External signing PDF with iText (2) [closed]

问题

Following External signing PDF with iText by Grazina, and reading the mkl answer on that post, I currently have invalid signature (syntax error), even after adding the hash prefix.

Current code:

private void SignPDFCore(string pdfFilePath)
    {
        var chain = this.GetUserCertificates(userId, ProgramV2.TempCertificatesLogFilePath);
        //var certificate = chain.First();

        var signedPdfFilePath = $"{pdfFilePath}.signed.pdf";

        using (PdfReader reader = new PdfReader(pdfFilePath))
        {
            using (FileStream baos = File.OpenWrite(signedPdfFilePath))
            {
                PdfStamper pdfStamper = PdfStamper.CreateSignature(reader, baos, '\0', null, true);
                PdfSignatureAppearance sap = pdfStamper.SignatureAppearance;
                //sap.Certificate = certificate; //Chain[0];
                sap.SetVisibleSignature(new iTextSharp.text.Rectangle(36, 720, 160, 780), 1, signatureFieldName);
                //sap.SetVisibleSignature(signatureFieldName);
                //sap.SignDate = DateTime.Now;
                PdfSignature dic = new PdfSignature(PdfName.ADOBE_PPKLITE, PdfName.ADBE_PKCS7_DETACHED);
                //dic.Date = new PdfDate(sap.SignDate);
                dic.Name = "Name Test"; //CertificateInfo.GetSubjectFields(certificate).GetField("CN");
                sap.CryptoDictionary = dic;
                sap.Acro6Layers = true;
                //sap.CertificationLevel = PdfSignatureAppearance.CERTIFIED_FORM_FILLING_AND_ANNOTATIONS;
                sap.Reason = "test";
                sap.Location = "test";

                IExternalSignature signatureExternal = new RemoteSignature(this, signedPdfFilePath);

                MakeSignature.SignDetached(sap, signatureExternal, chain, null, null, null, 8192, CryptoStandard.CMS);
            }
        }
    }    

public class RemoteSignature : IExternalSignature {
        private string FilePath { get; set; }

        public RemoteSignature(string filePath){
            this.FilePath = filePath;
        }

        public virtual byte[] Sign(byte[] message){
            IDigest messageDigest = DigestUtilities.GetDigest(GetHashAlgorithm());
            byte[] messageHash = DigestAlgorithms.Digest(messageDigest, message);

            //Add prefix to hash
            byte[] sha256Prefix = { 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20 };
            byte[] digestInfo = new byte[sha256Prefix.Length + messageHash.Length];
            sha256Prefix.CopyTo(digestInfo, 0);
            messageHash.CopyTo(digestInfo, sha256Prefix.Length);

            // Request signature for hash value messageHash and return signature bytes               
            var signedHash = CALL_WEBSERVICE_TO_SIGN_HASH(digestInfo);

            return signedHash;
        }

        public virtual String GetHashAlgorithm() {
            return "SHA-256";
        }

        public virtual String GetEncryptionAlgorithm() {
            return "RSA";
        }
    }

RESULTS OBTAINED

MESSAGE HASH (BASE 64):

YY4Zu4mx+HLFVDQiUZ7fFB+b6q47tsUgFeVuRVyKhT0=

MESSAGE HASH WITH PREFIX (BASE 64):

MDEwDQYJYIZIAWUDBAIBBQAEIGGOGbuJsfhyxVQ0IlGe3xQfm+quO7bFIBXlbkVcioU9

SIGNED HASH (base 64):

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

Can mkl please help checking the signed hash, like did in Grazina's answer? Thanks.

来源：https://stackoverflow.com/questions/59669163/external-signing-pdf-with-itext-2

标签

pdf

itext

digital-signature