Tag Archives: classifier security model

Causal Insulation

I just came across an essay by Wolter Pieters that complements my 2009 NSPW paper (mentioned here and here in this blog before) in style and content. In The (social) construction of information security (author’s version as PDF), Pieters discusses security in terms of causal insulation. This notion has its roots in Niklas Luhmann’s sociological theory of risk. Causal insulation means that to make something secure, one needs to isolate it from undesired causes, in the case of security from those that attackers would intentionally produce.On the other hand, some causes need to be allowed as they are  necessary for the desired functioning of a system.

I used a similar idea as the basis of my classifier model. A system in an environment creates a range of causalities—cause-effect relationships—to be considered. A security policy defines which of the causes are allowed and which ones are not, splitting the overall space into two classes. This is the security problem. Enforcing this policy is the objective of the security design of a system, its security mechanisms and other security design properties.

A security mechanism, modeled as a classifier, enforces some private policy in a mechanism-dependent space, and maps the security problem to this private space through some kind of feature extraction. In real-world scenarios, any mechanism is typically less complex than the actual security problem. The mapping implies loss of information and may be inaccurate and partial; as a result, the solution of the security problem by a mechanism or a suite of mechanisms becomes inaccurate even if the mechanism works perfectly well within its own reference model. My hope is that the theory of classifiers lends us some conceptual tools to analyze the degree and the causes of such inaccuracies.

What my model does not capture very well is the fact that any part of a system does not only classify causalities but also defines new causalities, I’m still struggling with this. I also struggle with practical applicability, as the causality model for any serious example quickly explodes in size.

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Using an Inappropriate Classifier As a Security Mechanism

Zed Shaw has a story to tell about ACLs (Access Control Lists) as a common security mechanism and how they are incapable of modeling actual regulatory requirements:

(Vimeo: The ACL is Dead)

It’s not really a talk about ACLs, it’s really about how companies work and how to survive and stay sane inside enterprises. I’ll focus here, however, on the technical issue that he uses as a hook.

He poses the technical problem as »ACLs not being Turing-complete«. According to my favorite abstraction of security mechanisms, the classifier model, ACL access control schemes are a type of classifier that does not fit the problem. All security mechanisms distinguish deny from allow, just in different sets of entities and with different boundaries between the two subsets. A low complexity classifier can handle only subsets with a simple boundary between them—most entities have only neighbors of the same class, and those near the boundary have other-class neighbors only in one direction—whereas a complex classifier can model more complex class distinctions. The most complex classification would be a random assignment of classes to entities.

Two things (at least) affect the complexity that a classifier can handle: classifier design and feature extraction. Classifier design defines the boundaries that a classifier can model. Feature extraction defines the parameters or dimensions available to the classifier, the degree of abstraction with which the classifier sees the world. Authentication for instance has a high degree of abstraction, it can distinguish entities but nothing else. Access control is richer in the parameters it uses, including besides the identity of entitites also properties of objects and actions. Yet, as the talk illustrates, these dimensions are not sufficient to properly express regulatory requirements. Whether this is a problem of the mechanism or a problem of the requirements I leave for the reader to ponder.