C# 如何对数字签名的pdf进行密码保护？

我正在使用c#，itextsharp创建和签署pdf。现在我正在使用此代码进行密码保护

有人能告诉我为什么会这样吗

谢谢

string passprotectedfile = filename;

using (Stream input = new FileStream(signedfile, FileMode.Open, FileAccess.Read,
                                     FileShare.Read))
{
    using (Stream output = new FileStream(passprotectedfile, FileMode.Create, 
                                          FileAccess.Write, FileShare.None))
    {
        PdfReader reader = new PdfReader(input);
        PdfEncryptor.Encrypt(reader, output, true, regno.ToString(), "",
                             PdfWriter.ALLOW_SCREENREADERS);
    }
}

---

数字签名的全部目的是确保没有人篡改文件的内容。通过添加密码，您正在修改密码，使应用的数字签名不再有效，这就是错误消息告诉您的。修改PDF文件后，您需要退出该文件。

对不起。我不知道发生了什么事。昨天，在密码保护生效后，我对PDF进行了签名。但现在，生成的PDF在打开时不要求任何密码。那是在对之前应用的密码保护进行了签名之后。我确实发布了我的代码这里..欢迎来到SO！请在发布代码片段时始终添加一些解释。此外，当问题集中在iTextSharp上时，代码似乎没有使用iTextSharp。并且op开始有一个他想要密码保护的签名文件（如果不破坏签名，这是不可能的）。另一方面，答案是两个步骤相同。

private void Generatedigitalsignatureandpassprotected(string filepath, string filepass)
    {
        Document _document = new Document();

        _document.Title = "PdfDigitalSignature - Sample";
        _document.Author = "dbAutoTrack Ltd, USA";
        _document.Creator = "dbAutoTrack.PDFWriter";
        PDFDigitalSignature digitalSignature = new PDFDigitalSignature();
        //  digitalSignature.X509Certificate = new X509Certificate2(MapPath("../data/DigitalSignature.pfx"), "password", X509KeyStorageFlags.MachineKeySet |X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable););
        string executingFolder = System.IO.Path.GetDirectoryName(Application.ExecutablePath);
        var digitalsignaturefilepath = "E:\\wwwroot\\NaveenWindowsAppFile\\WindowsFormsApplication1\\digitalsignaturefile\\DigitalSignature.pfx";
        digitalSignature.X509Certificate = new X509Certificate2(digitalsignaturefilepath, "password", X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);
        digitalSignature.Date = DateTime.Now;
        digitalSignature.Location = "Location ";
        digitalSignature.Reason = "Reason ";
        digitalSignature.ContactInfo = "Convert Digital Code by Cstech";
        digitalSignature.DetachSignature = false;
        digitalSignature.RootTrusted = false;
        _document.DigitalSignature = digitalSignature;
        //password protection code//
        SecurityManager _securityManager = new SecurityManager();
        _securityManager.Encryption = Encryption.Use128BitKey;
        _securityManager.OwnerPassword = filepass;
        _securityManager.UserPassword = filepass;
        _securityManager.AllowCopy = true;
        _securityManager.AllowEdit = true;
        //end//
        //Assign SecurityManager to the Document.
        _document.SecurityManager = _securityManager;
        PDFFont _font1 = new PDFFont(StandardFonts.TimesRoman, FontStyle.Regular);
        dbAutoTrack.PDFWriter.Page page1 = new dbAutoTrack.PDFWriter.Page(PageSize.A4);
        PDFGraphics graphics1 = page1.Graphics;

        PdfSignatureField signatureField = graphics1.AddDigitalSignature("name", new RectangleF(50, 50, 150, 75), _font1, 8f);
        signatureField.DigitalSignature = digitalSignature;
        _document.Pages.Add(page1);

        using (FileStream outFile = new FileStream(filepath, FileMode.Create, FileAccess.ReadWrite))
        {
            _document.Generate(outFile);
        }
    }