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Applied Cryptography: Protocols, Algorithms, and Source Code in C, Second Edition by Bruce Schneier

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CHAPTER 24

Example Implementations

It's one thing to design protocols and algorithms, but another thing to field them in operational systems. In theory, theory and practice are the same; in practice they are different. Often ideas that look good on paper don't work in real life. Maybe the bandwidth requirements are too large; maybe the protocol is too slow. Chapter 10 discusses some of the issues related to using cryptography; this chapter gives examples of how it has been done in practice.

24.1 IBM SECRET-KEY MANAGEMENT PROTOCOL

In the late 1970s IBM developed a complete key management system for communications and file security on a computer network, using only symmetric cryptography [515, 1027]. This protocol is less important in the actual mechanisms and more in its overall philosophy: By automating the generation, distribution, installation, storage, changing, and destruction of keys, the protocol went a long way to ensure the security of the underlying cryptographic algorithms.

This protocol provides three things: secure communications between a server and several terminals, secure file storage at the server, and secure communication among servers. The protocol doesn't really provide for direct terminal-to-terminal communication, although it can be modified to do that.

Each server on the network is attached to a cryptographic facility, which does all of the encrypting and decrypting. Each server has a Master Key, KM0, and two variants, KM1 and KM2, both of which are simple ...

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