iTextSharp 5多个签名-最后一个签名使以前的签名无效-文档已被更改或损坏

Question

我正在尝试使用iTextSharp 5.5.13.1添加多个签名。仅最后一个签名有效。并且所有以前的签名对消息均无效：“自签名以来，文档已被更改或损坏”-修改了1页

我不一定需要经过认证的签名。我使用签名追加模式，但仍无法弄清楚是什么修改了文档。在记事本中，文档的第一部分直到第二个签名似乎都不变。

我使用的代码是：

private string SignFile(string fileToSign, string certname, float xPercent, float yPercent, int page)
{
    string signedFile = fileToSign.Replace(".pdf", ".signed.pdf");

    using (PdfReader pdfReader = new PdfReader(fileToSign))
    {
        int pages = pdfReader.NumberOfPages;
        var currentSignaturesCount = pdfReader.AcroFields.GetSignatureNames().Count();

        using (FileStream signedPdf = new FileStream(signedFile, FileMode.Create, FileAccess.ReadWrite))
        {
            string tempDir = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), ".tempfiles");
            Directory.CreateDirectory(tempDir);
            string tempFileName = Path.Combine(tempDir, Guid.NewGuid().ToString("N") + ".pdf");
            if (!File.Exists(tempFileName))
                File.Create(tempFileName).Close();

            using (PdfStamper pdfStamper = PdfStamper.CreateSignature(pdfReader, signedPdf, '\0', tempFileName, true))  // Append mode
            {
                // Add signature image
                if (page <= pages && page > 0)
                {
                    var pdfContentByte = pdfStamper.GetOverContent(page);

                    var pageSize = pdfReader.GetPageSize(i);
                    float pageWidth = pageSize.Width;
                    float pageHeight = pageSize.Height;

                    // GenerateStamp() = simplified function that will get a custom bitmap (code not included here)
                    System.Drawing.Bitmap img = GenerateStamp();

                    var image = iTextSharp.text.Image.GetInstance(img, true);

                    image.SetAbsolutePosition(xPercent * pageWidth, pageHeight - yPercent * pageHeight - image.ScaledHeight);

                    pdfContentByte.AddImage(image);
                }
                //Also tried adding the image directly to signatureAppearance
                //signatureAppearance.SignatureGraphic = image;
                //signatureAppearance.SetVisibleSignature(rectangle, page, signatureFieldName);
                // and getting the error "Document has been altered or corrupted since it was signed"


                PdfSignatureAppearance signatureAppearance = pdfStamper.SignatureAppearance;

                signatureAppearance.Reason = "Test";
                signatureAppearance.SignDate = DateTime.Now;
                signatureAppearance.SignatureRenderingMode = PdfSignatureAppearance.RenderingMode.GRAPHIC;
                signatureAppearance.Acro6Layers = false;

                //Also tried like this:
                //signatureAppearance.CertificationLevel = currentSignaturesCount == 0 ? PdfSignatureAppearance.CERTIFIED_FORM_FILLING_AND_ANNOTATIONS : PdfSignatureAppearance.NOT_CERTIFIED;
                // with message: "There have been changes made to this document that invalidate the signature"

                // sign document
                try
                {
                    X509Certificate2 cert = GetCertificateByName(certname);
                    Org.BouncyCastle.X509.X509CertificateParser cp = new Org.BouncyCastle.X509.X509CertificateParser();
                    Org.BouncyCastle.X509.X509Certificate[] chain = new Org.BouncyCastle.X509.X509Certificate[] { cp.ReadCertificate(cert.RawData) };
                    IExternalSignature externalSignature = new X509Certificate2Signature(cert, "SHA-256");

                    MakeSignature.SignDetached(signatureAppearance, externalSignature, chain, null, null, null, 0, CryptoStandard.CMS);
                }
                catch (Exception ex)
                {
                    throw;
                }

            }
        }
    }

    return signedFile;
}

感谢您的帮助。谢谢

Answer 1

您的代码原样将图像添加到页面的静态内容。禁止对签名文件执行此操作。有关对签名的PDF进行允许和禁止的更改的详细信息，请阅读this answer。

不过，根据您的代码注释，您还尝试将图像添加到签名外观中。因此，这是不被禁止的。但是，通过分析提供的示例PDF，很明显，在这种尝试中，其他内容流已添加到页面。即使它们本质上是空的，这也被认为是页面内容的更改，这是不允许的。

事实证明，您这次没有将图像添加到页面内容中，但是仍然检索了页面的OverContent：

var pdfContentByte = pdfStamper.GetOverContent(page);

此操作已经在页面上添加了额外的内容流，以供OverContent使用。因此，严格来说，该方法应命名为CreateOrGetOverContent。

[删除GetOverContent呼叫签名后，不会再破坏以前的签名。