It's obvious that after arresting someone for a minor offense, police can't use the charge as a pretense to enter the suspect's house and rifle his desk in search of whatever comes up.

Rather, if police suspect the detained person of another crime and want to conduct a search of his residence, they must obtain a court-approved search warrant based on probable cause. The Fourth Amendment protects people against unreasonable searches and seizures.

In the digital age millions of people, in effect, carry their desks with them. Cellphones and highly sophisticated devices contain extraordinary volumes of information about their owners' personal and business lives - and, in some cases, about criminal conduct.

Across the country, police in many jurisdictions have taken routinely to searching suspects' cellphones without obtaining warrants.

This week, the Supreme Court of the United States struck a rare and sweeping blow for privacy in the digital age when, unanimously, it ruled that police must obtain a warrant before searching a cellphone.

Chief Justice John Roberts, who wrote the opinion, crafted the decision as the application of age-old principle to new technology. He noted that the Fourth Amendment is rooted in the Founders' disdain for "general warrants" that British officers had used to rummage through colonists' homes in search of crimes.

The technology is different, he said, but access to personal information remains the issue.

The case leading to the decision arose in California, where police obtained an attempted murder conviction in an investigation that began with a warrantless cellphone search. That search, the court found, was illegal - just as if police randomly had searched the suspect's home without a warrant.

The decision sets the appropriate standard for personal privacy protection from police intrusion in the digital age.