@AccessesPartialKey
  public static DeterministicAead create(AesSivKey key) throws GeneralSecurityException {
    return new AesSiv(
        key.getKeyBytes().toByteArray(InsecureSecretKeyAccess.get()), key.getOutputPrefix());
  }
The key used for the CTR encryption
java_src/src/main/java/com/google/crypto/tink/subtle/AesSiv.java