Add backup rules for CredentialIdentityStore kotlin implementation (#520)

apps/mobile-app/android/app/src/main/AndroidManifest.xml

@@ -12,7 +12,7 @@
       <data android:scheme="https"/>
     </intent>
   </queries>
-  <application android:name=".MainApplication" android:label="@string/app_name" android:icon="@mipmap/ic_launcher" android:roundIcon="@mipmap/ic_launcher_round" android:allowBackup="true" android:theme="@style/AppTheme" android:supportsRtl="true" android:usesCleartextTraffic="true" android:localeConfig="@xml/locales_config" android:networkSecurityConfig="@xml/network_security_config">
+  <application android:name=".MainApplication" android:label="@string/app_name" android:icon="@mipmap/ic_launcher" android:roundIcon="@mipmap/ic_launcher_round" android:allowBackup="true" android:fullBackupContent="@xml/backup_rules" android:dataExtractionRules="@xml/data_extraction_rules" android:theme="@style/AppTheme" android:supportsRtl="true" android:usesCleartextTraffic="true" android:localeConfig="@xml/locales_config" android:networkSecurityConfig="@xml/network_security_config">
     <meta-data android:name="expo.modules.updates.ENABLED" android:value="false"/>
     <meta-data android:name="expo.modules.updates.EXPO_UPDATES_CHECK_ON_LAUNCH" android:value="ALWAYS"/>
     <meta-data android:name="expo.modules.updates.EXPO_UPDATES_LAUNCH_WAIT_MS" android:value="0"/>

apps/mobile-app/android/app/src/main/java/net/aliasvault/app/credentialprovider/CredentialIdentityStore.kt

@@ -16,6 +16,12 @@ import org.json.JSONObject
  * This class stores passkey metadata (rpId, userName, displayName, credentialId, userHandle)
  * in SharedPreferences so that passkeys can be displayed without unlocking the vault.
  *
+ * Security features:
+ * - Uses MODE_PRIVATE for app-private storage (no other apps can access)
+ * - Excluded from device backups (see backup_rules.xml and data_extraction_rules.xml)
+ * - Contains no passwords, only metadata (usernames, rpIds, displayNames)
+ * - Cleared when credential provider is disabled by user
+ *
  * Similar to iOS ASCredentialIdentityStore, but using SharedPreferences as Android's
  * Credential Manager API doesn't provide a system-level identity store.
  */
@@ -41,6 +47,8 @@ class CredentialIdentityStore private constructor(context: Context) {
         }
     }
 
+    // MODE_PRIVATE ensures only this app can access the data
+    // Backup exclusion rules prevent data from being backed up
     private val prefs: SharedPreferences = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE)
 
     /**
@@ -154,6 +162,21 @@ class CredentialIdentityStore private constructor(context: Context) {
             .apply()
     }
 
+    /**
+     * Check if the credential identity store is empty.
+     * This is used to determine if initial sync is needed.
+     * @return true if the store has no credential identities
+     */
+    fun isStoreEmpty(): Boolean {
+        return try {
+            val jsonString = prefs.getString(KEY_PASSKEY_IDENTITIES, null)
+            jsonString == null || JSONArray(jsonString).length() == 0
+        } catch (e: Exception) {
+            Log.e(TAG, "Error checking if store is empty", e)
+            true // Assume empty if we can't read
+        }
+    }
+
     /**
      * Create a PasskeyIdentity from a Passkey and its parent Credential.
      */

apps/mobile-app/android/app/src/main/res/xml/backup_rules.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  Backup rules for Android API < 31 (Android 11 and below).
+  Excludes sensitive credential identity data from device backups.
+
+  The credential_identities file stores user metadata (usernames, email addresses, service names)
+  that should not leave the device, matching iOS ASCredentialIdentityStore behavior which
+  explicitly excludes its data from device backups.
+
+  Security rationale:
+  - Contains sensitive user information (though no passwords)
+  - Should remain device-local for privacy
+  - Can be re-synced from vault on new device
+-->
+<full-backup-content>
+    <!-- Exclude the credential identity store from backups -->
+    <exclude domain="sharedpref" path="credential_identities.xml"/>
+</full-backup-content>

apps/mobile-app/android/app/src/main/res/xml/data_extraction_rules.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  Data extraction rules for Android API 31+ (Android 12 and above).
+  Excludes sensitive credential identity data from cloud backups and device transfers.
+
+  The credential_identities file stores user metadata (usernames, email addresses, service names)
+  that should not leave the device, matching iOS ASCredentialIdentityStore behavior which
+  explicitly excludes its data from device backups.
+
+  Security rationale:
+  - Contains sensitive user information (though no passwords)
+  - Should remain device-local for privacy
+  - Can be re-synced from vault on new device
+-->
+<data-extraction-rules>
+    <cloud-backup>
+        <!-- Exclude credential identity store from cloud backups -->
+        <exclude domain="sharedpref" path="credential_identities.xml"/>
+    </cloud-backup>
+    <device-transfer>
+        <!-- Exclude credential identity store from device-to-device transfers -->
+        <exclude domain="sharedpref" path="credential_identities.xml"/>
+    </device-transfer>
+</data-extraction-rules>