/// <summary>
/// Sets plaintext = DecryptWithAd(h, ciphertext),
/// calls MixHash(ciphertext), and returns plaintext.
/// </summary>
public int DecryptAndHash(ReadOnlySpan <byte> ciphertext, Span <byte> plaintext)
{
var bytesRead = state.DecryptWithAd(h, ciphertext, plaintext);
MixHash(ciphertext);
return(bytesRead);
}
