I am writing a demo proof of concept to add electronic signatures to existing PDFs. I am running into one weird problem that I don't get. It seems that adding the signature to some documents works fine while adding one to others does not work and a corrupt file is generated that Adobe Reader can not open.
Here is my code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using iText;
using iText.Kernel.Pdf;
using System.IO;
using iText.Layout;
using iText.Layout.Element;
using iText.Kernel.Geom;
using Org.BouncyCastle.Crypto.Tls;
using iText.Signatures;
using System.Collections.ObjectModel;
using Org.BouncyCastle.Pkcs;
using System.Security.Cryptography.X509Certificates;
using Org.BouncyCastle.Crypto;
using System.Security.Cryptography;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Math;
using iText.IO.Image;
namespace LTVSkilrikjaDemo
{
class Program
{
static void Main(string[] args)
{
string welcomeText = "Welcome to LTVSkilríkjaDemotolid!";
string pressEnterToTry = "Commands: 's' - sign, 'stop' - stops the programme";
Console.WriteLine(welcomeText);
Console.WriteLine(pressEnterToTry);
// Base directory prepared
string basedir = AppDomain.CurrentDomain.BaseDirectory;
int index = basedir.IndexOf(@"bin\");
basedir = basedir.Remove(index);
string readString = Console.ReadLine().ToLower();
while(!readString.Equals("stop"))
{
if(readString.Equals("c"))
{
string inFile = "Infile2.pdf";
string outFile = "Outfile2.pdf";
// Open PDF document and decide where to write the new document
PdfWorker worker = new PdfWorker();
worker.ReadPdf(basedir + "App_Data\\InFiles\\" + inFile, basedir + "App_Data\\OutFiles\\" + outFile);
// Start working on certificate
X509Store store = new X509Store("My");
store.Open(OpenFlags.ReadOnly);
Collection<Org.BouncyCastle.X509.X509Certificate> xcertificates = new Collection<Org.BouncyCastle.X509.X509Certificate>();
foreach (X509Certificate2 mCert in store.Certificates)
{
if (mCert.Subject.IndexOf("CN=Róbert") > -1)
{
xcertificates.Add(Org.BouncyCastle.Security.DotNetUtilities.FromX509Certificate(mCert));
}
}
Org.BouncyCastle.X509.X509Certificate[] certificatesProcessed = new Org.BouncyCastle.X509.X509Certificate[xcertificates.Count];
for(int i = 0; i < xcertificates.Count; i++)
{
certificatesProcessed[i] = xcertificates[i];
}
var pk = Org.BouncyCastle.Security.DotNetUtilities.GetKeyPair(store.Certificates[5].PrivateKey).Private;
try
{
worker.Sign(certificatesProcessed, pk, DigestAlgorithms.SHA1, PdfSigner.CryptoStandard.CADES, "No apparent raisin!", "Lost in Iceland", null, null, null, 0, true, basedir);
}
catch(Exception ex)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine("Error! " + ex.Message + "\n\r" + ex.StackTrace);
if(ex.InnerException != null)
{
Console.WriteLine("Inner exception: " + ex.InnerException.Message);
}
Console.ForegroundColor = ConsoleColor.Gray;
}
}
else if(!readString.Equals("stop"))
{
Console.WriteLine("Command not understood. Understand only 's' and 'stop'.");
}
readString = Console.ReadLine();
}
Console.WriteLine("Goodbye!");
System.Threading.Thread.Sleep(500);
}
}
public class PdfWorker
{
private PdfDocument _document;
private string _source;
private string _dest;
public void ReadPdf(string source, string dest)
{
_source = source;
_dest = dest;
}
public void Sign(Org.BouncyCastle.X509.X509Certificate[] chain, Org.BouncyCastle.Crypto.ICipherParameters pk,
string digestAlgorithm, PdfSigner.CryptoStandard subfilter, string reason,
string location, Collection<ICrlClient> crlList, IOcspClient ocspClient, ITSAClient tsaClient,
int estimatedSize, bool initial, string baseDir)
{
File.Copy(_source, _dest, true);
FileStream f = new FileStream(_dest, FileMode.Append);
try
{
PdfSigner signer = new PdfSigner(new PdfReader(_source), f, true);
_document = signer.GetDocument();
_document.AddNewPage();
// Work the last page
Rectangle pageSize = _document.GetLastPage().GetPageSizeWithRotation();
//PdfWriter w = _document.GetWriter();
//long currentPos = w.GetCurrentPos();
float llx = pageSize.GetWidth() - 350 - 20; //pageSize.GetWidth() / 2 - 350 / 2;
float lly = pageSize.GetHeight() - 50 - 20; // pageSize.GetHeight() / 2 - 150 / 2;
float urx = 350; //llx + 350;
float ury = 50; //lly + 150;
PdfSignatureAppearance appearance = signer.GetSignatureAppearance();
appearance.SetPageRect(new Rectangle(llx, lly, urx, ury));
appearance.SetReason(reason);
appearance.SetLocation(location);
byte[] imagebytes = File.ReadAllBytes(baseDir + "App_Data\\UndirskriftDemo.png");
// It is not possible to use the path as it contains Icelandic characters
// which itext chokes on. We use byte array instead
ImageData imgData = ImageDataFactory.Create(imagebytes);
Image img = new Image(imgData);
img = img.ScaleToFit(350.0f, 50.0f);
appearance.SetImage(imgData);
int pageCount = _document.GetNumberOfPages();
// Creating the appearance
if(initial == true)
{
signer.SetCertificationLevel(PdfSigner.CERTIFIED_FORM_FILLING_AND_ANNOTATIONS);
}
appearance.SetPageNumber(pageCount);
Rectangle rect = new Rectangle(10, 50, 350, 50);
appearance.SetPageRect(rect).SetPageNumber(pageCount);
appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.NAME_AND_DESCRIPTION);
signer.SetFieldName(signer.GetNewSigFieldName());
// Creating the signature
IExternalSignature pks = new PrivateKeySignature(pk, digestAlgorithm);
signer.SignDetached(pks, chain, crlList, ocspClient, tsaClient, estimatedSize, subfilter);
Console.WriteLine("Signing successful!");
}
catch(Exception ex)
{
throw ex;
}
finally
{
_document.Close();
f.Close();
}
}
}
}
Please see the documents here then. Infile1.pdf I can not sign but Infile2.pdf is signed fine. Outfile1.pdf is the corrupt file. https://app.box.com/s/52jqe8qirl80km6hunxucs00dntx70o5
What causes this? Is there something about the input PDF file or the above programme?
The problem is in your program, more exactly in your PdfWorker.Sign method:
File.Copy(_source, _dest, true);
FileStream f = new FileStream(_dest, FileMode.Append);
try
{
PdfSigner signer = new PdfSigner(new PdfReader(_source), f, true);
...
Here you first copy the file to sign to the destination and then append the PdfSigner output to it.
But that PdfSigner output is the complete signed PDF, i.e. the source plus an additional revision in which the signature is added. Thus, in the destination file you eventually have two copies of the source and then some signing additions generated under the assumptions that only one copy of the source precedes.
To fix this problem, simply remove the File.Copy(_source, _dest, true) operation and don't open the FileStream with FileMode.Append.
Even after this correction, signing of the OP's sample file "Infile1.PDF" still creates a broken PDF while signing the sample file "Infile2.pdf" now succeeds. The cause is the iText 7.0.0 bug explained in this answer which (as far as I can see) is fixed in 7.0.1. As the "Infile1.PDF" uses object streams while "Infile2.pdf" does not, only the former file is affected.