The Unfalsifiability of Security Claims

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2016/05/the_unfalsifiab.html

Interesting research paper: Cormac Herley, “Unfalsifiability of security claims:

There is an inherent asymmetry in computer security: things can be declared insecure by observation, but not the reverse. There is no observation that allows us to declare an arbitrary system or technique secure. We show that this implies that claims of necessary conditions for security (and sufficient conditions for insecurity) are unfalsifiable. This in turn implies an asymmetry in self-correction: while the claim that countermeasures are sufficient is always subject to correction, the claim that they are necessary is not. Thus, the response to new information can only be to ratchet upward: newly observed or speculated attack capabilities can argue a countermeasure in, but no possible observation argues one out. Further, when justifications are unfalsifiable, deciding the relative importance of defensive measures reduces to a subjective comparison of assumptions. Relying on such claims is the source of two problems: once we go wrong we stay wrong and errors accumulate, and we have no systematic way to rank or prioritize measures.

This is both true and not true.

Mostly, it’s true. It’s true in cryptography, where we can never say that an algorithm is secure. We can either show how it’s insecure, or say something like: all of these smart people have spent lots of hours trying to break it, and they can’t — but we don’t know what a smarter person who spends even more hours analyzing it will come up with. It’s true in things like airport security, where we can easily point out insecurities but are unable to similarly demonstrate that some measures are unnecessary. And this does lead to a ratcheting up on security, in the absence of constraints like budget or processing speed. It’s easier to demand that everyone take off their shoes for special screening, or that we add another four rounds to the cipher, than to argue the reverse.

But it’s not entirely true. It’s difficult, but we can analyze the cost-effectiveness of different security measures. We can compare them with each other. We can make estimations and decisions and optimizations. It’s just not easy, and often it’s more of an art than a science. But all is not lost.

Still, a very good paper and one worth reading.