A Few Thoughts on Cryptographic Engineering

The limitations of Android N Encryption

Over the past few years we’ve heard more about smartphone encryption than, quite frankly, most of us expected to hear in a lifetime. We learned that proper encryption can slow down even sophisticated decryption attempts if done correctly. We’ve also learned that incorrect implementations can undo most of that security.

In other words, phone encryption is an area where details matter. For the past few weeks I’ve been looking a bit at Android Nougat’s new file-based encryption to see how well they’ve addressed some of those details in their latest release. The answer, unfortunately, is that there’s still lots of work to do. In this post I’m going to talk about a bit of that.

(As an aside: the inspiration for this post comes from Grugq, who has been loudly and angrily trying to work through these kinks to develop a secure Android phone. So credit where credit is due.)

Background: file and disk encryption 

Disk encryption is much older than smartphones. Indeed, early encrypting filesystems date back at least to the early 1990s and proprietary implementations may go back before that. Even in the relatively new area of PCs operating systems, disk encryption has been a built-in feature since the early 2000s.

The typical PC disk encryption system operates as follows. At boot time you enter a password. This is fed through a key derivation function to derive a cryptographic key. If a hardware co-processor is available (e.g., a TPM), your key is further strengthened by “tangling” it with some secrets stored in the hardware. This helps to lock encryption to a particular device.

The actual encryption can be done in one of two different ways:

  1. Full Disk Encryption (FDE) systems (like TruecryptBitLocker and FileVault) encrypt disks at the level of disk sectors. This is an all-or-nothing approach, since the encryption drivers won’t necessarily have any idea what files those sectors represent. At the same time, FDE is popular — mainly because it’s extremely easy to implement.
  2. File-based Encryption (FBE) systems (like EncFS and eCryptFS) encrypt individual files. This approach requires changes to the filesystem itself, but has the benefit of allowing fine grained access controls where individual files are encrypted using different keys.

Most commercial PC disk encryption software has historically opted to use the full-disk encryption (FDE) approach. Mostly this is just a matter of expediency: FDE is just significantly easier to implement. But philosophically, it also reflects a particular view of what disk encryption was meant to accomplish.

In this view, encryption is an all-or-nothing proposition. Your machine is either on or off; accessible or inaccessible. As long as you make sure to have your laptop stolen only when it’s off, disk encryption will keep you perfectly safe.

So what does this have to do with Android?

Android’s early attempts at adding encryption to their phones followed the standard PC full-disk encryption paradigm. Beginning in Android 4.4 (Kitkat) through Android 6.0 (Marshmallow), Android systems shipped with a kernel device mapper called dm-crypt designed to encrypt disks at the sector level. This represented a quick and dirty way to bring encryption to Android phones, and it made sense — if you believe that phones are just very tiny PCs.

The problem is that smartphones are not PCs.

The major difference is that smartphone users are never encouraged to shut down their device. In practice this means that — after you enter a passcode once after boot — normal users spend their whole day walking around with all their cryptographic keys in RAM. Since phone batteries live for a day or more (a long time compared to laptops) encryption doesn’t really offer much to protect you against an attacker who gets their hands on your phone during this time.

Of course, users do lock their smartphones. In principle, a clever implementation could evict sensitive cryptographic keys from RAM when the device locks, then re-derive them the next time the user logs in. Unfortunately,  Android doesn’t do this — for the very simple reason that Android users want their phones to actually work. Without cryptographic keys in RAM, an FDE system loses access to everything on the storage drive. In practice this turns it into a brick.

For this very excellent reason, once you boot an Android FDE phone it will never evict its cryptographic keys from RAM. And this is not good.

So what’s the alternative?

Android is not the only game in town when it comes to phone encryption. Apple, for its part, also gave this problem a lot of thought and came to a subtly different solution.

Starting with iOS 4, Apple included a “data protection” feature to encrypt all data stored a device. But unlike Android, Apple doesn’t use the full-disk encryption paradigm. Instead, they employ a file-based encryption approach that individually encrypts each file on the device.

In the Apple system, the contents of each file is encrypted under a unique per-file key (metadata is encrypted separately). The file key is in turn encrypted with one of several “class keys” that are derived from the user passcode and some hardware secrets embedded in the processor.

iOS data encryption. Source: iOS Security Guide.

The main advantage of the Apple approach is that instead of a single FDE key to rule them all, Apple can implement fine-grained access control for individual files. To enable this, iOS provides an API developers can use to specify which class key to use in encrypting any given file. The available “protection classes” include:

By giving developers the option to individually protect different files, Apple made it possible to build applications that can work while the device is locked, while providing strong protection for files containing sensitive data.

Apple even created a fourth option for apps that simply need to create new encrypted files when the class key has been evicted from RAM. This class uses public key encryption to write new files. This is why you can safely take pictures even when your device is locked.

Apple’s approach isn’t perfect. What it is, however, is the obvious result of a long and careful thought process. All of which raises the following question…

Why the hell didn’t Android do this as well?

