The historic decision handed down by D.C. federal judge Richard Leon last week that found the NSA's bulk collection of phone records likely violated the Fourth Amendment is a crucial first step towards protecting digital privacy from suspicionless government searches. But the importance of the decision extends beyond the NSA's surveillance programs. Judge Leon made two important findings on how the Fourth Amendment right to be free from unreasonable searches should apply at a time when technology can make invasive surveillance -- once considered the stuff of science fiction -- a part of every day life.

The Increasing Irrelevancy of Smith v. Maryland

Critically, Judge Leon's opinion recognized that relying on the Supreme Court's 1979 decision in Smith v. Maryland to look at new forms of electronic surveillance was foolish. Smith ruled that a person did not have a Fourth Amendment expectation of privacy in the phone numbers he dialed since he had turned that information over to the phone company in order to make the call. Since Smith was decided, courts have extended it to defeat Fourth Amendment challenges to many forms of warrantless surveillance such as acquiring historical cell site records, information turned over to Internet companies like Twitter, and even customer records held by a utility. So when defending the NSA's surveillance, the government naturally argued that Smith controlled and no warrant was needed to access the phone records.

As technology has advanced, however, courts have become leery of relying on Smith to justify endless amounts of electronic surveillance. In 2010, the Sixth Circuit Court of Appeals ruled in United States v. Warshak that people have an expectation of privacy in their email even though the messages are stored on a service provider's server. In 2012, Supreme Court Justice Sonia Sotomayor noted in her concurring opinion in United States v. Jones that the doctrine was "ill suited to the digital age, in which people reveal a great deal of information about themselves to third parties in the course of carrying out mundane tasks." Earlier this year, the New Jersey Supreme Court ruled under its state constitution that police needed a search warrant to obtain cell site records since the expectation of privacy in a person's location is not extinguished merely because a cell phone company has the cell site records.

Judge Leon was also unconvinced that Smith controlled, finding the opinion did not justify the NSA's records grab for reasons that extend to all sorts of surveillance practices conducted by the NSA and other law enforcement agencies.

Technological Changes Since Smith Matter

One of the fundamental reasons Judge Leon found Smith didn't control was because technology today has far surpassed what the Supreme Court was considering in 1979.

Smith was dealing with a much more limited form of surveillance -- the phone numbers dialed by one customer over a two-week period -- than what the NSA was doing. Indeed, the Supreme Court in 1979 could not have envisioned the type of bulk collection and analysis of phone records the NSA was conducting, let alone the fact that since cell phones are so ubiquitous, the police could use them to recreate a person's location and movements over an extended period of time. Plus, there has been a significant change in the government's relationship with the telephone companies since Smith. Rather than asking the phone company to engage in limited surveillance of one suspect over a short time period, the government was ordering Verizon to disclose to the government every record of every customer every day on a continuing basis since 2006. The government's ability to collect phone records in bulk allowed it to learn more information than merely collecting information about one particular user in a narrower time frame. Indeed, that's precisely why the government wants the records: to determine connections and associations between people in a way never before imaginable.

Ultimately, this difference in scale has constitutional implications.

The underlying rationale of Smith is that once customers exposed information to the phone company, they lost the ability to control that information and ran the risk that the company would turn that information over to someone else, including the government. Ultimately, what matters is "exposure" but whether someone "exposes" something is dependent on realistic, not theoretical possibilities. For Judge Leon, even if you "expose" some numbers to the phone company to make a phone call, you don't actually expect and thus don't "expose" all of the numbers you've dialed (or which have dialed you) for the last five years to the NSA.

This is perhaps the most important takeaway from Judge Leon's decision. As technology advances are allowing police to take information that is "exposed" in discrete ways for limited purposes, and aggregate that information in a way that invades upon privacy expectations, constitutional protections are triggered. So even though you may "expose" the front door of your house to your next door neighbor for fleeting moments, since you don't expect to have your neighbor sit outside your door all day for a month to watch everything you do, you likewise don't "expose" your home to around the clock video surveillance. While you may expose your location to a person driving by you on the street for a few moments until you make a left turn, you don't expect that person to follow you for weeks and months, and therefore you don't "expose" your location to continuous GPS tracking.

As police departments throughout the country get their hands on all sort of digital surveillance goodies from drones to facial recognition devices to mesh networks, it's important for courts to keep in mind that just because surveillance is possible doesn't mean its reasonable. Thankfully, the court that will review Judge Leon's decision has already indicated its sympathy to this approach. Hopefully the other courts hearing challenges to the NSA's phone records collection program, as well as those reviewing Fourth Amendment challenges to new technologies in the coming new year, will reach the same result.