This United States Supreme Court got it right in a unanimous decision on a key privacy issue in the 21st century (PDF). Today, the highest court in the land held that “the police generally may not, without a warrant, search digital information on a cell phone seized from an individual who has been arrested.”

The closely watched case, which the Court decided to take up in January 2014 and heard oral arguments in April 2014 (PDF), sets a landmark precedent for the digital due process that law enforcement officers must accord citizens, creating a legal requirement for the search of a private device similar to the one that exists for a private home.

The decision will have a far-reaching impact upon hundreds of millions of people, extending Fourth Amendment protections to a device that the vast majority of Americans carry everywhere today. Until today, whether police can search your phone when they arrest you was subject to the state you were in at the time, as Kashmir Hill reported at Forbes. By shielding the privacy of cell phones and thus their owners, the court decided upon an issue that directly affects the 91% of American adults who now carry cell phones and use them for professional and personal applications.

In Riley v. California (PDF), a lower court had ruled that a law enforcement officer could not only take and hold a suspect’s cell phone pursuant to an arrest but could also search the contents of that device without a warrant or probable cause. The consequences of the Supreme Court getting this decision wrong in this case would have been immense, given how much information mobile devices contain about where we have been and when, what we have communicated and to whom, or what we have searched for or the subsequent sites we have visited to shop.

The Supreme Court ruling extending this “broad cloak of privacy” for cell phones, as Lyle Denniston put it at SCOTUSblog, “put strict new technology-based limits on a century-old doctrine on police authority to search”:

“…the idea that, once police had arrested an individual, they could search items that person had brought along, in person or within reach in a car. Such a ‘search incident to arrest’ exception to the Fourth Amendment warrant requirement had been a feature of criminal law since it was first mentioned by the Court in 1914.”

In today’s Supreme Court decision, Chief Justice John Roberts, writing for the majority, recognized something that many people understand quite well: the smartphones that 58% of American adults have today are not just phones.

“Cell phones differ in a both a quantitative and a qualitative sense from other objects that might be kept on an arrestee’s person. The term ‘cell phone’ is itself misleading shorthand; many of these devices are in fact minicomputers that also happen to have the capacity to be used as a telephone. They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps, or newspapers.”

In the past, the Supreme Court has made exceptions for warrantless searches at the time of an arrest. This decision, however, recognized a critical distinction: “the possible intrusion on privacy is not physically limited in the same way when it comes to cell phones.” That potential is made possible through immense capacity of these devices, which goes far beyond the physical effects that an arrested person might carry in his or her pockets. This court’s understanding of this reality was reflected in the decision’s citation of an amicus brief filed by the Electronic Frontier Foundation and the Center for Democracy & Technology.

As Roberts pithily put it, to assert that a search of all of the data stored on a mobile phone is “materially indistinguishable” from searches of the same sorts of physical items “is like saying a ride on horseback is materially indistinguishable from a flight to the moon.”

What that means in practical terms is that a police officer using a “Universal Forensic Extraction Device” to scrape all of the data from a cell phone obtained from someone who committed, say, a minor traffic violation, is unconstitutional.

The requirement that law enforcement must get a warrant prior to search of a mobile device protects more than the media stored in its memory — it also extends to other data collected by it, in particular by location services. A recent research paper published in the New York University Journal of Law and Liberty found that just a week of location data stored on a mobile device may constitute unreasonable search and seizure. In this context, today’s ruling complements a similarly historic unanimous decision in United States vs. Jones (PDF) two years ago, in which the Supreme Court found that “the Government’s attachment of the GPS device to the vehicle, and its use of that device to monitor the vehicle’s movements, constitutes a search under the Fourth Amendment.” The result of that ruling was that the federal government had to get a probable cause warrant from a judge before they started tracking the movements of an American citizen using a Global Positioning System device.

The decision did take note of a practical concern that law enforcement officers may have regarding waiting for a warrant: an arrestee or his or her agent might wipe the device remotely, removing critical evidence in an investigation or pursuant to a crime. On that count, one technical detail jumped out from the decision that should be of particular interest to slightly geekier readers: the use of Faraday cages to isolate devices from remote access or control. Again, Roberts:

“Remote wiping can be fully prevented by disconnecting a phone from the network. There are at least two simple ways to do this: First, law enforcement officers can turn the phone off or remove its battery. Second, if they are concerned about encryption or other potential problems, they can leave a phone powered on and place it in an enclosure that isolates the phone from radio waves. …

Such devices are commonly called ‘Faraday bags,’ after the English scientist Michael Faraday. They are essentially sandwich bags made of aluminum foil: cheap, lightweight, and easy to use. They may not be a complete answer to the problem, but at least they are a reasonable response. In fact, a number of law enforcement agencies around the country already use Faraday bags.”

Such details aside, the Court specifically recognized today that “privacy comes at a cost,” in this case, an impact upon the ability of law enforcement authorities to fight crime. The decision specifically noted, however, that past Supreme Court cases had found that warrant requirement for searches was an important part of the machinery of government, not merely an inconvenience. The Court also noted existing exceptions for “exigent circumstances,” in which a suspect was fleeing, to assist a severely injured person or those threatened with imminent injuries. These exceptions also apply to mobile devices. The difference is that, unless such circumstances are present, the police need to get a warrant to search a phone.

Near the end of the decision, Roberts notes that the Fourth Amendment was drafted in response to the reviled “general warrants” and “writs of assistance” used by British officers to search homes in the colonial era. “Opposition to such searches was in fact one of the driving forces behind the [American] Revolution itself,” wrote the Chief Justice.

Extending the protections that the founding generation and those that followed enacted and fought to defend to today’s context bridges the gap between the 18th and 21st centuries. While disruptive innovations may pose difficult ethical questions for society, the laws that govern us do not cease to apply because technology outpaces them. As I’ve argued in the past, We the People need our Bill of Rights to apply in digital domains. Today’s Supreme Court decision in a landmark ruling does exactly that, clearly and unanimously:

“Modern cell phones are not just another technological convenience. With all they contain and all they may reveal, they hold for many Americans ‘the privacies of life,’ Boyd, supra, at 630. The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought. Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple — get a warrant.”