Google Online Security Blog: Pixel Security: Better, Faster, Stronger

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Pixel Security: Better, Faster, Stronger

17. November 2016

Posted by Paul Crowley, Senior Software Engineer and Paul Lawrence, Senior Software Engineer[Cross-posted from the Android Developers Blog]security improvements

File-Based Encryption Direct Boot experiencefile-based encryptionDirect boot
Enhanced with TrustZone® security

TrustZone enforces the Verified Boot process. If TrustZone detects that the operating system has been modified, it won't decrypt disk encryption keys; this helps to secure device encrypted (DE) data.

TrustZone enforces a waiting period between guesses at the user credential, which gets longer after a sequence of wrong guesses. With 1624 valid four-point patterns and TrustZone's ever-growing waiting period, trying all patterns would take more than four years. This improves security for all users, especially those who have a shorter and more easily guessed pattern, PIN, or password.

Encryption on Pixel phones
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.