One of the FBI’s Major Claims in the iPhone Case Is Fraudulent

“We have enormous computing power in the US government, but we need to be able to bring it to bear without the phone killing itself.”
                                  - FBI Director James Comey, March 1, 2016.

In the FBI’s court order requesting Apple's assistance in unlocking the work iPhone 5c used by the San Bernardino shooter, the bureau's first and most urgent demand is that Apple disable the iPhone's “auto-erase” security feature. This feature (which is not enabled by default on most iPhones) protects user data on a device from would-be snoops by wiping the phone after 10 failed passcode attempts. This protects you and me from thieves trying to guess our passcodes and access our data for identify theft, for example.

But the truth is that even if this feature is enabled on the device in question, the FBI doesn't need to worry about it, because they can already bypass it by backing up part of the phone (called the “Effaceable Storage”) before attempting to guess the passcode. I'll go into the technical details (which the FBI surely already knows) below.

How the FBI describes the “auto-erase” feature

Let's look at how the FBI describes the situation. The court order's first and most urgently phrased request is to ask Apple to “bypass or disable the auto-erase function whether or not it has been enabled.”

A few days after the court order was issued, but before Apple had formally responded, the government filed a strongly worded motion to compel, which contained this description of the feature:

The FBI has been unable to make attempts to determine the passcode to access the SUBJECT DEVICE because Apple has written, or “coded,” its operating systems with a user-enabled “auto-erase function” that would, if enabled, result in the permanent destruction of the required encryption key material after 10 failed attempts at the [sic] entering the correct passcode (meaning that, after 10 failed attempts, the information on the device becomes permanently inaccessible)…

In sum, the government seeks the ability to make multiple attempts at determining the passcode without risk that the data subject to search under the warrant would be rendered permanently inaccessible after 10 wrong attempts.

To add urgency to their attempt to compel Apple to abuse their software signing keys, the FBI is painting a picture of “permanently inaccessible” data. But if its agents are doing their job, that's just not the case.

How the “auto-erase” feature actually works

Here's where the technical details come in. The iPhone protects its user's data with a complex hierarchy of cryptographic keys. Some data is protected by multiple keys. Imagine a pile of letters and photos placed inside a locked box, with the box itself placed inside a locked filing cabinet. You'd have to have keys to the filing cabinet and the box to read any of the letters or see any of the photos. If either of these keys is destroyed, the letters and photos are lost forever.

When iOS decides to wipe out user data because the passcode guess limit has been reached (or for any other reason), it doesn’t actually erase all the data from its underlying storage; that would actually take several minutes. Instead, it just destroys one of the keys that protects the data, rendering that data permanently unreadable. The key that is erased in this case is called the “file system key”—and (unlike the hardwired “UID” key that we discussed in our previous blog post) it is not burned into the phone’s processor, but instead merely stored in what Apple calls “Effaceable Storage,” which is just a term for part of the flash memory of the phone designed to be easily erasable. Apple's iOS Security Guide explains:

Since it’s stored on the device, this key is not used to maintain the confidentiality of data; instead, it’s designed to be quickly erased on demand (by the user, with the “Erase all content and settings” option, or by a user or administrator issuing a remote wipe command…. Erasing the key in this manner renders all files cryptographically inaccessible.

The file system key is like the key to the filing cabinet in our example: a small thing that is easy to destroy, which disables access to the rest of the information.

Why the FBI can easily work around “auto-erase”

So the file system key (which the FBI claims it is scared will be destroyed by the phone’s auto-erase security protection) is stored in the Effaceable Storage on the iPhone in the “NAND” flash memory. All the FBI needs to do to avoid any irreversible auto erase is simple to copy that flash memory (which includes the Effaceable Storage) before it tries 10 passcode attempts. It can then re-try indefinitely, because it can restore the NAND flash memory from its backup copy.

Here's a picture of the front and back of main circuit board inside the iPhone 5c:

iPhone interior

Image credit: http://www.mobpart.com/iphone-5c-c-61_63

The large chip on the front marked A6 is the processor -- a custom chip designed by Apple specifically for its devices. It contains the CPU, BootROM, RAM, crypto engines, Apple's public signing key (used to verify software updates), and the UID key (see our previous blog post).

The largest chip on the back (outlined in red above) is the NAND flash, where all the data is stored, including both the encrypted filesystem and the Effaceable Storage.

The FBI can simply remove this chip from the circuit board (“desolder” it), connect it to a device capable of reading and writing NAND flash, and copy all of its data. It can then replace the chip, and start testing passcodes. If it turns out that the auto-erase feature is on, and the Effaceable Storage gets erased, they can remove the chip, copy the original information back in, and replace it. If they plan to do this many times, they can attach a “test socket” to the circuit board that makes it easy and fast to do this kind of chip swapping.

