perform a series of steps to accomplish a task (like baking a cake), but this is not a protocol. (Someone else must eat the cake to make it a protocol.) Finally, “designed to accomplish a task” means that the protocol must achieve something. Something that looks like a protocol but does not accomplish a task is not a protocol—it's a waste of time.

Protocols have other characteristics as well:

  • Everyone involved in the protocol must know the protocol and all of the steps to follow in advance.
  • Everyone involved in the protocol must agree to follow ...

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