對現有PDF文檔簽名有時會導致損壞的文件

Question

我正在編寫概念驗證示例，以便將電子簽名添加到現有PDF。我遇到了一個我不明白的問題。看起來，將簽名添加到某些文檔可以正常工作，而向其他人添加簽名不起作用，並且會生成Adobe Reader無法打開的損壞文件。

這裏是我的代碼：

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(); 
      } 
     } 
    } 
} 

請參閱文件在這裏呢。 Infile1.pdf我無法簽名，但Infile2.pdf簽署的很好。 Outfile1.pdf是損壞的文件。 https://app.box.com/s/52jqe8qirl80km6hunxucs00dntx70o5

這是什麼原因造成的？是否有關於輸入PDF文件或上述程序的內容？

Answer 1

的問題是在你的程序，更準確地在您的PdfWorker.Sign方法：

File.Copy(_source, _dest, true); 
FileStream f = new FileStream(_dest, FileMode.Append); 
try 
{ 
    PdfSigner signer = new PdfSigner(new PdfReader(_source), f, true); 
    ... 

在這裏，你先複製登錄到目的地，然後PdfSigner輸出追加到它的文件。

但是，PdfSigner輸出是完整簽名的PDF，即源加上添加簽名的額外修訂。因此，在目標文件中，您最終將有兩個源副本，然後在假設只有一個源副本的前提下生成一些簽名添加。

要解決此問題，只需刪除File.Copy(_source, _dest, true)操作並且不要打開FileStream與FileMode.Append。

---

即使這樣修正之後，OP的樣本文件「Infile1.PDF」的簽署仍創建了一個破碎的PDF，同時簽署樣本文件「Infile2.pdf」現在成功了。原因是在this answer中解釋的iText 7.0.0錯誤（據我所見）在7.0.1中修復。由於「Infile1.PDF」使用對象流，而「Infile2.pdf」不使用，因此只有前一個文件受到影響。