I've just gotten around to reading Orin Kerr's fine paper "Applying the Fourth Amendment to the Internet: A General Approach." Like most everything he writes on the topic of technology and privacy, it is thoughtful and worth reading. Here, from the abstract, are the main conclusions:
First, the traditional physical distinction between inside and outside should be replaced with the online distinction between content and non-content information. Second, courts should require a search warrant that is particularized to individuals rather than Internet accounts to collect the contents of protected Internet communications. These two principles point the way to a technology-neutral translation of the Fourth Amendment from physical space to cyberspace.
I'll let folks read the full arguments to these conclusions in Orin's own words, but I want to suggest a clarification and a tentative objection. The clarification is that, while I think the right level of particularity is, broadly speaking, the person rather than the account, search warrants should have to specify in advance either the accounts covered (a list of e-mail addresses) or the method of determining which accounts are covered ("such accounts as the ISP identifies as belonging to the target," for instance). Since there's often substantial uncertainty about who is actually behind a particular online identity, the discretion of the investigator in making that link should be constrained to the maximum practicable extent.
The objection is that there's an important ambiguity in the physical-space "inside/outside" distinction, and how one interprets it matters a great deal for what the online content/non-content distinction amounts to. The crux of it is this: Several cases suggest that surveillance conducted "outside" a protected space can nevertheless be surveillance of the "inside" of that space. The grandaddy in this line is, of course, Katz v. United States, which held that wiretaps and listening devices may constitute a "search" though they do not involve physical intrusion on private property. Kerr can accomodate this by noting that while this is surveillance "outside" physical space, it captures the "inside" of communication contents. But a greater difficulty is presented by another important case, Kyllo v. United States, with which Kerr deals rather too cursorily.
In Kyllo, the majority—led, perhaps surprisingly, by Justice Scalia!—found that the use without a warrant of a thermal imaging scanner to detect the use of marijuana growing lights in a private residence violated the Fourth Amendment. As Kerr observes, the crux of the disagreement between the majority and the dissent had to do with whether the scanner should be considered to be gathering private information about the interior of the house, or whether it only gathered information (about the relative warmth of certain areas of the house) that might have been obtained by ordinary observation from the exterior of the house. No great theoretical problem, says Kerr: That only shows that the inside/outside line will sometimes be difficult to draw in novel circumstances. Online, for instance, we may be unsure whether to regard the URL of a specific Web page as mere "addressing" information or as "content"—first, because it typically makes it trivial to learn the content of what a user has read, and second, because URLs often contain the search terms manually entered by users. A similar issue arose with e-mail subject lines, which now seem by general consensus to be regarded as "content" even though they are transmitted in the "header" of an e-mail.
Focus on this familiar (if thorny) line drawing problem, however, misses what is important about the Kyllo case, and the larger problem it presents for Kerr's dichotomy: Both the majority and the dissent seemed to agree that a more sophisticated scanner capable of detecting, say, the movements of persons within the house, would have constituted a Fourth Amendment search. But reflect, for a moment, on what this means given the way thermal imaging scanners operate. Infrared radiation emitted by objects within the house unambiguously ends up "outside" the house: A person standing on the public street cannot help but absorb some of it. What all the justices appeared to agree on, then, is that the collection and processing of information that is unambiguously outside the house, and is conducted entirely outside the house, may nevertheless amount to a search because it is surveillance of and yields information about the inside of the house. This means that there is a distinction between the space where information is acquired and the space about which it is acquired.
This matters for Kerr's proposed content/non-content distinction, because in very much the same way, sophisticated measurement and analysis of non-content information may well yield information about content. A few examples may help to make this clear. Secure Shell (SSH) is an encrypted protocol for secure communications. In its interactive mode, SSH transmits each keystroke as a distinct packet—and this packet transmission information is non-content information of the sort that might be obtained, say, using a so-called pen/trap order, issued using a standard of mere "relevance" to an investigation, rather than the "probable cause" required for a full Fourth Amendment search—the same standard Kerr agrees should apply to communications. Yet there are strong and regular patterns in the way human beings type different words on a standard keyboard, such that the content of what is typed—under SSH or any realtime chat protocol that transmits each keystroke as a packet—may be deducible from the non-content packet transmission data given sufficiently advanced analytic algorithms. The analogy to the measurement and analysis of infrared radiation in Kyllo is, I think, quite strong.
It is not hard to come up with a plethora of similar examples. By federal statute, records of the movies a person rents enjoy substantial privacy protection, and the standard for law enforcement to obtain them—probable cause showing of "relevance" and prior notice to the consumer—is higher than required for a mere pen/trap. Yet precise analysis of the size of a file transmitted from a service like Netflix or iTunes could easily reveal either the specific movie or program downloaded, or at the least narrow it down to a reasonably small field of possibilities. Logs of the content-sensitive advertising served by a service like Gmail to a particular user may reveal general information about the contents of user e-mails. Sophisticated social network analysis based on calling or e-mailing patterns of multiple users may reveal, not specific communications contents, but information about the membership and internal structure of various groups and organizations. That amounts to revealing the "contents" of group membership lists, which could have profound First Amendment implications in light of a string of Supreme Court precedents making it clear that state compelled disclosure of such lists may impermissibly burden the freedom of expressive association even when it does not run afoul of Fourth Amendment privacy protections. And running back to Kyllo, especially as "smart" appliances and ubiquitous networked computing become more pervasive, analysis of non-content network traffic may reveal enormous amounts of information about the movements and activities of people within private homes.
Here's one way to describe the problem here: The combination of digital technology and increasingly sophisticated analytic methods have complicated the intuitive link between what is directly observed or acquired and what is ultimately subject to surveillance in a broader sense. The natural move here is to try to draw a distinction between what is directly "acquired" and what is learned by mere "inference" from the information acquired. I doubt such a distinction will hold up. It takes a lot of sophisticated processing to turn ambient infrared radiation into an image of the interior of a home; the majority in Kyllo was not sympathetic to the argument that this was mere "inference." Strictly speaking, after all, the data pulled off an Internet connection is nothing but a string of ones and zeroes. It is only a certain kind of processing that renders it as the text of an e-mail or an IM transcript. If a different sort of processing can derive the same transcript—or at least a fair chunk of it—from the string of ones and zeroes representing packet transmission timing, should we presume there's a deep constitutional difference?
That is not to say there's anything wrong with Kerr's underyling intuition. But it does, I think, suggest that new technologies will increasingly demand that privacy analysis not merely look at what is acquired but at what is done with it. In a way, the law's hyperfocus on the moment of acquisition as the unique locus of Fourth Amendment blessing or damnation is the shadow of the myopically property-centric jurisprudence the Court finally found to be inadequate in Katz. As Kerr intimates in his paper, shaking off the digital echoes of that legacy—with its convenient bright lines—is apt to make things fiendishly complex, at least in the initial stages. But I doubt it can be avoided much longer.