Migrate backup passphrase to be keystore-encrypted when available.

src/org/thoughtcrime/securesms/backup/BackupDialog.java

@@ -34,7 +34,7 @@ public class BackupDialog {
       button.setOnClickListener(v -> {
         CheckBox confirmationCheckBox = dialog.findViewById(R.id.confirmation_check);
         if (confirmationCheckBox.isChecked()) {
-          TextSecurePreferences.setBackupPassphrase(context, Util.join(password, " "));
+          BackupPassphrase.set(context, Util.join(password, " "));
           TextSecurePreferences.setBackupEnabled(context, true);
           LocalBackupListener.schedule(context);
 
@@ -75,7 +75,7 @@ public class BackupDialog {
                    .setMessage(R.string.BackupDialog_disable_and_delete_all_local_backups)
                    .setNegativeButton(android.R.string.cancel, null)
                    .setPositiveButton(R.string.BackupDialog_delete_backups_statement, (dialog, which) -> {
-                     TextSecurePreferences.setBackupPassphrase(context, null);
+                     BackupPassphrase.set(context, null);
                      TextSecurePreferences.setBackupEnabled(context, false);
                      BackupUtil.deleteAllBackups();
                      preference.setChecked(false);

src/org/thoughtcrime/securesms/backup/BackupPassphrase.java

@@ -0,0 +1,47 @@
+package org.thoughtcrime.securesms.backup;
+
+import android.content.Context;
+import android.os.Build;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
+
+import org.thoughtcrime.securesms.crypto.KeyStoreHelper;
+import org.thoughtcrime.securesms.logging.Log;
+import org.thoughtcrime.securesms.util.TextSecurePreferences;
+
+/**
+ * Allows the getting and setting of the backup passphrase, which is stored encrypted on API >= 23.
+ */
+public class BackupPassphrase {
+
+  private static final String TAG = BackupPassphrase.class.getSimpleName();
+
+  public static String get(@NonNull Context context) {
+    String passphrase          = TextSecurePreferences.getBackupPassphrase(context);
+    String encryptedPassphrase = TextSecurePreferences.getEncryptedBackupPassphrase(context);
+
+    if (Build.VERSION.SDK_INT < 23 || (passphrase == null && encryptedPassphrase == null)) {
+      return passphrase;
+    }
+
+    if (encryptedPassphrase == null) {
+      Log.i(TAG, "Migrating to encrypted passphrase.");
+      set(context, passphrase);
+      encryptedPassphrase = TextSecurePreferences.getEncryptedBackupPassphrase(context);
+    }
+
+    KeyStoreHelper.SealedData data = KeyStoreHelper.SealedData.fromString(encryptedPassphrase);
+    return new String(KeyStoreHelper.unseal(data));
+  }
+
+  public static void set(@NonNull Context context, @Nullable String passphrase) {
+    if (passphrase == null || Build.VERSION.SDK_INT < 23) {
+      TextSecurePreferences.setBackupPassphrase(context, passphrase);
+      TextSecurePreferences.setEncryptedBackupPassphrase(context, null);
+    } else {
+      KeyStoreHelper.SealedData encryptedPassphrase = KeyStoreHelper.seal(passphrase.getBytes());
+      TextSecurePreferences.setEncryptedBackupPassphrase(context, encryptedPassphrase.serialize());
+      TextSecurePreferences.setBackupPassphrase(context, null);
+    }
+  }
+}

src/org/thoughtcrime/securesms/jobs/LocalBackupJob.java

@@ -5,6 +5,7 @@ import android.Manifest;
 import android.content.Context;
 import android.support.annotation.NonNull;
 
+import org.thoughtcrime.securesms.backup.BackupPassphrase;
 import org.thoughtcrime.securesms.jobmanager.SafeData;
 import org.thoughtcrime.securesms.logging.Log;
 
@@ -68,7 +69,7 @@ public class LocalBackupJob extends ContextJob {
                                                  R.drawable.ic_signal_backup);
 
     try {
-      String backupPassword  = TextSecurePreferences.getBackupPassphrase(context);
+      String backupPassword  = BackupPassphrase.get(context);
       File   backupDirectory = StorageUtil.getBackupDirectory();
       String timestamp       = new SimpleDateFormat("yyyy-MM-dd-HH-mm-ss", Locale.US).format(new Date());
       String fileName        = String.format("signal-%s.backup", timestamp);

src/org/thoughtcrime/securesms/util/TextSecurePreferences.java

@@ -138,10 +138,11 @@ public class TextSecurePreferences {
   private static final String ACTIVE_SIGNED_PRE_KEY_ID = "pref_active_signed_pre_key_id";
   private static final String NEXT_SIGNED_PRE_KEY_ID   = "pref_next_signed_pre_key_id";
 
-  public  static final String BACKUP_ENABLED    = "pref_backup_enabled";
-  private static final String BACKUP_PASSPHRASE = "pref_backup_passphrase";
-  private static final String BACKUP_TIME       = "pref_backup_next_time";
-  public  static final String BACKUP_NOW        = "pref_backup_create";
+  public  static final String BACKUP_ENABLED              = "pref_backup_enabled";
+  private static final String BACKUP_PASSPHRASE           = "pref_backup_passphrase";
+  private static final String ENCRYPTED_BACKUP_PASSPHRASE = "pref_encrypted_backup_passphrase";
+  private static final String BACKUP_TIME                 = "pref_backup_next_time";
+  public  static final String BACKUP_NOW                  = "pref_backup_create";
 
   public static final String SCREEN_LOCK         = "pref_android_screen_lock";
   public static final String SCREEN_LOCK_TIMEOUT = "pref_android_screen_lock_timeout";
@@ -231,6 +232,14 @@ public class TextSecurePreferences {
     return getStringPreference(context, BACKUP_PASSPHRASE, null);
   }
 
+  public static void setEncryptedBackupPassphrase(@NonNull Context context, @Nullable String encryptedPassphrase) {
+    setStringPreference(context, ENCRYPTED_BACKUP_PASSPHRASE, encryptedPassphrase);
+  }
+
+  public static @Nullable String getEncryptedBackupPassphrase(@NonNull Context context) {
+    return getStringPreference(context, ENCRYPTED_BACKUP_PASSPHRASE, null);
+  }
+
   public static void setBackupEnabled(@NonNull Context context, boolean value) {
     setBooleanPreference(context, BACKUP_ENABLED, value);
   }