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

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 ...