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## 9.9\. Data Storage Encryption
If device implementations support a secure lock screen as described in
[section 9.11.1](#9_11_1_secure_lock_screen), they:
* [C-1-1] MUST support data storage encryption of the application private
data (`/data partition`), as well as the application shared storage partition
(`/sdcard partition`) if it is a permanent, non-removable part of the device.
If device implementations support a secure lock screen as described in
[section 9.11.1](#9_11_1_secure_lock_screen) and support data storage
encryption with Advanced Encryption Standard (AES) crypto performance
above 50MiB/sec, they:
* [C-2-1] MUST enable the data storage encryption by default at the time
the user has completed the out-of-box setup experience. If device
implementations are already launched on an earlier Android version with
encryption disabled by default, such a device cannot meet the requirement
through a system software update and thus MAY be exempted.
* SHOULD meet the above data storage encryption
requirement via implementing [File Based Encryption](
https://source.android.com/security/encryption/file-based.html) (FBE).
### 9.9.1\. Direct Boot
Device implementations:
* [C-0-1] MUST implement the [Direct Boot mode](
http://developer.android.com/preview/features/direct-boot.html) APIs even if
they do not support Storage Encryption.
* [C-0-2] The [`ACTION_LOCKED_BOOT_COMPLETED`](
https://developer.android.com/reference/android/content/Intent.html#ACTION_LOCKED_BOOT_COMPLETED)
and [`ACTION_USER_UNLOCKED`](https://developer.android.com/reference/android/content/Intent.html#ACTION_USER_UNLOCKED)
Intents MUST still be broadcast to signal Direct Boot aware applications that
Device Encrypted (DE) and Credential Encrypted (CE) storage locations are
available for user.
### 9.9.2\. File Based Encryption
If device implementations support FBE, they:
* [C-1-1] MUST boot up without challenging the user for credentials and
allow Direct Boot aware apps to access to the Device Encrypted (DE) storage
after the `ACTION_LOCKED_BOOT_COMPLETED` message is broadcasted.
* [C-1-2] MUST only allow access to Credential Encrypted (CE) storage after
the user has unlocked the device by supplying their credentials
(eg. passcode, pin, pattern or fingerprint) and the `ACTION_USER_UNLOCKED`
message is broadcasted.
* [C-1-3] MUST NOT offer any method to unlock the CE protected storage
without the user-supplied credentials.
* [C-1-4] MUST support Verified Boot and ensure that DE keys are
cryptographically bound to the device's hardware root of trust.
* [C-1-5] MUST support encrypting file contents using AES with a key length
of 256-bits in XTS mode.
* [C-1-6] MUST support encrypting file name using AES with a key length of
256-bits in CBC-CTS mode.
* The keys protecting CE and DE storage areas:
* [C-1-7] MUST be cryptographically bound to a hardware-backed Keystore.
* [C-1-8] CE keys MUST be bound to a user's lock screen credentials.
* [C-1-9] CE keys MUST be bound to a default passcode when the user has
not specified lock screen credentials.
* [C-1-10] MUST be unique and distinct, in other words no user's CE or DE
key matches any other user's CE or DE keys.
* SHOULD make preloaded essential apps (e.g. Alarm, Phone, Messenger)
Direct Boot aware.
* MAY support alternative ciphers, key lengths and modes for file content
and file name encryption, but MUST use the mandatorily supported ciphers, key
lengths and modes by default.
The upstream Android Open Source project provides a preferred implementation of
this feature based on the Linux kernel ext4 encryption feature.
### 9.9.3\. Full Disk Encryption
If device implementations support [full disk encryption](
http://source.android.com/devices/tech/security/encryption/index.html)
(FDE), they:
* [C-1-1] MUST use AES with a key of 128-bits (or greater) and a mode
designed for storage (for example, AES-XTS, AES-CBC-ESSIV).
* [C-1-2] MUST use a default passcode to wrap the encryption key and
MUST NOT write the encryption key to storage at any time
without being encrypted.
* [C-1-3] MUST provide the user the possibility to AES encrypt the
encryption key, except when it is in active use, with the lock screen
credentials stretched using a slow stretching algorithm
(e.g. PBKDF2 or scrypt).
* [C-1-4] The above default password stretching algorithm MUST be
cryptographically bound to that keystore when the user has not specified a lock
screen credentials or has disabled use of the passcode for encryption and
the device provides a hardware-backed keystore.
* [C-1-5] MUST NOT send encryption key off the the device
(even when wrapped with the user passcode and/or hardware bound key).
The upstream Android Open Source project provides a preferred implementation
of this feature, based on the Linux kernel feature dm-crypt.