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With this post, I’m launching a new, (very) occasional series I’m calling YANAL, for “You Are Not A Lawyer.” In this series, I will try to disabuse computer scientists and other technically minded people of some commonly held misconceptions about the law (and the legal system).

I start with something from criminal law. As you probably already know, in the American criminal law system, as in most others, a jury must find a defendant guilty “beyond a reasonable doubt” to convict. “Beyond a reasonable doubt” is a famously high standard, and many guilty people are free today only because the evidence against them does not meet this standard.

When techies think about criminal law, and in particular crimes committed online, they tend to fixate on this legal standard, dreaming up ways people can use technology to inject doubt into the evidence to avoid being convicted. I can’t count how many conversations I have had with techies about things like the “open wireless access point defense,” the “trojaned computer defense,” the “NAT-ted firewall defense,” and the “dynamic IP address defense.” Many people have talked excitedly to me about tools like TrackMeNot or more exotic methods which promise, at least in part, to inject jail-springing reasonable doubt onto a hard drive or into a network.

People who place stock in these theories and tools are neglecting an important drawback. There are another set of legal standards–the legal standards governing search and seizure–you should worry about long before you ever get to “beyond a reasonable doubt”. Omitting a lot of detail, the police, even without going to a judge first, can obtain your name, address, and credit card number from your ISP if they can show the information is relevant to a criminal investigation. They can obtain transaction logs (think apache or sendmail logs) after convincing a judge the evidence is “relevant and material to an ongoing criminal investigation.” If they have probable cause–another famous, but often misunderstood standard–they can read all of your stored email, rifle through your bedroom dresser drawers, and image your hard drive. If they jump through a few other hoops, they can wiretap your telephone. Some of these standards aren’t easy to meet, but all of them are well below the “beyond a reasonable doubt” standard for guilt.

So by the time you’ve had your Perry Mason moment in front of the jurors, somehow convincing them that the fact that you don’t enable WiFi authentication means your neighbor could’ve sent the death threat, your life will have been turned upside down in many ways: The police will have searched your home and seized all of your computers. They will have examined all of the files on your hard drives and read all of the messages in your inboxes. (And if you have a shred of kiddie porn stored anywhere, the alleged death threat will be the least of your worries. I know, I know, the virus on your computer raises doubt that the kiddie porn is yours!) They will have arrested you and possibly incarcerated you pending trial. Guys with guns will have interviewed you and many of your friends, co-workers, and neighbors.

In addition, you will have been assigned an overworked public defender who has no time for far-fetched technological defenses and prefers you take a plea bargain, or you will have paid thousands of dollars to a private attorney who knows less than the public defender about technology, but who is “excited to learn” on your dime. Maybe, maybe, maybe after all of this, your lawyer convinces the judge or the jury. You’re free! Congratulations?

The police and prosecutors run into many legal standards, many of which are much easier to satisfy than “beyond a reasonable doubt” and most of which are met long before they see an access point or notice a virus infection. By meeting any of these standards, they can seriously disrupt your life, even if they never end up putting you away.