Breaking the code, according to an Apple technical guide, could take “more than 5 1/2 years to try all combinations of a six-character alphanumeric passcode with lowercase letters and numbers.” (Computer security experts question that figure, because Apple does not fully realize how quickly the N.S.A. supercomputers can crack codes.)…

“Terrorists will figure this out,” along with savvy criminals and paranoid dictators, one senior official predicted, and keep their data just on the iPhone 6. Another said, “It’s like taking out an ad that says, ‘Here’s how to avoid surveillance — even legal surveillance.’ ”

The move raises a critical issue, the intelligence officials say: Who decides what kind of data the government can access? Until now, those decisions have largely been a matter for Congress, which passed the Communications Assistance for Law Enforcement Act in 1994, requiring telecommunications companies to build into their systems an ability to carry out a wiretap order if presented with one. But despite intense debate about whether the law should be expanded to cover email and other content, it has not been updated, and it does not cover content contained in a smartphone.