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Trusted – by who, why, for what?

September 21, 2013

All too commonly, security architectures are simplified, dividing entities and components into exactly two categories: the trusted and the untrusted.  Sometimes the trusted are distinguished by their placement (e.g., one or the other side of a firewall), or sometimes by possession of a key.  It’s problematic, however, to have exactly two and only two categories.  It’s also important to understand who is trusting a particular entity, what their basis for that trust is, and what properties are being trusted.  The fact that you’re able to verify the identity of a site that you’re communicating with can help to place it in the context of a relationship or reputation.  Certification authorities (CAs) can be important intermediaries in enabling that verification, constraints outside this post’s scope notwithstanding.

The ability to authenticate an identity, though, isn’t sufficient to ensure that it’s an identity that you’ll necessarily want to have anything else to do with.  Even if you do want to interact with it, the fact of its authentication doesn’t guarantee that its processing will do exactly what you want and expect, and without adding and performing additional operations that you didn’t request or anticipate.  I might choose to trust entity A and not entity B based on information that I’ve obtained or experienced, or you might make the opposite choice. The fact that both A and B have certified keys can provide useful input to overall trust decisions but doesn’t render those decisions moot. 

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