Android N runs in a secure, Direct Boot mode when the device has been powered on but the user has not unlocked the device. To support this, the system provides two storage locations for data:
- Credential encrypted storage, which is the default storage location and only available after the user has unlocked the device.
- Device encrypted storage, which is a storage location available both during Direct Boot mode and after the user has unlocked the device.
By default, apps do not run during Direct Boot mode. If your app needs to take action during Direct Boot mode, you can register app components that should be run during this mode. Some common use cases for apps needing to run during Direct Boot mode include:
- Apps that have scheduled notifications, such as alarm clock apps.
- Apps that provide important user notifications, like SMS apps.
- Apps that provide accessibility services, like Talkback.
If your app needs to access data while running in Direct Boot mode, use device encrypted storage. Device encrypted storage contains data encrypted with a key that is only available after a device has performed a successful verified boot.
For data that should be encrypted with a key associated with user credentials, such as a PIN or password, use credential encrypted storage. Credential encrypted storage is only available after the user has successfully unlocked the device, up until when the user restarts the device again. If the user enables the lock screen after unlocking the device, this doesn't lock credential encrypted storage.
Requesting Access to Run During Direct Boot
Apps must register their components with the system before they
can run during Direct Boot mode or access device encrypted
storage. Apps register with the system by marking components as
encryption aware. To mark your component as encryption aware, set the
android:directBootAware attribute to true in your manifest.
Encryption aware components can register to receive a
LOCKED_BOOT_COMPLETED broadcast message from the
system when the device has been restarted. At this point device encrypted
storage is available, and your component can execute tasks that need to be
run during Direct Boot mode, such as triggering a scheduled alarm.
The following code snippet is an example of how to register a
BroadcastReceiver as encryption aware, and add an
intent filter for
LOCKED_BOOT_COMPLETED, in the app manifest:
<receiver android:directBootAware="true" > ... <intent-filter> <action android:name="android.intent.action.LOCKED_BOOT_COMPLETED" /> </intent-filter> </receiver>
Once the user has unlocked the device, all components can access both the device encrypted storage as well as credential encrypted storage.
Accessing Device Encrypted Storage
To access device encrypted storage, create a second
Context instance by calling
Context.createDeviceProtectedStorageContext(). All storage API
calls made using this context access the device encrypted storage. The
following example accesses the device encrypted storage and opens an existing
app data file:
Context directBootContext = appContext.createDeviceProtectedStorageContext(); // Access appDataFilename that lives in device encrypted storage FileInputStream inStream = directBootContext.openFileInput(appDataFilename); // Use inStream to read content...
Use device encrypted storage only for information that must be accessible during Direct Boot mode. Don't use device encrypted storage as a general-purpose encrypted store. For private user information, or encrypted data that isn't needed during Direct Boot mode, use credential encrypted storage.
Getting Notified of User Unlock
When the user unlocks the device after restart, your app can switch to accessing credential encrypted storage and use regular system services that depend on user credentials.
To get notified when the user unlocks the device after a reboot,
BroadcastReceiver from a running component
to listen for unlock notification messages. When the user unlocks the device
- If your app has foreground processes that need immediate notification,
listen for the
- If your app only uses background processes that can act on a delayed
notification, listen for the
If the user has unlocked the device, you can find out by calling
Migrating Existing Data
If a user updates their device to use Direct Boot mode, you might have
existing data that needs to get migrated to device encrypted storage. Use
Context.moveDatabaseFrom() to migrate preference and database
data between credential encrypted storage and device encrypted storage.
Use your best judgment when deciding what data to migrate from credential encrypted storage to device encrypted storage. You should not migrate private user information, such as passwords or authorization tokens, to device encrypted storage. In some scenarios, you might need to manage separate sets of data in the two encrypted stores.
Testing Your Encryption Aware App
Test your encryption aware app using the new Direct Boot mode. There are two ways to enable Direct Boot.
Caution: Enabling Direct Boot wipes all user data on the device.
On supported devices with Android N installed, enable Direct Boot by doing one of the following:
- On the device, enable Developer options if you haven't already by going to Settings > About phone, and tapping Build number seven times. Once the developer options screen is available, go to Settings > Developer options and select Convert to file encryption.
- Use the following adb shell commands to enable Direct Boot mode:
$ adb reboot-bootloader $ fastboot --wipe-and-use-fbe
An emulated Direct Boot mode is also available, in case you need to switch modes on your test devices. Emulated mode should only be used during development and may cause data loss. To enable emulated Direct Boot mode, set a lock pattern on the device, choose "No thanks" if prompted for a secure start-up screen when setting a lock pattern, and then use the following adb shell command:
$ adb shell sm set-emulate-fbe true
To turn off emulated Direct Boot mode, use the following command:
$ adb shell sm set-emulate-fbe false
Using these commands causes the device to reboot.
Checking Device Policy Encryption Status
Device administration apps can use
DevicePolicyManager.getStorageEncryptionStatus() to check the current
encryption status of the device. If your app is targeting an API level
lower than Android N,
getStorageEncryptionStatus() will return
ENCRYPTION_STATUS_ACTIVE if the device is either using full-disk encryption,
or file-based encryption with Direct Boot. In both of these cases, data is
always stored encrypted at rest. If your app is targeting an API level of
Android N or higher,
getStorageEncryptionStatus() will return
ENCRYPTION_STATUS_ACTIVE if the device is using full-disk encryption. It will
ENCRYPTION_STATUS_ACTIVE_PER_USER if the device is using file-based encryption
with Direct Boot.