The FBI's new method for unlocking iPhones won't work on most models, FBI Director Comey said in a speech last night at Kenyon University. "It's a bit of a technological corner case, because the world has moved on to sixes," Comey said, describing the bug in response to a question. "This doesn't work on sixes, doesn't work on a 5s. So we have a tool that works on a narrow slice of phones." He continued, "I can never be completely confident, but I'm pretty confident about that." The exchange can be found at 52:30 in the video above.
Comey did not elaborate on why the new method did not work on more advanced phones, but it's very likely related to the Secure Enclave protections that were implemented with the 5s' A7 chip, present in all subsequent iPhones. If true, that would severely limit the impact of the FBI's new method. As of September, roughly a quarter of American iPhone users were using phones without the Secure Enclave, a number that has likely dropped in the time since.
That limited scope also makes it less likely that the FBI will be compelled to disclose the bug. The FBI recently disclosed the method to legislators working on an encryption bill, but has yet to successfully deploy it in another investigation. Speaking to the crowd at Kenyon, Comey said he was confident the Bureau could keep the method secret if necessary. "The FBI is very good at keeping secrets," Comey said. "And the people we bought this from, I know a fair amount about them, and I have a high degree of confidence that they are good at protecting it."