Cryptography and Network Security (4th Edition)

Each of the messages, like each one he had ever read of Stern's commands, began with a number and ended with a number or row of numbers. No efforts on the part of Mungo or any of his experts had been able to break Stern's code, nor was there any clue as to what the preliminary number and those ultimate numbers signified.

Talking to Strange Men, Ruth Rendell

The Douglas Squirrel has a distinctive eating habit. It usually eats pine cones from the bottom end up. Partially eaten cones can indicate the presence of these squirrels if they have been attacked from the bottom first. If, instead, the cone has been eaten from the top end down, it is more likely to have been a crossbill finch that has been doing the dining.

Squirrels: A Wildlife Handbook, Kim Long

In this chapter, we look at important examples of both secure hash algorithms and message authentication codes (MACs). Most important modern hash functions follow the basic structure of Figure 11.9. This has proved to be a fundamentally sound structure, and newer designs simply refine the structure and add to the hash code length. Within this basic structure, two approaches have been followed in the design of the compression function, which is the basic building block of the hash function. Traditionally, most hash functions that have achieved widespread use rely on a compression function specifically designed for the hash function. Typically, the compression function makes use of modular arithmetic and logical binary operations. Another approach is to use a symmetric block cipher as the compression function. In this chapter, we examine perhaps the most important example of each approach: the Secure Hash Algorithm (SHA) and Whirlpool.

MACs also conveniently fall into two categories based on their fundamental building block. One popular approach is to use a hash algorithm such as SHA as the core of the MAC algorithm. Another approach is to use a symmetric block cipher in a cipher block chaining mode. Again, we look at perhaps the most important example of each approach: HMAC and CMAC.