Mastering Algorithms with Perl

Chapter 13.  Cryptography

Quis custodiet ipsos custodes?

—Juvenal (c. 60-140), Satires

Do you lock your door when you leave the house? Do you write personal information on a postcard? Do you pay bills without verifying the amounts? Cryptography is the science of ensuring that important activities can only be carried out by authorized entities. There are many variations on this theme: identification, permissions, and, of course, secrecy.

Cryptography is diverse, because there are many ways that people and computers can gain unauthorized access to data. The access can be passive (reading data) or active (transforming data). This chapter deals with a number of topics which may seem independent, but there are frequent and surprising connections among them. They have a lot to do with number theory, probability (especially random numbers), and compression (which treats redundancy as an opportunity, while cryptography treats redundancy as a hazard). Some of the topics are:

• Logging on to a computer

• Determining whether a file’s contents have changed

• Sending a secret message that intermediate parties will not be able to read

• Legal issues involved in writing, distributing, and using cryptography

To prevent a bad guy from gaining access to an important resource, you can either guard the resource or lock it. Guarding controls the individual; a guard challenges individuals, requiring them to verify their authority. Locking controls the resource; a physical lock prevents access ...