public static void Sign(string input, string output, ImageData stamper, ICipherParameters privateKey, X509Certificate[] chain, string flag)
{
PdfDocument document = new PdfDocument(new PdfReader(input));
PdfAcroForm acroForm = PdfAcroForm.GetAcroForm(document, false);
bool append = (acroForm != null && acroForm.GetSignatureFlags() != 0);
int pageNumber = document.GetNumberOfPages();
RegexBasedLocationExtractionStrategy strategy = new RegexBasedLocationExtractionStrategy(flag);
PdfDocumentContentParser parser = new PdfDocumentContentParser(document);
parser.ProcessContent(pageNumber, strategy);
var locations = new List <IPdfTextLocation>(strategy.GetResultantLocations());
document.Close();
StampingProperties properties = new StampingProperties();
properties = append ? properties.UseAppendMode() : properties;
PdfSigner signer = new PdfSigner(new PdfReader(input), new FileStream(output, FileMode.Create), properties);
signer.SetCertificationLevel(PdfSigner.CERTIFIED_NO_CHANGES_ALLOWED);
PdfSignatureAppearance appearance = signer.GetSignatureAppearance();
appearance.SetPageNumber(pageNumber);
int size = locations.Count;
if (size != 0)
{
IPdfTextLocation location = locations[size - 1];
float flagX = location.GetRectangle().GetX();
float flagY = location.GetRectangle().GetY();
float width = stamper.GetWidth();
float height = stamper.GetHeight();
float x = flagX - width / 2;
float y = flagY - height / 2;
appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.GRAPHIC);
appearance.SetSignatureGraphic(stamper);
appearance.SetPageRect(new Rectangle(x, y, width, height));
}
PrivateKeySignature signature = new PrivateKeySignature(privateKey, DigestAlgorithms.SHA256);
signer.SignDetached(signature, chain, null, null, null, 0, PdfSigner.CryptoStandard.CADES);
}