@AccessesPartialKey
  public static Aead create(XChaCha20Poly1305Key key) throws GeneralSecurityException {
    return new XChaCha20Poly1305(
        key.getKeyBytes().toByteArray(InsecureSecretKeyAccess.get()),
        key.getOutputPrefix().toByteArray());
  }
XChaCha20Poly1305 AEAD construction, as described in
https://tools.ietf.org/html/draft-arciszewski-xchacha-01.
java_src/src/main/java/com/google/crypto/tink/subtle/XChaCha20Poly1305.java