Only use MasterSecret for local message encryption.

Not for the axolotl store.

// FREEBIE

src/org/thoughtcrime/securesms/crypto/MediaKey.java

@@ -0,0 +1,31 @@
+package org.thoughtcrime.securesms.crypto;
+
+import android.support.annotation.NonNull;
+
+import org.thoughtcrime.securesms.util.Base64;
+import org.whispersystems.libaxolotl.InvalidMessageException;
+
+import java.io.IOException;
+
+public class MediaKey {
+
+  public static String getEncrypted(@NonNull MasterSecretUnion masterSecret, @NonNull byte[] key) {
+    if (masterSecret.getMasterSecret().isPresent()) {
+      return Base64.encodeBytes(new MasterCipher(masterSecret.getMasterSecret().get()).encryptBytes(key));
+    } else {
+      return "?ASYNC-" + Base64.encodeBytes(new AsymmetricMasterCipher(masterSecret.getAsymmetricMasterSecret().get()).encryptBytes(key));
+    }
+  }
+
+  public static byte[] getDecrypted(@NonNull MasterSecret masterSecret,
+                                    @NonNull AsymmetricMasterSecret asymmetricMasterSecret,
+                                    @NonNull String encodedKey)
+      throws IOException, InvalidMessageException
+  {
+    if (encodedKey.startsWith("?ASYNC-")) {
+      return new AsymmetricMasterCipher(asymmetricMasterSecret).decryptBytes(Base64.decode(encodedKey.substring("?ASYNC-".length())));
+    } else {
+      return new MasterCipher(masterSecret).decryptBytes(Base64.decode(encodedKey));
+    }
+  }
+}