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Secrets and Lies: Digital Security in a Networked World, 15th Anniversary Edition by Bruce Schneier

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21

Attack Trees

Danaë was the daughter of Acrisius. An oracle warned Acrisius that Danaë’s son would someday kill him, so Acrisius shut Danaë in a bronze room, away from anything even remotely masculine. Zeus had the hots for Danaë, so he penetrated the bronze room through the roof, in the form of a shower of gold that poured down into her lap. Danaë gave birth to Perseus, and you can probably guess the end of the story.

Threat modeling is, for the most part, ad hoc. You think about the threats until you can't think of any more, then you stop. And then you're annoyed and surprised when some attacker thinks of an attack you didn't. My favorite example is a band of California art thieves that would break into people's houses by cutting a hole in their walls with a chainsaw. The attacker completely bypassed the threat model of the defender. The countermeasures that the homeowner put in place were door and window alarms; they didn't make a difference to this attack.

To help the process, I invented something called an attack tree. Attack trees provide a methodical way of describing threats against, and countermeasures protecting, a system. By extension, attack trees provide a methodical way of representing the security of systems. They allow you to make calculations about security, compare the security of different systems, and do a whole bunch of other cool things.

Basically, you represent attacks against a system in a tree structure, with the goal as the root node and different ways ...

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