Digital Signatures

itextsharp

Introduction

In this article, I will present a simple source code allowing you to digitally sign a PDF document and modify its meta data. I will use the excellent and free port of the iText library: iTextSharp, that can be downloaded here. You'll need Visual Studio 2008 to be able to open and build the project.

If you don’t know what digital signatures are or how they work, you can go here or here, or simply ask Google :).

iTextSharp provides a lot of interesting features to create and manipulate PDF documents, but in this article, we will only use the digital signature functions. I will also use some functions to manipulate PKCS#12 certificates. The only thing you need to know here is that our digital signature will use a private key extracted from a PKCS#12 certificate.

Background

I prefer e-signing of an iTextSharp document first, and then doing any change/modification as per requirements.

Using the code

string signerName = 
       PdfPKCS7.GetSubjectFields(this.myCert.Chain[0]).GetField("CN");
PdfTemplate template = st.SignatureAppearance.GetLayer(2);
template.MoveTo(0, 200);
template.LineTo(500, 0);
template.Stroke();
template.BeginText();
BaseFont bf1 = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, 
                                   BaseFont.NOT_EMBEDDED);
template.SetFontAndSize(bf1, 10);
template.SetTextMatrix(1, 1);
template.ShowTextAligned(PdfContentByte.ALIGN_LEFT, "" + 
                         signerName + "", 0, 40, 0);

BaseFont bf = BaseFont.CreateFont(BaseFont.HELVETICA, 
              BaseFont.CP1252, BaseFont.NOT_EMBEDDED);
template.SetFontAndSize(bf, 7);
template.SetTextMatrix(1, 1);
template.ShowTextAligned(PdfContentByte.ALIGN_LEFT, "" + 
         System.DateTime.Now.ToString("yyyy.MM.dd HH:mm:ss zzz") + "", 
         50, 30, 0);

template.SetFontAndSize(bf, 7);
template.SetTextMatrix(1, 1);
template.ShowTextAligned(PdfContentByte.ALIGN_LEFT, "Signer:", 0, 25, 0);

template.SetFontAndSize(bf, 7);
template.SetTextMatrix(1, 1);
template.ShowTextAligned(PdfContentByte.ALIGN_LEFT, "" + "CN=" + 
         PdfPKCS7.GetSubjectFields(myCert.Chain[0]).GetField("CN") + "", 
         10, 17, 0);



template.SetFontAndSize(bf, 7);
template.SetTextMatrix(1, 1);
template.ShowTextAligned(PdfContentByte.ALIGN_LEFT, "" + "C=" + 
                         PdfPKCS7.GetSubjectFields(myCert.Chain[0]).GetField("C") + "", 
                         10, 10, 0);

template.EndText();