When I teach computer security, one of the first lessons is on the need to have a clear threat model, that is, a clearly defined statement of which harms you are trying to prevent, and what assumptions you are making about the capabilities and motivation of the adversaries who are trying to cause those harms. Many security failures stem from threat model confusion. Conversely, a good threat model often shapes the solution.
The same is true for security research: the solutions you develop will depend strongly on what threat you are trying to address.
Lately I’ve noticed more and more papers in the computer security research literature that include subpoenas and/or search warrants as part of their threat model. For example, the Vanish paper, which won Best Student Paper (the de facto best paper award) at the recent Usenix Security symposium, uses the word “subpoena” 13 times, in passages like this:
Attackers. Our motivation is to protect against retroactive data disclosures, e.g., in response to a subpoena, court order, malicious compromise of archived data, or accidental data leakage. For some of these cases, such as the subpoena, the party initiating the subpoena is the obvious “attacker.” The final attacker could be a user’s ex-husband’s lawyer, an insurance company, or a prosecutor. But executing a subpoena is a complex process involving many other actors …. For our purposes we define all the involved actors as the “adversary.”
(I don’t mean to single out this particular paper. This is just the paper I had at hand — others make the same move.)
Certainly, subpoenas are no fun for any of the parties involved. They’re costly to deal with, not to mention the ick factor inherent in compelled disclosure to a stranger, even if you’re totally blameless. And certainly, subpoenas are sometimes used to harass, rather than to gather legitimately relevant evidence. But are subpoenas really the biggest threat to email confidentiality? Are they anywhere close to the biggest threat? Almost certainly not.
Usually when the threat model mentions subpoenas, the bigger threats in reality come from malicious intruders or insiders. The biggest risk in storing my documents on CloudCorp’s servers is probably that somebody working at CloudCorp, or a contractor hired by them, will mess up or misbehave.
So why talk about subpoenas rather than intruders or insiders? Perhaps this kind of talk is more diplomatic than the alternative. If I’m talking about the risks of Gmail, I might prefer not to point out that my friends at Google could hire someone who is less than diligent, or less than honest. If I talk about subpoenas as the threat, nobody in the room is offended, and the security measures I recommend might still be useful against intruders and insiders. It’s more polite to talk about data losses that are compelled by a mysterious, powerful Other — in this case an Anonymous Lawyer.
Politeness aside, overemphasizing subpoena threats can be harmful in at least two ways. First, we can easily forget that enforcement of subpoenas is often, though not always, in society’s interest. Our legal system works better when fact-finders have access to a broader range of truthful evidence. That’s why we have subpoenas in the first place. Not all subpoenas are good — and in some places with corrupt or evil legal systems, subpoenas deserve no legitimacy at all — but we mustn’t lose sight of society’s desire to balance the very real cost imposed on the subpoena’s target and affected third parties, against the usefulness of the resulting evidence in administering justice.
The second harm is to security. To the extent that we focus on the subpoena threat, rather than the larger threats of intruders and insiders, we risk finding “solutions” that fail to solve our biggest problems. We might get lucky and end up with a solution that happens to address the bigger threats too. We might even design a solution for the bigger threats, and simply use subpoenas as a rhetorical device in explaining our solution — though it seems risky to mislead our audience about our motivations. If our solution flows from our threat model, as it should, then we need to be very careful to get our threat model right.