c# itextsharp how to get digital signature image

问题

Is it possible to get the image of any digital signatures in a pdf file with itextsharp using c# code?

PdfReader pdf = new PdfReader("location.pdf");
AcroFields acroFields = pdf.AcroFields;
List<string> names = acroFields.GetSignatureNames();

foreach (var name in names)
{
    PdfDictionary dict = acroFields.GetSignatureDictionary(name);
}

With this simple lines i can get the signature dictionaries but from this object i am not able to get the content of the image. Can anyone help?

回答1:

I answer my own question... if it could be usefull to someone else i did it like this. I found a Java class to do what i was looking for and I translated it in C#.

class XyzmoSignatureDataExtractor
{
    private PdfReader reader;

    public XyzmoSignatureDataExtractor(PdfReader reader)
    {
        this.reader = reader;
    }

    public PdfImageObject extractImage(String signatureName) 
    {
        MyImageRenderListener listener = new MyImageRenderListener();

        PdfDictionary sigFieldDic = reader.AcroFields.GetFieldItem(signatureName).GetMerged(0);
        PdfDictionary appearancesDic = sigFieldDic.GetAsDict(PdfName.AP);
        PdfStream normalAppearance = appearancesDic.GetAsStream(PdfName.N);

        PdfDictionary resourcesDic = normalAppearance.GetAsDict(PdfName.RESOURCES);

        PdfContentStreamProcessor processor = new PdfContentStreamProcessor(listener);
        processor.ProcessContent(ContentByteUtils.GetContentBytesFromContentObject(normalAppearance), resourcesDic);        

        return listener.image;
    }

    class MyImageRenderListener : IRenderListener
    {
        public void BeginTextBlock() { }

        public void EndTextBlock() { }

        public void RenderImage(ImageRenderInfo renderInfo)
        {
            try
            {
                image = renderInfo.GetImage();
            }
            catch (Exception e)
            {
                throw new Exception("Failure retrieving image", e);
            }
        }

        public void RenderText(TextRenderInfo renderInfo) { }

        public PdfImageObject image = null;
    }

}

To use the class and save the image i just do like that:

PdfReader reader = new PdfReader("location.pdf");
XyzmoSignatureDataExtractor extractor = new XyzmoSignatureDataExtractor(reader);
AcroFields acroFields = reader.AcroFields;

foreach (string name in  acroFields.GetSignatureNames())
{

  PdfImageObject image = extractor.extractImage(name);

  var _image = image.GetDrawingImage();
  string file_name = "sig." + image.GetFileType();
  _image.Save(file_name);
}

来源：https://stackoverflow.com/questions/38430974/c-sharp-itextsharp-how-to-get-digital-signature-image

标签

pdf

itext

digital-signature