Security is layered like an onion. On the outside are the users: How they use the system, who they trust, what they do when the system fails. Inside that are the security relationships between the user and the system, and between different systems. Further inside is the software, those bug-riddled pieces of code that are expected to enforce whatever security rules we have. That software works on networks and computers. Looking further in toward the theoretical are the idealized protocols that the computers run. And in the center (sometimes) is the cryptography: the mathematical equations that enforce security.
Security is a process, not a product. As a process, it has many components. And like any process, some of these components are sturdier, more reliable, more oiled, more secure. Moreover, the components have to fit together. The better they fit together, the better the process works. Often it's the interfaces between components that are the least secure.
Security is also like a chain. It is composed of many links, and each one of them is essential to the strength of the chain. And like a chain, security is only as strong as the weakest link. In this part, we look at the different security technologies that make up a chain, looking from the inside of the onion to the outside.
And we try not to mix metaphors quite so badly anymore.