In its second major ruling of the day, the Supreme Court has decided that, unless under certain extreme circumstances, law enforcement may not search cellphones without a warrant. The ruling, written by Chief Justice John Roberts, was reached unanimously by the court, and brings a resolution to a long-standing civil liberties debate with regard to digital privacy.

The landmark decision's summary is fairly clear on the issue: "The police generally may not, without a warrant, search digital information on a cellphone seized from an individual who has been arrested." While the Court does allow for warrantless searches in certain "exigent circumstances" like kidnappings and bomb threats, they are limited and do not apply to searches after arrests. The decision doesn't mince words in its kicker, though, stating simply: "Get a warrant."

"A visitor from Mars might conclude phones are an important feature of human anatomy."

The Supreme Court took on the warrantless search issue in April, after two contentious cases on how cellphones may be treated by the police — namely Riley v. California and United States v. Wurie — wound their way through the appeals courts in the past decade. Prosecutors and police argued that cellphones ought to be treated as address books that may contain evidence useful during investigations, while opponents contended that phones have increasingly become computers containing personal information protected by the Fourth Amendment. Justice Roberts came down in favor of the latter view, stating that phones are now "such a pervasive and insistent part of daily life that a proverbial visitor from Mars might conclude they were an important feature of human anatomy."

Roberts even took proponents of warrantless searches to task in the decision, writing: "The United States asserts that a search of all data stored on a cellphone is 'materially indistinguishable' from searches of these sorts of physical items. That is like saying a ride on horseback is materially indistinguishable from a flight to the moon." Indeed, phones now can and often do carry enough data to reveal a person's private life, including location data and photos. "The fact that a search in the pre-digital era could have turned up a photograph or two in a wallet," he writes, "does not justify a search of thousands of photos in a digital gallery."

Privacy advocates are trumpeting the decision. In an official statement, ACLU legal director Steven Shapiro said:

By recognizing that the digital revolution has transformed our expectations of privacy, today’s decision is itself revolutionary and will help to protect the privacy rights of all Americans. We have entered a new world but, as the court today recognized, our old values still apply and limit the government’s ability to rummage through the intimate details of our private lives.

EFF staff attorney Hanni Fakhoury tells The Verge that the organization is pleased because it shows the court's recognition of the privacy concerns inherent to today's changing technology. "Recognizing that a cellphone is a miniature computer that stores enormous amounts of sensitive data is important, as the government continues to develop technological capabilities to gather wide amounts of information," he says, "whether it be the government’s use of a Stingray, the use of cell site location information to track a person’s location or the NSA’s bulk collection of Internet and phone records."