The Supreme Court has eschewed the “reasonable expectation of privacy” test in its most important recent Fourth Amendment cases. It’s not certain that the trend away from the so‐​called “Katz test,” largely driven by Justice Scalia, will continue, and nobody knows what will replace it. But doctrinal shift is in the air. Courts are searching for new and better ways to administer the Fourth Amendment.

A good example is the Tenth Circuit’s decision last week in U.S. v. Ackerman. That court found that opening an email file was a Fourth Amendment “search,” both as a matter of reasonable expectations doctrine and the “distinct line of authority” that is emerging from the Supreme Court’s 2012 decision in U.S. v. Jones.

Here are the facts: AOL scans outgoing emails for child porn by comparing hashes of files sent through its network to hashes of known child porn. When it becomes aware of child porn, it is required by law to report them to the National Center for Missing and Exploited Children. NCMEC is a governmental entity and agent. (That point takes up the bulk of the decision; Congress has made huge grants of governmental power to the organization.) NCMEC opened the file without a warrant.

Nobody in the case disputed that the email is a “paper” or “effect” for Fourth Amendment purposes. The remaining question was whether the opening of the email was a search. If it was, it required a warrant.

The government persuaded the court below that NCMEC did no more than AOL did. The “private search” doctrine suggests that Ackerman can’t complain of the government learning what AOL told it by looking again.

But the Tenth Circuit overruled the district court because of the difference between observing the similarity between the hashes of two files and looking at the file, the text of the email to which it was attached, and three other attachments. NCMEC’s observation of all the email’s content was a search, which requires a warrant.

But the private search doctrine’s leading case, U.S. v. Jacobsen (1984), allowed the government to chemically test a powdery substance that FedEx had reported after they opened a damaged parcel. Such a test exposes only contraband, in which there cannot be a reasonable expectation of privacy, so such test is not a search under “reasonable expectation of privacy” doctrine. Wouldn’t the analogy allow the government to look at an email that “tested positive” for child porn?

Not really. Looking at the rest of an email after one file has tested positive is an additional search of distinct things that have not tested positive.

But the court continued in a way that has been called puzzling. It bolstered its decision with reasoning that seems to recognize property rights in information.

“Jones explained that government conduct can constitute a Fourth Amendment search either when it infringes on a reasonable expectation of privacy or when it involves a physical intrusion (trespass) on a constitutionally protected space or thing (‘persons, houses, papers, and effects’) for the purpose of obtaining information.” Destroying powder to test it would seem to be a “trespass to chattels” that might not survive Jones. Opening an email would be the digital/​informational equivalent—also disallowed in light of Jones.

It is a puzzling decision, but that’s because Fourth Amendment doctrine is so convoluted. There are good ways to make sense of it.

An email is a communication analogous to a letter sent in the mail. Rather than writing words on paper, folding the paper, and sealing it so that its contents can’t be seen, the words are typed into a computer and (along with other content) encoded as digital signals. The signals pass invisibly along wires, cables, and radio waves like letters travel by road, rail, and air—unavailable to strangers. Emails are reconstructed into visible material at the receiving end like letters are unsealed by their proper recipients. Emails and their contents are papers and effects.

In general, AOL and other service providers have bound themselves by contract to maintain the confidentiality of such communications, but AOL does not permit transmission of child porn and it monitors for the stuff. When it discovers it, AOL is required by law to turn it over to NCMEC. (That reporting obligation is of doubtful constitutionality to me, but I assume that citizenship/​market pressure would cause AOL to report such crimes voluntarily.)

Ackerman doesn’t have a complaint about his information being absconded with by AOL. But he has a complaint when the government uses his email file to recreate the content of his email and peruse it. Distinct from possessing it, the government’s use of Ackerman’s data requires a warrant. The right to use is another of the sticks in the “bundle” that comprise property rights. AOL hadn’t converted the file to human‐​readable form; NCMEC did. It should have gotten a warrant.

This is how the case squares with Jones, which at its heart is a property case. Just like popping a GPS device onto a car converts it to the government’s purposes without affecting possession, making use of a file takes its “benefits” for the government—a seizure—regardless of possession.

(Courts and commentators routinely equivocate between property and “trespass,” as if the trespass cause of action triggers constitutional analysis. No, the seizure of a property right—any property right—is what should trigger further Fourth Amendment analysis.)

Making Ackerman’s data file into something humans can perceive facilitated the search that immediately followed. That’s the Kyllo case, which pairs with Jones as the foundation of a juridical method for interpreting the Fourth Amendment. There’s seizure founded in common law property rights, and search founded in common experience with sensing (and here data processing) that has a purpose of finding particular things.

In Kyllo, it wasn’t digitized email content, but analog infrared waves that government agents converted to perceptible form. As a Fourth Amendment scholar better recognized than I said of Kyllo, “the transformation of [a] signal into a form that communicates information to a person … constitutes the search.”

In U.S. v. Ackerman, the government in the person of NCMEC rightly came into possession of a file that probably contained child porn. Making use of that file was a seizure, which, though minor, required a warrant. That seizure faciliated a search, which would have been covered by the same, constitutionally required warrant.

Though it has called the “reasonable expectation of privacy” test and its odd corollaries into question, the Supreme Court has not provided a clear path for lower courts. Judge Gorsuch’s opinion in U.S. v. Ackerman is a game attempt at finding the right path. Here’s hoping the Supreme Court better fleshes out the contours of seizure (Jones) and search (Kyllo) in cases to come.