Second, some readers argued that a Tor user loses a reasonable expectation of privacy in IP addresses because the user must disclose his true IP address to Tor. This is essentially the argument the government (briefly) makes in Michaud: By using Tor, you are sending your IP address to Tor, which is ultimately hosted by “an unknowable collection of strangers” who are running Tor exit nodes. You have put out your IP address to lots of people, which means that you have no expectation of privacy in it.
That argument doesn’t work. Fourth Amendment law regulates how the government learns information, not what information it learns. Where the government gets the information controls what is a search. If the government wants to obtain public information that I have stored in a private place, such as in my house, it’s a search for the government to enter that private place. Conversely, if the government wants to obtain private information that I have put in a public place, it’s not a search for the government to go to the public place and observe it. It’s the place that matters, not the abstract status of the information. Government access to information stored inside a suspect’s computer without permission is a search regardless of whether the information has been voluntarily revealed in some other way to someone else.
A physical world analogy might make the point more intuitive. Imagine I want to invite lots of people to my birthday party. I mail out 300 paper invitations, and I have 20 invitations left over that I keep in my kitchen drawer. The police are investigating me for drug offenses, and they hear through the grapevine that the invitation says that there will be lots of cocaine at the party. The police want to get a copy of the invitation so they can get a warrant to search the place where the party will be held.
Whether the government conducts a search of my house, papers or effects when they obtain a copy of the invitation depends entirely on which copy of the invitation they obtain. On one hand, if the police obtain one of the invitations that I mailed out and that has already been delivered, then obtaining the invitation is not a search. Upon delivery of a letter, my Fourth Amendment rights as a sender extinguish. The police are free to seize as many copies of already-delivered letters as they want and it won’t implicate my Fourth Amendment rights.
On the other hand, the fact that there are hundreds of invitations circulating doesn’t mean that the police can break into my house and search my kitchen drawer to find the 20 invitations I never sent. Breaking into my house and opening my kitchen drawer is a search, no matter whether the information found inside there is something that I have disclosed elsewhere. If the government tried to argue that no search occurred when they broke into my house and rifled through my kitchen, because, well, they could have obtained the invitation some other way that is not a search, no judge would be convinced.
The same principle applies to computer searches, and that makes this an easy issue. What matters is how the government obtained the information, not whether it could have obtained the information some other way that would not be a search. If the police think they can obtain the information another way that isn’t a search, they are free to try. But if the police fail in their efforts, and the only way they can get the information is by accessing a Fourth-Amendment-protected space, then accessing that space is a search.