使用itextsharp从签名图像中获取Layer2文本（签名描述） [英] Get Layer2 Text (Signature Description) from signature image using itextsharp

问题描述

我需要从签名中检索layer2文本。如何使用itextsharp获取描述（在签名图像下）？下面是我用来获取签名日期和用户名的代码：

I need to retrieve the layer2 text from a signature. How can I get the description (under the signature image) using itextsharp? below is the code I'm using to get the sign date and username:

        PdfReader reader = new PdfReader(pdfPath, System.Text.Encoding.UTF8.GetBytes(MASTER_PDF_PASSWORD));
        using (MemoryStream memoryStream = new MemoryStream())
        {
            PdfStamper stamper = new PdfStamper(reader, memoryStream);
            AcroFields acroFields = stamper.AcroFields;
            List<String> names = acroFields.GetSignatureNames();
            foreach (String name in names)
            {
                PdfPKCS7 pk = acroFields.VerifySignature(name);
                String userName = PdfPKCS7.GetSubjectFields(pk.SigningCertificate).GetField("CN");
                Console.WriteLine("Sign Date: " + pk.SignDate.ToString() + " Name: " + userName);
               // Here i need to retrieve the description underneath the signature image
            }
            reader.RemoveUnusedObjects();
            reader.Close();
            stamper.Writer.CloseStream = false;
            if (stamper != null)
            {
                stamper.Close();
            }
        }

以下是我用来设置描述的代码

and below is the code I used to set the description

PdfStamper st = PdfStamper.CreateSignature(reader, memoryStream, '\0', null, true);
PdfSignatureAppearance sap = st.SignatureAppearance;
sap.Render = PdfSignatureAppearance.SignatureRender.GraphicAndDescription;
sap.Layer2Font = font;
sap.Layer2Text = "Some text that i want to retrieve";

谢谢。

推荐答案

虽然Bruno从包含第2层的PDF开始解决了这个问题，但请允许我首先说明在PDF签名中使用这些签名图层不是 PDF规范要求，规范实际上根本不知道这些层！因此，如果您尝试解析特定图层，您可能找不到这样的图层，或者更糟糕的是，找到一些看起来像该图层（名为 n2 的XObject）的内容包含错误的数据。

While Bruno addressed the issue starting with a PDF containing a "layer 2", allow me to first state that using these "signature layers" in PDF signature appearances is not required by the PDF specification, the specification actually does not even know these layers at all! Thus, if you try to parse a specific layer, you may not find such a "layer" or, even worse, find something that looks like that layer (a XObject named n2) which contains the wrong data.

尽管如此，无论您是从第2层查找文本还是从签名外观中查找文本，都可以使用iTextSharp文本提取功能。我使用Bruno的代码作为检索 n2 图层的基础。

That been said, though, Whether you look for text from a layer 2 or from the signature appearance as a whole, you can use iTextSharp text extraction capabilities. I used Bruno's code as base for retrieving the n2 layer.

public static void ExtractSignatureTextFromFile(FileInfo file)
{
    try
    {
        Console.Out.Write("File: {0}\n", file);
        using (var pdfReader = new PdfReader(file.FullName))
        {
            AcroFields fields = pdfReader.AcroFields;
            foreach (string name in fields.GetSignatureNames())
            {
                Console.Out.Write("  Signature: {0}\n", name);
                iTextSharp.text.pdf.AcroFields.Item item = fields.GetFieldItem(name);
                PdfDictionary widget = item.GetWidget(0);
                PdfDictionary ap = widget.GetAsDict(PdfName.AP);
                if (ap == null)
                    continue;
                PdfStream normal = ap.GetAsStream(PdfName.N);
                if (normal == null)
                    continue;
                Console.Out.Write("    Content of normal appearance: {0}\n", extractText(normal));

                PdfDictionary resources = normal.GetAsDict(PdfName.RESOURCES);
                if (resources == null)
                    continue;
                PdfDictionary xobject = resources.GetAsDict(PdfName.XOBJECT);
                if (xobject == null)
                    continue;
                PdfStream frm = xobject.GetAsStream(PdfName.FRM);
                if (frm == null)
                    continue;
                PdfDictionary res = frm.GetAsDict(PdfName.RESOURCES);
                if (res == null)
                    continue;
                PdfDictionary xobj = res.GetAsDict(PdfName.XOBJECT);
                if (xobj == null)
                    continue;
                PRStream n2 = (PRStream) xobj.GetAsStream(PdfName.N2);
                if (n2 == null)
                    continue;
                Console.Out.Write("    Content of normal appearance, layer 2: {0}\n", extractText(n2));
            }
        }
    }
    catch (Exception ex)
    {
        Console.Error.Write("Error... " + ex.StackTrace);
    }
}

public static String extractText(PdfStream xObject)
{
    PdfDictionary resources = xObject.GetAsDict(PdfName.RESOURCES);
    ITextExtractionStrategy strategy = new LocationTextExtractionStrategy();

    PdfContentStreamProcessor processor = new PdfContentStreamProcessor(strategy);
    processor.ProcessContent(ContentByteUtils.GetContentBytesFromContentObject(xObject), resources);
    return strategy.GetResultantText();
}

对于示例文件 signature_n2.pdf 布鲁诺用你得到这个：

For the sample file signature_n2.pdf Bruno used you get this:

File: ...\signature_n2.pdf
  Signature: Signature1
    Content of normal appearance: This document was signed by Bruno
Specimen.
    Content of normal appearance, layer 2: This document was signed by Bruno
Specimen.

由于此示例使用了OP期望的第2层，因此它已包含相关文本。

As this sample uses the layer 2 as the OP expects, it already contains the text in question.

这篇关于使用itextsharp从签名图像中获取Layer2文本（签名描述）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！