private void signDetached(PdfSignatureAppearance signAppearance)
{
signAppearance.CertificationLevel = PdfSignatureAppearance.CERTIFIED_NO_CHANGES_ALLOWED;
var es = new PrivateKeySignature(_asymmetricKeyParameter, "SHA-256");
MakeSignature.SignDetached(signAppearance, es, _chain, null, null, null, 0, CryptoStandard.CMS);
}