Synchronize Cipher in KeystoreHelper

app/src/main/java/org/thoughtcrime/securesms/crypto/KeyStoreHelper.java

@@ -45,44 +45,52 @@ public final class KeyStoreHelper {
   private static final String ANDROID_KEY_STORE = "AndroidKeyStore";
   private static final String KEY_ALIAS         = "SignalSecret";
 
-  @RequiresApi(Build.VERSION_CODES.M)
+  private static final Object lock = new Object();
+
   public static SealedData seal(@NonNull byte[] input) {
     SecretKey secretKey = getOrCreateKeyStoreEntry();
 
     try {
-      Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
-      cipher.init(Cipher.ENCRYPT_MODE, secretKey);
+      // Cipher operations are not thread-safe so we synchronize over them through doFinal to
+      // prevent crashes with quickly repeated encrypt/decrypt operations
+      // https://github.com/mozilla-mobile/android-components/issues/5342
+      synchronized (lock) {
+        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
+        cipher.init(Cipher.ENCRYPT_MODE, secretKey);
 
-      byte[] iv   = cipher.getIV();
-      byte[] data = cipher.doFinal(input);
+        byte[] iv = cipher.getIV();
+        byte[] data = cipher.doFinal(input);
 
-      return new SealedData(iv, data);
+        return new SealedData(iv, data);
+      }
     } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException | IllegalBlockSizeException | BadPaddingException e) {
       throw new AssertionError(e);
     }
   }
 
-  @RequiresApi(Build.VERSION_CODES.M)
   public static byte[] unseal(@NonNull SealedData sealedData) {
     SecretKey secretKey = getKeyStoreEntry();
 
     try {
-      Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
-      cipher.init(Cipher.DECRYPT_MODE, secretKey, new GCMParameterSpec(128, sealedData.iv));
-
-      return cipher.doFinal(sealedData.data);
+      // Cipher operations are not thread-safe so we synchronize over them through doFinal to
+      // prevent crashes with quickly repeated encrypt/decrypt operations
+      // https://github.com/mozilla-mobile/android-components/issues/5342
+      synchronized (lock) {
+        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
+        cipher.init(Cipher.DECRYPT_MODE, secretKey, new GCMParameterSpec(128, sealedData.iv));
+
+        return cipher.doFinal(sealedData.data);
+      }
     } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException | InvalidAlgorithmParameterException | IllegalBlockSizeException | BadPaddingException e) {
       throw new AssertionError(e);
     }
   }
 
-  @RequiresApi(Build.VERSION_CODES.M)
   private static SecretKey getOrCreateKeyStoreEntry() {
     if (hasKeyStoreEntry()) return getKeyStoreEntry();
     else                    return createKeyStoreEntry();
   }
 
-  @RequiresApi(Build.VERSION_CODES.M)
   private static SecretKey createKeyStoreEntry() {
     try {
       KeyGenerator keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, ANDROID_KEY_STORE);
@@ -99,7 +107,6 @@ public final class KeyStoreHelper {
     }
   }
 
-  @RequiresApi(Build.VERSION_CODES.M)
   private static SecretKey getKeyStoreEntry() {
     KeyStore keyStore = getKeyStore();
 
@@ -137,7 +144,6 @@ public final class KeyStoreHelper {
     }
   }
 
-  @RequiresApi(Build.VERSION_CODES.M)
   private static boolean hasKeyStoreEntry() {
     try {
       KeyStore ks = KeyStore.getInstance(ANDROID_KEY_STORE);