Lawyers representing a man convicted of six robberies in the Detroit area have now filed their opening brief at the Supreme Court in one of the most important digital privacy cases in recent years.

This case, Carpenter v. United States, asks a simple question: is it OK for police to seize and search 127 days of cell-site location information (CSLI) without a warrant?

Previously, lower courts have said that such practices are compatible with current law. But the fact that the Supreme Court agreed to hear the case suggests that at least four justices feel that perhaps the law should be changed.

In Carpenter, as is the case in countless modern criminal cases, law enforcement was able to obtain the relevant records directly from the mobile phone provider with a court order that has less stringent requirements than a warrant.

This is not a trivial distinction. A so-called "d-order" can be circumspect with how information is obtained by authorities. It does not, as the Fourth Amendment demands, require as much particularity. A warrant, unlike a d-order application, also mandates a signed and sworn affidavit ("on oath or affirmation"), as the Constitution requires, which describes the "places to be searched and the things to be seized."

Carpenter's attorneys, many of whom are from the American Civil Liberties Union, argue in their filing that the current legal standard gives the government too much leeway.

"If the Court were to accept this argument, the government could use this tool to monitor the minute-by-minute whereabouts of anyone—from ordinary citizens to prominent businesspersons to leaders of social movements," they wrote in their August 7 brief.

"Ill-suited"

Previously, the Supreme Court found that there is no privacy interest in "business records" disclosed to a company—like location data, for instance—under the so-called "third-party doctrine."

However, in 2012, Justice Sonia Sotomayor wrote in her concurrence of United States v. Jones that the third-party doctrine is "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."

The Supreme Court has also recently shown more deference to digital privacy in the Riley v. California (2014) case: there, the nine justices unanimously agreed that, during an arrest, police could not search a mobile phone without a warrant.

Justice Sotomayor alluded to what is often referred to in legal academic circles as the "mosaic theory," the notion that the sum total of data gathered is often more revelatory than the discrete data collection. As such, some privacy activists argue, potentially revelatory data—like CSLI—should require a warrant.

However, Orin Kerr—a law professor at George Washington University and one of the nation’s top scholars on digital privacy law—has previously written that implementing the "mosaic theory" into practice is unworkable, as it opens a slew of questions that are not easily answered by judges, legislators, or law enforcement. In short, there is no "bright line."

"For example, what is the standard for the mosaic?" he wrote in 2012. "How should courts aggregate conduct to know when a sufficient mosaic has been created? Which techniques should fall within the mosaic approach? Should mosaic searches require a warrant? If so, how can mosaic warrants satisfy the particularity requirement? Should the exclusionary rule apply to violations of the mosaic search doctrine? Who has standing to challenge mosaic searches?"

On Twitter, Kerr lambasted those who side with the "mosaic theory." Meanwhile, other lawyers have also taken to Twitter to disagree with their esteemed colleague.

OK, I can play that game -- my bright line is that you need a warrant for location data. I bet most Americans would like my rule better. — David Oscar Markus (@domarkus) August 8, 2017

Ok. I will play. Mosaic is when sufficient data collected to allow reasonable inference of fact that could not be determined by single /1 — Paul Rosenzweig (@RosenzweigP) August 8, 2017

The government will present its response in the coming months. Oral arguments are likely to be scheduled for early November 2017, with a decision anticipated in the spring of 2018.