Posted by Paul Crowley, Senior Software Engineer and Paul Lawrence, Senior Software Engineer
Encryption protects your data if your phone falls into someone else's hands. The new Google Pixel and Pixel XL are encrypted by default to offer strong data protection, while maintaining a great user experience with high I/O performance and long battery life. In addition to encryption, the Pixel phones debuted running the Android Nougat release, which has even more security improvements.
This blog post covers the encryption implementation on Google Pixel devices and how it improves the user experience, performance, and security of the device.
One of the security features introduced in Android Nougat was file-based encryption. File-based encryption (FBE) means different files are encrypted with different keys that can be unlocked independently. FBE also separates data into device encrypted (DE) data and credential encrypted (CE) data.
Direct boot uses file-based encryption to allow a seamless user experience when a device reboots by combining the unlock and decrypt screen. For users, this means that applications like alarm clocks, accessibility settings, and phone calls are available immediately after boot.
Modern processors provide a means to execute code in a mode that remains secure even if the kernel is compromised. On ARM®-based processors this mode is known as TrustZone. Starting in Android Nougat, all disk encryption keys are stored encrypted with keys held by TrustZone software. This secures encrypted data in two ways:
Protecting different folders with different keys required a distinct approach from full-disk encryption (FDE). The natural choice for Linux-based systems is the industry-standard eCryptFS. However, eCryptFS didn't meet our performance requirements. Fortunately one of the eCryptFS creators, Michael Halcrow, worked with the ext4 maintainer, Ted Ts'o, to add encryption natively to ext4, and Android became the first consumer of this technology. ext4 encryption performance is similar to full-disk encryption, which is as performant as a software-only solution can be.
Additionally, Pixel phones have an inline hardware encryption engine, which gives them the ability to write encrypted data at line speed to the flash memory. To take advantage of this, we modified ext4 encryption to use this hardware by adding a key reference to the bio structure, within the ext4 driver before passing it to the block layer. (The bio structure is the basic container for block I/O in the Linux kernel.) We then modified the inline encryption block driver to pass this to the hardware. As with ext4 encryption, keys are managed by the Linux keyring. To see our implementation, take a look at the source code for the Pixel kernel.
While this specific implementation of file-based encryption using ext4 with inline encryption benefits Pixel users, FBE is available in AOSP and ready to use, along with the other features mentioned in this post.
Android Studio 2.2 launched recently with many new and improved features. Some of the changes are easy to miss because they happened under the hood in the Android Gradle plugin, such as the newly rewritten integrated APK packaging and signing step.
With the introduction of the new APK Signature Scheme v2 in Android 7.0 Nougat, we decided to rewrite how assembling APKs works in the Android Gradle plugin. You can read all about the low-level technical details of v2 signatures in the documentation, but here's a quick tl;dr summary of the info you need as an Android app developer:
Why introduce this change to how Android verifies APKs? Firstly, for enhanced security and extensibility of this new signing format, and secondly for performance - the new signatures take significantly less time to verify on the device (no need for costly decompression), resulting in faster app installation times.
The consequence of this new signing scheme, however, is that there are new constraints on the APK creation process. Since only uncompressed file contents were verified in v1, that allowed for quite a lot of modifications to be made after APK signing - files could be moved around or even recompressed. In fact, the zipalign tool which was part of the build process did exactly that - it was used to align ZIP entries on correct byte boundaries for improved runtime performance.
zipalign
Because v2 signatures verify all bytes in the archive and not individual ZIP entries, running zipalign is no longer possible after signing. That's why compression, aligning and signing now happens in a single, integrated step of the build process.
If you have any custom tasks in your build process that involve tampering with or post-processing the APK file in any way, please make sure you disable them or you risk invalidating the v2 signature and thus making your APKs incompatible with Android 7.0 and above.
Should you choose to do signing and aligning manually (such as from the command line), we offer a new tool in the Android SDK, called apksigner, that provides both v1 and v2 APK signing and verification. Note that you need to run zipalign before running apksigner if you are using v2 signatures. Also remember the jarsigner tool from the JDK is not compatible with Android v2 signatures, so you can't use it to re-sign your APKs if you want to retain the v2 signature.
apksigner
jarsigner
In case you want to disable adding v1 or v2 signatures when building with the Android Gradle plugin, you can add these lines to your signingConfig section in build.gradle:
signingConfig
v1SigningEnabled false v2SigningEnabled false
Note: both signing schemes are enabled by default in Android Gradle plugin 2.2.
We took this opportunity when rewriting the packager to make some optimizations to the size of release APKs, resulting in faster downloads, smaller delta updates on the Play Store, and less wasted space on the device. Here are some of the changes we made:
android:extractNativeLibs="false"
These changes help make your releases as small as possible so that users can download and update your app even on a slower connection or on less capable devices. But what about debug builds?
When developing apps you want to keep the iteration cycle fast - change code, build, and deploy on a connected device or emulator. Since Android Studio 2.0 we've been working to make all the steps as fast as possible. With Instant Run we're now able to update only the changed code and resources during runtime, while the new Emulator brings multi-processor support and faster ADB speeds for quicker APK transfer and installation. Build improvements can cut that time even further and in Android Studio 2.2 we're introducing incremental packaging and parallel compression for debug builds. Together with other features like selectively packaging resources for the target device density and ABI this will make your development even faster.
A word of caution: the APK files created for Instant Run or by invoking a debug build are not meant for distribution on the Play Store! They contain additional instrumentation code for Instant Run and are missing resources for device configurations other than the one that was connected when you started the build. Make sure you only distribute release versions of the APK which you can create using the Android Studio Generate Signed APK command or the assembleRelease Gradle task.
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