If the FBI doesn't have the equipment or expertise to do this, they can hire any one of dozens of data recovery firms that specialize in information extraction from digital devices.

NAND flash storage is an extremely common component. It's found in USB thumb drives, mobile phones, portable music players, low-end laptops—pretty much every portable device. Desoldering a chip from the circuitboard is straightforward enough that there are many clips on YouTube showing the practice, and reading and writing a bare NAND chip requires a minor investment in hardware and training that the FBI has probably already made.

What's really going on here?

If this generally useful security feature is actually no threat to the FBI, why is it painting it in such a scary light that some commentators have even called it a “doomsday mechanism”? The FBI wants us to think that this case is about a single phone, used by a terrorist. But it's a power grab: law enforcement has dozens of other cases where they would love to be able to compel software and hardware providers to build, provide, and vouch for deliberately weakened code. The FBI wants to weaken the ecosystem we all depend on for maintenance of our all-too-vulnerable devices. If they win, future software updates will present users with a troubling dilemma. When we're asked to install a software update, we won’t know whether it was compelled by a government agency (foreign or domestic), or whether it truly represents the best engineering our chosen platform has to offer.

In short, they're asking the public to grant them significant new powers that could put all of our communications infrastructure at risk, and to trust them to not misuse these powers. But they're deliberately misleading the public (and the judiciary) to try to gain these powers. This is not how a trustworthy agency operates. We should not be fooled.

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Anonymous

Thank you for providing this information to the public

Eruanion

So, wait, they don't even know if the 10 guess security is enabled or not? They haven't even tried to break into the phone? Definitely a power grab. I believe, even if it is enabled, that there must be a reset timer on the 10 guesses. If you get to 9 guesses and leave it alone, it should, eventually, go back to having 10 guesses. I can't see Apple intentionally bricking people's phones , or maybe I can, so they can sell them a new one. As for the data being unrecoverable,, that is a lie too, These people have ways to read intentionally erased drives and memory as well, so the encryption would be a pain to crack, but not impossible.

Zaph

The user can set the phone to erase the content on 10 failed attempts, the phone is not "bricked" it is reset to a new phone state. If the erase option is not set attempts past 10 are limited to one per hour.

Anonymous

What's the point of having a 17 agency, trillion dollar "intelligence" apparatus if the employees of said agencies can't be bothered to put in a little work?

Citizen

So you're proposing that Apple brute force the device by manually making 9 password entry attempts via the phone's UI, then after those attempts all fail, remove the chip, reflash it, then replace the chip and make 9 more attempts, then rinse and repeat. This must be joking to insinuate that this has any hope of working. Using your technique, the FBI would break the password at about the same time that you get a clue and invent warp drive. Part of the FBI's problem that you haven't addressed within is the speed and frequency at which attempts can be made. For brute force to be practical, you need to be able to try millions of passwords in a relatively short period of time. The method described here doesn't fit the likely success criteria.

Anonymous

It's a four digit pin. That's 10,000 attempts, not millions. It would be a matter of days to retrieve the PIN, if not less.

Informed Citzen

They can simply attach the Iphones hardware to a terminal service and run the brute force through a program. Or they can just decrypt the NAND memory using a similar method -> attach it to a memory reader and use a decryption program. This is extremely basic stuff.

If you dont think they have the computing power to do this, then you really have to ask yourself how they can possibly ensure national safety from custom programs and hardware found in foreign countries defense programs.

Suomynona

Sounds like you're part of the FBI team working on this case... Read the article...
Brute force will probably be done with a TransLock utility tool. Swapping the NAND will be done by an external 'test socket' (as stated in the article). So no manual typing on the screen...

Anonymous

"For brute force to be practical, you need to be able to try millions of passwords in a relatively short period of time."
...except in this case they don't, because the password is a 4-digit PIN with exactly 10,000 possible combinations.

Anonymous

There is no need for millions of passwords. It's a 4 digit pass code. 9,999 possible permutations divided by 9 (10 attempts is what the article said, I believe, but we can use 9) is 1,111 attempts, maximum, before the repeatedly reflashed chip is finally accessed. In short, if it takes 5 minutes for 9 attempts (30 seconds) plus reflashing the drive (4:30 minutes... total guess, but how long do updates on our phones take?), then they'll be done in 5,555 minutes; otherwise know as Sunday. If they want to stop terrorists, they'll do this, but it'll make the court case moot, so they won't.

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