SAN FRANCISCO • The Federal Bureau of Investigation's method for breaking into a locked iPhone 5c is unlikely to stay secret for long, according to senior Apple engineers and outside experts.

Once it is exposed, Apple should be able to plug the encryption hole, comforting iPhone users worried that losing physical possession of their devices will leave them vulnerable to hackers.

When Apple does fix the flaw, it is expected to announce it to customers and thereby extend the rare public battle over security holes, a debate that typically rages out of public view.

The FBI last week dropped its courtroom quest to force Apple to hack into the iPhone of one of the San Bernardino shooters, saying an unidentified party provided a method for getting around the deceased killer's unknown passcode.

If the government pursues a similar case seeking Apple's help in New York, the court could make the FBI disclose its new trick.

But even if the government walks away from that battle, the growing number of state and local authorities seeking the FBI's help with locked phones in criminal probes increases the likelihood that the FBI will have to provide it.

When that happens, defence attorneys will cross-examine the experts involved.

Although each lawyer would mainly be interested in whether evidence-tampering may have occurred, the process would likely reveal enough about the method for Apple to block it in future versions of its phones, an Apple employee said.

"The FBI would need to resign itself to the fact that such an exploit would only be viable for a few months if released to other departments," said Mr Jonathan Zdziarski, an independent forensics expert who has helped police get into many devices. "It would be a temporary Vegas jackpot that would quickly get squandered on the case backlog."

In a memo to police obtained by Reuters last Friday, the FBI said it would share the tool "consistent with our legal and policy constraints."

REUTERS