Unlocking an iPhone for investigators impossible in 'in most cases,' Apple tells court

The DoJ's unlock request would normally be "substantially burdensome," the company said in a Monday court filing seen by the Wall Street Journal. Apple pointed out that the full-disk encryption feature in iOS 8 and 9 should prevent anyone from accessing an iPhone's data without the passcode, including Apple itself.

The phone the Justice Department wants to investigate, however, is running iOS 7, and Apple admitted that it has the capacity to extract "certain categories of unencrypted data" — excluding email, calendar entries, and third-party app data.

It nevertheless said it would prefer to avoid pulling information from the phone unless forced to, since that might "threaten the trust between Apple and its customers and substantially tarnish the Apple brand."

The company submitted the brief at the request of Magistrate Judge James Orenstein, who is known to favor limits on government surveillance. In an earlier ruling, he suggested that he may not have the authority to force Apple's cooperation, something the Justice Department disputes.

Since 2013 leaks by NSA whistleblower Edward Snowden, Apple has made privacy one of its key marketing points, attacking rival products as insecure. CEO Tim Cook repeated this position at conference on Monday, making a particular point to oppose the backdoors demanded by some U.S. government officials. These would make it easier to fulfill legitimate search warrants, but critics have complained that it would also make mass surveillance easier and expose devices to hackers.