# What is Security?

Security in general is about achieving some goal when there is an adversary present.

A secure system is one that can actually do something, regardless of what the bad guy is trying to do to you.

The general way to think about security at a high lovel is to break it in three parts:

1. Policy: statement about what I want my system to be able to do. Typical policies is concerned with the confidentiality of data, the integrity of data nad availability of data.
2. Threat model: a set of assumptions about the adversary (see ). As a general rule it’s much better to err on the side of caution.
3. Mechanism: the software, hardware or system, implementation etc. that’s going to try to make sure our policy is followed as long as the bad guys follow the threat model.

If our threat model is correct, hopefully we’ll satisfy our policy. And the mechanism should work as well.

## Why is it Hard?

In theory the rules are pretty simple, but in practice breaches and bugs are fairly common.

The reason is policy is a negative goal: the security policy must works regardless of what the attacker can do.

For example for a file system, it’s easier to have a rule that says “$$x$$ and $$y$$ can access the file” than “no one other than $$x$$ or $$y$$ can access the file” it’s a much more difficult problem to solve.

There is no right answer, a clear cut as to what the right sets of assumptions to make is: it is an iterative process.

In practice, people often get all three points above (policy, threat model and mechanism) wrong.

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