Computer security is different from cryptography. It often uses cryptography, but its scope is much broader. General computer security includes such diverse things as controlling authorized (and unauthorized) computer access, managing computer accounts and user privileges, copy protection, virus protection, software metering, and database security. More generally, it also includes defenses against computers across network connections, password sniffers, and network worms, but we'll discuss those sorts of things in the chapters on network security. In the age of the Internet, computer security and network security have blurred considerably; but for the purposes of this book, I'll draw the somewhat arbitrary line between computer and network security as “whether or not the security problem affects any computer, as opposed to just a computer attached to networks.” General computer security, which can be defined as the prevention and/or detection of unauthorized actions by users of a computer system, seems a whole lot harder than the simple mathematics of cryptography. And it is.
Philosophically, the problem is that the defender doesn't have mathematics on his side. The mathematics of cryptography gives the defender an enormous advantage over the attacker. Add one bit to the key, double the work to break the algorithm. Add ten bits, multiply the work by a thousand. Computer security is more balanced: attackers and defenders can get similar advantages from technology. ...