The short answer is Android is trying to. Sort of. Let me explain.

As of Android 7.0 (Nougat), Google has moved away from full-disk encryption as the primary mechanism for protecting data at rest. If you set a passcode on your device, Android N systems can be configured to support a more Apple-like approach that uses file encryption. So far so good.

The new system is called Direct Boot, so named because it addresses what Google obviously saw as fatal problem with Android FDE — namely, that FDE-protected phones are useless bricks following a reboot. The main advantage of the new model is that it allows phones to access some data even before you enter the passcode. This is enabled by providing developers with two separate “encryption contexts”:

Direct Boot even provides separate encryption contexts for different users on the phone — something I’m not quite sure what to do with. But sure, why not?

If Android is making all these changes, what’s the problem?

One thing you might have noticed is that where Apple had four categories of protection, Android N only has two. And it’s the two missing categories that cause the problems. These are the “complete protection” categories that allow the user to lock their device following first user authentication — and evict the keys from memory.

Of course, you might argue that Android could provide this by forcing application developers to switch back to “device encrypted storage” following a device lock. The problem with this idea is twofold. First, Android documentation and sample code is explicit that this isn’t how things work:

Moreover, a quick read of the documentation shows that even if you wanted to, there is no unambiguous way for Android to tell applications when the system has been re-locked. If keys are evicted when the device is locked, applications will unexpectedly find their file accesses returning errors. Even system applications tend to do badly when this happens.

And of course, this assumes that Android N will even try to evict keys when you lock the device. Here’s how the current filesystem encryption code handles locks:

While the above is bad, it’s important to stress that the real problem here is not really in the cryptography. The problem is that since Google is not giving developers proper guidance, the company may be locking Android into years of insecurity. Without (even a half-baked) solution to define a “complete” protection class, Android app developers can’t build their apps correctly to support the idea that devices can lock. Even if Android O gets around to implementing key eviction, the existing legacy app base won’t be able to handle it — since this will break a million apps that have implemented their security according to Android’s current recommendations.

In short: this is a thing you get right from the start, or you don’t do at all. It looks like — for the moment — Android isn’t getting it right.

Are keys that easy to steal?

Of course it’s reasonable to ask whether it’s having keys in RAM is that big of concern in the first place. Can these keys actually be accessed?

The answer to that question is a bit complicated. First, if you’re up against somebody with a hardware lab and forensic expertise, the answer is almost certainly “yes”. Once you’ve entered your passcode and derived the keys, they aren’t stored in some magically secure part of the phone. People with the ability to access RAM or the bus lines of the device can potentially nick them.

But that’s a lot of work. From a software perspective, it’s even worse. A software attack would require a way to get past the phone’s lockscreen in order to get running code on the device. In older (pre-N) versions of Android the attacker might need to then escalate privileges to get access to Kernel memory. Remarkably, Android N doesn’t even store its disk keys in the Kernel — instead they’re held by the “vold” daemon, which runs as user “root” in userspace. This doesn’t make exploits trivial, but it certainly isn’t the best way to handle things.

Of course, all of this is mostly irrelevant. The main point is that if the keys are loaded you don’t need to steal them. If you have a way to get past the lockscreen, you can just access files on the disk.

What about hardware?

Although a bit of a tangent, it’s worth noting that many high-end Android phones use some sort of trusted hardware to enable encryption. The most common approach is to use a trusted execution environment (TEE) running with ARM TrustZone.

This definitely solves a problem. Unfortunately it’s not quite the same problem as discussed above. ARM TrustZone — when it works correctly, which is not guaranteed — forces attackers to derive their encryption keys on the device itself, which should make offline dictionary attacks on the password much harder. In some cases, this hardware can be used to cache the keys and reveal them only when you input a biometric such as a fingerprint.

The problem here is that in Android N, this only helps you at the time the keys are being initially derived. Once that happens (i.e., following your first login), the hardware doesn’t appear to do much. The resulting derived keys seem to live forever in normal userspace RAM. While it’s possible that specific phones (e.g., Google’s Pixel, or Samsung devices) implement additional countermeasures, on stock Android N phones hardware doesn’t save you.

So what does it all mean?

How you feel about this depends on whether you’re a “glass half full” or “glass half empty” kind of person.

If you’re an optimistic type, you’ll point out that Android is clearly moving in the right direction. And while there’s a lot of work still to be done, even a half-baked implementation of file-based implementation is better than the last generation of dumb FDE Android encryption. Also: you probably also think clowns are nice.

On the other hand, you might notice that this is a pretty goddamn low standard. In other words, in 2016 Android is still struggling to deploy encryption that achieves (lock screen) security that Apple figured out six years ago. And they’re not even getting it right. That doesn’t bode well for the long term security of Android users.

And that’s a shame, because as many have pointed out, the users who rely on Android phones are disproportionately poorer and more at-risk. By treating encryption as a relatively low priority, Google is basically telling these people that they shouldn’t get the same protections as other users. This may keep the FBI off Google’s backs, but in the long term it’s bad judgement on Google’s part.