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9.9. Data Storage Encryption
All devices MUST meet the requirements of section 9.9.1. Devices which launched on an API level earlier than that of this document are exempted from the requirements of sections 9.9.2 and 9.9.3; instead they MUST meet the requirements in section 9.9 of the Android Compatibility Definition document corresponding to the API level on which the device launched.
9.9.1. Direct Boot
Device implementations:
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[C-0-1] MUST implement the Direct Boot mode APIs even if they do not support Storage Encryption.
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[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
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. Encryption requirements
Device implementations:
- [C-0-1] MUST encrypt the application private
data (
/datapartition), as well as the application shared storage partition (/sdcardpartition) if it is a permanent, non-removable part of the device. - [C-0-2] MUST enable the data storage encryption by default at the time the user has completed the out-of-box setup experience.
- [C-0-3] MUST meet the above data storage encryption requirement via implementing File Based Encryption (FBE) and Metadata Encryption.
9.9.3. Encryption Methods
If device implementations are encrypted, they:
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[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_COMPLETEDmessage 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_UNLOCKEDmessage is broadcasted. -
[C-1-13] MUST NOT offer any method to unlock the CE protected storage without either the user-supplied credentials, a registered escrow key or a resume on reboot implementation meeting the requirements in section 9.9.4.
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[C-1-4] MUST use Verified Boot.
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[C-1-5] MUST encrypt file contents and filesystem metadata using AES-256-XTS or Adiantum. AES-256-XTS refers to the Advanced Encryption Standard with a 256-bit cipher key length, operated in XTS mode; the full length of the key is 512 bits. Adiantum refers to Adiantum-XChaCha12-AES, as specified at https://github.com/google/adiantum. Filesystem metadata is data such as file sizes, ownership, modes, and extended attributes (xattrs).
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[C-1-6] MUST encrypt file names using AES-256-CBC-CTS or Adiantum.
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[C-1-12] If the device has Advanced Encryption Standard (AES) instructions (such as ARMv8 Cryptography Extensions on ARM-based devices, or AES-NI on x86-based devices) then the AES-based options above for file name, file contents, and filesystem metadata encryption MUST be used, not Adiantum.
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[C-1-13] MUST use a cryptographically strong and non-reversible key derivation function (e.g. HKDF-SHA512) to derive any needed subkeys (e.g. per-file keys) from the CE and DE keys. "Cryptographically strong and non-reversible" means that the key derivation function has a security strength of at least 256 bits and behaves as a pseudorandom function family over its inputs.
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[C-1-14] MUST NOT use the same File Based Encryption (FBE) keys or subkeys for different cryptographic purposes (e.g. for both encryption and key derivation, or for two different encryption algorithms).
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The keys protecting CE and DE storage areas and filesystem metadata:
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[C-1-7] MUST be cryptographically bound to a hardware-backed Keystore. This keystore MUST be bound to Verified Boot and the device's hardware root of trust.
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[C-1-8] CE keys MUST be bound to a user's lock screen credentials.
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[C-1-9] CE keys MUST be bound to a default passcode when the user has not specified lock screen credentials.
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[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.
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[C-1-11] MUST use the mandatorily supported ciphers, key lengths and modes.
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SHOULD make preinstalled essential apps (e.g. Alarm, Phone, Messenger) Direct Boot aware.
The upstream Android Open Source project provides a preferred implementation of File Based Encryption based on the Linux kernel "fscrypt" encryption feature, and of Metadata Encryption based on the Linux kernel "dm-default-key" feature.
9.9.4. Resume on Reboot
Resume on Reboot allows unlocking the CE storage of all apps, including those that do not yet support Direct Boot, after a reboot initiated by an OTA. This feature enables users to receive notifications from installed apps after the reboot.
An implementation of Resume-on-Reboot must continue to ensure that when a device falls into an attacker’s hands, it is extremely difficult for that attacker to recover the user’s CE-encrypted data, even if the device is powered on, CE storage is unlocked, and the user has unlocked the device after receiving an OTA. For insider attack resistance, we also assume the attacker gains access to broadcast cryptographic signing keys.
Specifically:
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[C-0-1] CE storage MUST NOT be readable even for the attacker who physically has the device and then has these capabilities and limitations:
- Can use the signing key of any vendor or company to sign arbitrary messages.
- Can cause an OTA to be received by the device.
- Can modify the operation of any hardware (AP, flash etc) except as detailed below, but such modification involves a delay of at least an hour and a power cycle that destroys RAM contents.
- Cannot modify the operation of tamper-resistant hardware (eg Titan M).
- Cannot read the RAM of the live device.
- Cannot obtain the user’s credential (PIN, pattern, password) or otherwise cause it to be entered.
By way of example, a device implementation that implements and complies with all of the descriptions found here will be compliant with [C-0-1].