iText7 (.net) SignExternalSignatureContainer NullReferenceException

Question

我正在尝试创建一个可以对 PDF 文档进行数字签名的 C# 程序。为了将签名元素包含到 PDF 中，我使用 iText7。现在，如果我运行没有调试器的程序将被抛出 System.NullReferenceException 并且程序失败。但是如果我运行使用调试器的程序也会发生异常并且代码继续并正确签署 PDF...

我不确定这是 iText7 的问题还是我在创建签名字段时犯了错误或忘记了一些重要的事情。

有什么解决办法吗？

异常

System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt. bei iText.Signatures.PdfSignatureAppearance.GetAppearance() in C:\Development\Others\itext7\itext\itext.sign\itext\signatures\PdfSignatureAppearance.cs:Zeile 584. bei iText.Signatures.PdfSigner.PreClose(IDictionary`2 exclusionSizes) in C:\Development\Others\itext7\itext\itext.sign\itext\signatures\PdfSigner.cs:Zeile 808. bei iText.Signatures.PdfSigner.SignExternalContainer(IExternalSignatureContainer externalSignatureContainer, Int32 estimatedSize) in C:\Development\Others\itext7\itext\itext.sign\itext\signatures\PdfSigner.cs:Zeile 582. bei SignService.Engine.Core.PdfEngine.AddSignature(SignTask task) in C:\Development\Signature\SignService.Engine\Core\PdfEngine.cs:Zeile 122.

我的代码

/// <summary>
/// Add SignatureField to Pdf
/// and digitally sign it 
/// </summary>
public SignatureResult AddSignature(SignTask task)
{
    _logger.Info("Start Signing PDF");

    var prop = task.SignatureProperties;

    try
    {
        var reader = new PdfReader(new MemoryStream(prop.Document));
        var stream = new ByteArrayOutputStream();
        var signer = new PdfSigner(reader, stream, new StampingProperties().UseAppendMode());

        // set appearance
        var appearance = signer.GetSignatureAppearance();
        appearance.SetReason(prop.SignReason)
                  .SetLocation(prop.SignLocation)
                  .SetContact(prop.SignContact);

        // set rendering mode
        switch (_settings.Pdf.RenderingMode)
        {
            case SignatureRenderingMode.Description:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.DESCRIPTION);
                break;
            case SignatureRenderingMode.Graphic:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.GRAPHIC);
                break;
            case SignatureRenderingMode.GraphicAndDescription:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.GRAPHIC_AND_DESCRIPTION);
                break;
            case SignatureRenderingMode.NameAndDescription:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.NAME_AND_DESCRIPTION);
                break;
        }

        // set image
        if (!string.IsNullOrEmpty(_settings.Pdf.SignatureGraphicPath))
        {
            if (File.Exists(_settings.Pdf.SignatureGraphicPath))
            {
                var imageData = ImageDataFactory.Create(_settings.Pdf.SignatureGraphicPath);
                appearance.SetSignatureGraphic(imageData);
            }
            else if (_settings.Pdf.RenderingMode == SignatureRenderingMode.GraphicAndDescription || _settings.Pdf.RenderingMode == SignatureRenderingMode.GraphicAndDescription)
            {
                throw new Exception("Failed to create Signature Field.\nIf rendering mode is graphic or graphic and description, a signature image must be provided");
            }
        }

        // set visibility
        if (prop.Visible)
        {
            var rect = new Rectangle(prop.X, prop.Y, prop.Width, prop.Height);
            appearance.SetPageRect(rect);
            appearance.SetPageNumber(1); // todo create setting for this
        }

        signer.SetFieldName(_settings.Pdf.SignatureFieldName);

        // sign field
        var signatureContainer = new ExternalSignatureContainer(task, _settings.Ais);
        // **Exeption thrown here**
        signer.SignExternalContainer(signatureContainer, GetEstimatedSize(task.TimestampOnly));

        var tempResult = stream.ToArray();

        // set revocation info if active
        if (tempResult.Length > 0 && _settings.Ais.AddRevocationInfo)
        {
            tempResult = AddRevocationInfo(signatureContainer.Crl, signatureContainer.Ocsp, tempResult);
        }

        reader.Close();
        stream.Close();

        _logger.Info("Finished signing");

        // return sign result
        return tempResult.Length > 0 
          ? new SignatureResult
          {
               Message = "",
               Status = RequestStatus.Success,
               Document = stream.ToArray(),
               Id = prop.Id
          }
          : new SignatureResult
          {
               Message = "Failed to sign the Document",
               Status = RequestStatus.Failed,
               Document = null,
               Id = prop.Id
          };
    }
    catch (AisServiceException aisServiceException)
    {
        _logger.Error($"While requesting Signature an error occured: {aisServiceException.Message}", aisServiceException);
        throw;
    }
    catch (Exception exception)
    {
        _logger.Error($"While creating signed pdf an error occured: {exception.Message}", exception);
        throw new PdfException($"While creating Signed Pdf an error Occured: {exception.Message}");
    }
}

设置：

{
  "Pdf": {
    "Visible": true,
    "Position": {
      "X": 50,
      "Y": 50,
      "Height": 100,
      "Width": 200
    },
    "SignatureFieldName": "SignatureField",
    "SignatureGraphicPath": "",
    "RenderingMode": 1
  }
}

任务：

  "SignatureProperties": {
    "Id": "1",
    "Document": [DocumentAsByteArray],
    "Visible": true,
    "SignReason": "Test",
    "SignLocation": "Test",
    "SignContact": "Test",
    "Height": 100,
    "Width": 200,
    "X": 50,
    "Y": 50
  }

Answer 1

在您的代码中，您没有使用 PdfSignatureAppearance.SetCertificate 设置签署者证书。虽然在像您这样的 SignExternalContainer 用例的情况下，iText 确实不需要用于实际签名过程的证书，但它确实需要它来检索有关签名者的信息以用作名称，并在可见签名的情况下用于描述带有名称 and/or 描述（即任何渲染模式，但纯 GRAPHICS）。

您的设置 "RenderingMode": 1 和堆栈跟踪 PdfSignatureAppearance.cs:Zeile 584 表明您处于 NAME_AND_DESCRIPTION 用例中。因此，iText 尝试构建一个描述，访问它的 signCertificate，但失败了，因为该成员是 null.

要解决此问题，请同时使用 PdfSignatureAppearance.SetCertificate 设置签名者证书。

在 DESCRIPTION 和 GRAPHIC_AND_DESCRIPTION 的情况下，您也可以使用 PdfSignatureAppearance.SetLayer2Text.

将描述设置为预制值

不过，在 NAME_AND_DESCRIPTION 的情况下，iText 无法提供注入预先计算的名称的方法。

一个更通用的替代方法是自己创建签名外观，只需使用 GetLayer2 检索一个 PdfFormXObject，您可以在上面绘制任何您喜欢的可视化效果。在这种情况下，iText 不会尝试从证书中检索任何签名者信息。

iText7 (.net) SignExternalSignatureContainer NullReferenceException

iText7 (.net) SignExternalSignatureContainer NullReferenceException

c#

digital-signature

nullreferenceexception

itext7