吃瓜大本营

The FBI wants to establish the precedent that the government can conscript a technology supplier to break security features they've offered to their users. In particular, the legal order obtained by the FBI (directing Apple to assist the Bureau in accessing San Bernardino shooter Syed Rizwan Farook鈥檚 phone) asks Apple to misuse their software update mechanism鈥� a mechanism that every Apple user depends on.

But the FBI鈥檚 demand is premised on the notion that the FBI needs Apple鈥檚 help, and cannot access the encrypted data on Farook鈥檚 phone without Apple鈥檚 help. We were curious: is that actually the case? To answer requires looking at technical details of Apple鈥檚 encryption system and how it works.

The basic technological situation

The FBI can easily extract the raw data from the phone鈥檚 flash memory, but they can鈥檛 read it because the information on Farook鈥檚 phone is protected by encryption that ultimately depends on a strong key we can call the 鈥渦nlocking key.鈥� This key is so strong that it would take the FBI longer than the age of the universe to crack it using 鈥渂rute force鈥� methods (i.e. guessing every possible key)鈥攅ven with all the computers in the world dedicated to the task.

Of course, iPhone users don鈥檛 have to enter the unlocking key itself鈥攁 lengthy tangle of random digits鈥攖o open their phone. Instead, they just enter a shorter passcode. Passcodes can vary in length, but there鈥檚 a strong chance the FBI could brute-force the passcode, which is usually short and memorizeable by humans. The problem for the FBI is that the passcode by itself does not unlock the phone. Instead, the weak passcode is combined with a hard-wired 鈥渦nique ID鈥� (UID) that is embedded within the phone when it is manufactured. The UID is actually used as an encryption key itself鈥攁nd like the unlocking key, it is lengthy and cannot be brute-forced. When the passcode is entered it is combined with the hard-wired UID to open the unlocking key, which unlocks the phone鈥檚 data. In other words: passcode + UID = unlocking key and access to data.

In short, the phone鈥檚 UID is everything. With it, the FBI can get what they鈥檙e asking for. Without it, they can鈥檛.

If the FBI could extract the UID key from the phone, they could then take their own external copy of the phone鈥檚 data and bring supercomputers to bear on brute-forcing the passcode instead of using a single relatively weak phone processor. They could bypass the guessing-limit functions of Apple鈥檚 operating system without requiring Apple to create a whole new insecure operating system for them, and rapidly try combining billions of passcodes with the UID until they successfully unlock the data extracted from the phone. The FBI鈥檚 effort to conscript Apple would therefore be unnecessary.

That brings us to the question:

Is it really true that the FBI cannot access the UID key?

Certainly, preventing access to the UID is Apple's stated engineering goal. The company鈥檚 for iOS 9 states, that the UID (and another key we don鈥檛 need to worry about)

are AES 256-bit keys fused鈥�into the application processor鈥�during manufacturing. No software or firmware can read them directly; they can see only the results of encryption or decryption operations performed by dedicated AES engines implemented in silicon using the UID鈥�as a key.

The fuses in Apple鈥檚 chips are just like the fuses in your home's wiring鈥攅lectrical connections designed to 鈥渂low鈥� when too much current flows through them. Imagine 256 fuses in a row. For each fuse, flip a coin. If it comes up heads, send enough current across the fuse to make it blow. If it comes up tails, leave the fuse alone. This series of blown or not-blown fuses鈥攊n minature, inside the chip during Apple鈥檚 manufacturing process鈥攃onstitutes the UID key. Apple says that no one has a record of that UID key for any phone.

While the chip is designed to not expose the UID key via its electrical interfaces, there is actually an array of physical fuses inside of it. Through processes that are already available to , the FBI could etch off the surface of the chip, and then use a powerful microscope to inspect the silicon inside. If this succeeds in exposing the UID, then the device itself is no longer needed, and the FBI can proceed to crack the unlocking key by brute-forcing the passcode on their own hardware.

It would be extremely surprising if the FBI鈥攐r at least the NSA鈥攄id not already have the equipment and knowledge to perform this work. We know the FBI electron microscopes at least since 2002, for example. Certainly we鈥檙e not suggesting an expansion of NSA participation in domestic law enforcement investigations, but surely before any court considers setting such a dangerous precedent it should be certain that there aren鈥檛 less destructive alternatives.

It鈥檚 possible that the FBI has access to such capabilities but doesn鈥檛 want to advertise them because the facts around this case present them with a golden opportunity to establish a powerful new legal precedent鈥攖hat they can force a technology provider to abuse their software update mechanism at the request of the government. That鈥檚 a precedent that will give them a significant new power.

In the end, course, whether or not there is a way for the FBI to get that UID and Farook鈥檚 data without Apple, the order should still be overturned for the deeper reasons that our colleague Alex Abdo and others have laid out.

But, before seeking a radical and sweeping dangerous new legal precedent, the FBI should explain why they are unable to extract the UID. Not only journalists but also judges should press the FBI on whether techniques such as those suggested above鈥攐r any others鈥攃ould offer an alternative to the sweeping new power the Bureau is seeking.