External signature on a smart card

I am using a smart card that is signing a SHA-1 hash of a document, and compute a 256 bytes digital signature.

I am using the code posted on this question - iText signing PDF using external signature with smart card.

My problem is that I get the error:" The document has been altered or corrupted since the signature was applied".

I am using a GUI to create the hash and then send the signed 256 bytes that is computed on the card to the signing functions .

Here is my code:

hash creating code of filepath pdf document:

SHA1CryptoServiceProvider sha1 = new SHA1CryptoServiceProvider(); 
SHA256 sha2 = SHA256.Create(); 
//sha2.ComputeHash 
byte[] pdfBytes = System.IO.File.ReadAllBytes(filePath); 
byte[] hash = null; 
hash=  sha1.ComputeHash(pdfBytes); 

the above code is used in one of the GUI functions to create the hash of the document

namespace EIDSmartCardSign
{
    class PdfSignature
    {
        private string outputPdfPath;
        private string certPath;
        byte[] messageDigest;

        private string inputPdfPath;
        public PdfSignature(byte[] messageDigest, string inputPdfPath,string outputPdfPath)
        {
            this.messageDigest = messageDigest;
            this.outputPdfPath = outputPdfPath;
            this.inputPdfPath = inputPdfPath;
        }

        public void setCertPath(string certPath)
        {
            this.certPath = certPath;
        }

        public void signPdf()
        {
            X509Certificate2 cert = new X509Certificate2();

            cert.Import(certPath); // .cer file certificate obtained from smart card

            X509CertificateParser certParse = new Org.BouncyCastle.X509.X509CertificateParser();
                       Org.BouncyCastle.X509.X509Certificate[] chain = new Org.BouncyCastle.X509.X509Certificate[1] ;
            chain[0] = certParse.ReadCertificate(cert.RawData);

            X509Certificate2[] certs;

            PdfReader reader  = new PdfReader(inputPdfPath);
            FileStream fout = new FileStream(outputPdfPath,FileMode.Create);
            PdfStamper stamper = PdfStamper.CreateSignature(reader, fout, '\0',null,true);

            PdfSignatureAppearance appearance = stamper.SignatureAppearance;
            appearance.SignatureCreator = "Me";
            appearance.Reason = "Testing iText";
            appearance.Location = "On my Laptop";
            iTextSharp.text.Rectangle rec = new iTextSharp.text.Rectangle(50, 50, 250, 100);
            appearance.SetVisibleSignature(rec, 1, "Signature");
            IExternalSignature extSignature= new MyExternalSignature("SHA-1",this.messageDigest);


            MakeSignature.SignDetached(appearance, extSignature, chain, null, null, null, 0, CryptoStandard.CMS);
            //MakeSignature.
        }
    }
}

Copyright Notice：Content Author:「GLC」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/43974413/external-signature-on-a-smart-card