encryptCipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(key, "RC4"));
        byte[] encrypted = encryptCipher.doFinal(plaintext);

        Cipher decryptCipher = Crypto.getArcfour(key);
        decryptCipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(key, "RC4"));
        byte[] decrypted = decryptCipher.doFinal(encrypted);

        // Then
        assertNotNull(encrypted);
        assertNotNull(decrypted);

        int ms_usage = mapArcfourMD5KeyUsage(keyusage);
        Mac mac = Mac.getInstance("HmacMD5");
        MessageDigest md = MessageDigest.getInstance("MD5");
        mac.init(key);
        byte[] dk = mac.doFinal(MD5_CONSTANT);
        try {
            // little endian
            md.update((byte) (ms_usage & 0xFF));
            md.update((byte) (ms_usage >> 8 & 0xFF));
            md.update((byte) (ms_usage >> 16 & 0xFF));

      checkState(!done, "Cannot re-use a Hasher after calling hash() on it");
    }

    @Override
    public HashCode hash() {
      checkNotDone();
      done = true;
      return HashCode.fromBytesNoCopy(mac.doFinal());
    }
  }