CDD: Add per-user block-level encryption to storage encryption

Add an alternative section to define encryption requirements for
device implementations using per-user block-level encrypted

Bug: 184198954
Test: none
Change-Id: Icba5a5541c367f8863466b453e249800c1f6d9aa
(cherry picked from commit 2ea5b7f402881b48dcd02acbc12d1fc2348adb18)
diff --git a/9_security-model/ b/9_security-model/
index cbf4de5..0d89202 100644
--- a/9_security-model/
+++ b/9_security-model/
@@ -31,9 +31,13 @@
 *   [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](
+requirement by implementing one of the following two encryption methods:
+    *   [File Based Encryption](
+        (FBE) and
+        [Metadata Encryption](
+        as described in section
+    *   Per-User Block-Level Encryption as described in section
 ### 9.9.3\. Encryption Methods
@@ -51,6 +55,12 @@
 resume on reboot implementation meeting the requirements in
 [section 9.9.4](#9_9_4_resume_on_reboot).
 *    [C-1-4] MUST use Verified Boot.
+###\. File Based Encryption with Metadata Encryption
+If device implementations use File Based Encryption with Metadata Encryption,
 *    [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
@@ -94,6 +104,29 @@
 File Based Encryption based on the Linux kernel "fscrypt" encryption feature,
 and of Metadata Encryption based on the Linux kernel "dm-default-key" feature.
+###\. Per-User Block-Level Encryption
+If device implementations use per-user block-level encryption, they:
+*    [C-1-1] MUST enable multi-user support as described in section 9.5.
+*    [C-1-2] MUST provide per-user partitions, either using raw partitions or
+logical volumes.
+*    [C-1-3] MUST use unique and distinct encryption keys per-user for
+encryption of the underlying block devices.
+*    [C-1-4] MUST use AES-256-XTS for block-level encryption of the user
+*   The keys protecting the per-user block-level encrypted devices:
+   *   [C-1-5] 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.
+   *   [C-1-6] MUST be bound to the corresponding user's lock screen
+   credentials.
+Per-user block-level encryption can be implemented using the Linux kernel
+“dm-crypt” feature over per-user partitions.
 ### 9.9.4\. Resume on Reboot
 Resume on Reboot allows unlocking the CE storage of all apps, including those