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Why the FBI's request to Apple will affect civil rights for a generation

Rich Mogull | Feb. 18, 2016
No legal case applies in a vacuum, and in this case the FBI needs the precedence more than the evidence.

The FBI Director has been clear that the government wants back doors into our devices, even though the former head of the NSA disagrees and supports strong consumer encryption. One reason Apple is likely fighting this case so publicly is that it is a small legal step from requiring new circumvention technology, to building such access into devices. The FBI wants the precedence far more than they need the evidence, and this particular case is incredibly high profile and emotional.

The results will, without question, establish precedence beyond one killer’s iPhone.

The technical details

The court order is quite specific. It applies only to one iPhone, and requests Apple create a new version of the firmware that eliminates the existing feature that erases the iPhone after 10 failed attempts at entering the passcode. It further asks Apple to allow passcode attempts to be performed as rapidly as possible.

Apple has been prompting users to choose longer and more complicated—and harder to crack—iPhone passcodes.

Beginning with iOS 8, devices are encrypted using a key derived from your passcode. This is combined with a hardware key specific to the device. Apple has no way of knowing or circumventing that key. On newer devices, the hardware key is embedded in the device and is not recoverable. Thus the passcode must be combined with the device key in a chip on the phone, and that chip rate-limits passcode attempts to make a brute force attack slower.

Reading through the order, it seems the FBI thinks that a modified version of the operating system would allow them to engage in high-speed attacks, if the 10-tries limit was removed. The request indicates they likely can’t image the device and perform all the attacks on their own super-fast computers, due to that hardware key. With a four-character passcode the device could probably be cracked in hours. A six-character code might take days or weeks, and anything longer could take months or years.

Dan Guido over at Trail of Bits posted a great explanation:

As many jailbreakers are familiar, firmware can be loaded via Device Firmware Upgrade (DFU) Mode. Once an iPhone enters DFU mode, it will accept a new firmware image over a USB cable. Before any firmware image is loaded by an iPhone, the device first checks whether the firmware has a valid signature from Apple. This signature check is why the FBI cannot load new software onto an iPhone on their own—the FBI does not have the secret keys that Apple uses to sign firmware.

This opens up a few questions. Could this work on newer devices with the enhanced encryption of the Secure Enclave? How can Apple pair the device and replace the firmware in the first place? Would they be using the shooter’s computer? An over-the-air update? Sources at Apple declined to comment on the implications for other devices and the specific technical methods, a position I initially disagreed with, but on reflection is probably the right move for reasons we will get to in a moment.

 

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