@Override
  InsecureNonceChaCha20Base newChaCha20Instance(final byte[] key, int initialCounter)
      throws InvalidKeyException {
    return new InsecureNonceXChaCha20(key, initialCounter);
  }
XChaCha20Poly1305 AEAD construction, as described in
https://tools.ietf.org/html/draft-arciszewski-xchacha-01.
java_src/src/main/java/com/google/crypto/tink/aead/internal/InsecureNonceXChaCha20Poly1305.java