Ein Leitfaden für Entwickler zum digitalen Signieren von PDFs mit C#

Wie wende ich eine digitale Signatur mit einem Zertifikat auf ein PDF an?

Sie können eine digitale Signatur auf ein PDF-Dokument anwenden, indem Sie eine digitale Zertifikatsdatei (wie .pfx oder .p12) verwenden, um die Authentizität und Integrität des Dokuments zu garantieren. Dieser Prozess gewährleistet, dass das Dokument seit der Unterzeichnung nicht verändert wurde. Einen vollständigen Überblick über die Möglichkeiten des digitalen Signierens finden Sie in unserem umfassenden Leitfaden für digitale Signaturen.

IronPDF bietet eine einfache API für diesen Zweck und unterstützt mehrere Möglichkeiten, eine digitale Signatur anzuwenden. Das Herzstück dieser Funktionalität ist die PdfSignature Klasse, die das Zertifikat und alle zugehörigen Metadaten der Signatur kapselt.

Install-Package IronPdf

using IronPdf;
using IronPdf.Signing;
using System.Security.Cryptography.X509Certificates;

// Create a new PDF from an HTML string for demonstration.
var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Signed Document</h1><p>This document has been digitally signed.</p>");

// Load the certificate from a .pfx file with its password.
// The X509KeyStorageFlags.Exportable flag is crucial for allowing the private key to be used in the signing process.
var cert = new X509Certificate2("IronSoftware.pfx", "123456", X509KeyStorageFlags.Exportable);

// Create a PdfSignature object using the loaded certificate.
var signature = new PdfSignature(cert);

// Apply the signature to the PDF document.
pdf.Sign(signature);

// Save the securely signed PDF document.
pdf.SaveAs("Signed.pdf");

using IronPdf;
using IronPdf.Signing;
using System.Security.Cryptography.X509Certificates;

// Create a new PDF from an HTML string for demonstration.
var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Signed Document</h1><p>This document has been digitally signed.</p>");

// Load the certificate from a .pfx file with its password.
// The X509KeyStorageFlags.Exportable flag is crucial for allowing the private key to be used in the signing process.
var cert = new X509Certificate2("IronSoftware.pfx", "123456", X509KeyStorageFlags.Exportable);

// Create a PdfSignature object using the loaded certificate.
var signature = new PdfSignature(cert);

// Apply the signature to the PDF document.
pdf.Sign(signature);

// Save the securely signed PDF document.
pdf.SaveAs("Signed.pdf");

using IronPdf;
using IronPdf.Signing;
using IronSoftware.Drawing;
using System;

// Load an existing PDF document to be signed.
var pdf = PdfDocument.FromFile("invoice.pdf");

// Create a PdfSignature object directly from the certificate file and password.
var signature = new PdfSignature("IronSoftware.pfx", "123456");

// Add detailed metadata to the signature for a comprehensive audit trail.
// These properties enhance the signature's credibility and provide context
signature.SignatureDate = DateTime.Now;
signature.SigningContact = "legal@ironsoftware.com";
signature.SigningLocation = "Chicago, USA";
signature.SigningReason = "Contractual Agreement";

// Add a secure timestamp from a trusted Time Stamp Authority (TSA).
// This provides cryptographic proof of the signing time.
signature.TimeStampUrl = new Uri("[http://timestamp.digicert.com](http://timestamp.digicert.com)");
signature.TimestampHashAlgorithm = TimestampHashAlgorithms.SHA256;

// Apply a visual appearance to the signature. (More on this in the next section)
signature.SignatureImage = new PdfSignatureImage("assets/visual-signature.png", 0, new Rectangle(350, 750, 200, 100));

// Sign the PDF document with the configured signature object.
pdf.Sign(signature);

// Save the final, signed PDF document.
pdf.SaveAs("DetailedSignature.pdf");

using IronPdf;
using IronPdf.Signing;
using IronSoftware.Drawing;
using System;

// Load an existing PDF document to be signed.
var pdf = PdfDocument.FromFile("invoice.pdf");

// Create a PdfSignature object directly from the certificate file and password.
var signature = new PdfSignature("IronSoftware.pfx", "123456");

// Add detailed metadata to the signature for a comprehensive audit trail.
// These properties enhance the signature's credibility and provide context
signature.SignatureDate = DateTime.Now;
signature.SigningContact = "legal@ironsoftware.com";
signature.SigningLocation = "Chicago, USA";
signature.SigningReason = "Contractual Agreement";

// Add a secure timestamp from a trusted Time Stamp Authority (TSA).
// This provides cryptographic proof of the signing time.
signature.TimeStampUrl = new Uri("[http://timestamp.digicert.com](http://timestamp.digicert.com)");
signature.TimestampHashAlgorithm = TimestampHashAlgorithms.SHA256;

// Apply a visual appearance to the signature. (More on this in the next section)
signature.SignatureImage = new PdfSignatureImage("assets/visual-signature.png", 0, new Rectangle(350, 750, 200, 100));

// Sign the PDF document with the configured signature object.
pdf.Sign(signature);

// Save the final, signed PDF document.
pdf.SaveAs("DetailedSignature.pdf");

Wie füge ich eine visuelle Darstellung zu einer digitalen Signatur hinzu?

Während eine digitale Signatur kryptografisch in das PDF eingebettet ist, ist es oft nützlich, eine visuelle Darstellung auf der Seite zu haben. Dies kann ein Firmenlogo, ein handschriftliches Signaturbild oder andere Grafiken sein. IronPDF macht es einfach, ein Bild zu einem PdfSignature-Objekt hinzuzufügen.

Sie können ein Bild von einer Datei oder einem Stream laden und es präzise auf jeder Seite des PDFs positionieren. Zu den unterstützten Bildformaten gehören PNG, JPEG, GIF, BMP, TIFF und WebP. Diese Technik ist vergleichbar mit dem Stempeln von Text und Bildern in PDF-Dokumenten.

using IronPdf.Signing;
using IronSoftware.Drawing;

// This example demonstrates various ways to add a visual image to a PDF signature.

// Create a PdfSignature object.
var signature = new PdfSignature("IronSoftware.pfx", "123456");

// Define the position and size for the signature image on the first page (index 0).
// Rectangle parameters: x position, y position, width, height
var signatureRectangle = new Rectangle(350, 750, 200, 100);

// Option 1: Set the SignatureImage property directly.
// This is the most straightforward approach
signature.SignatureImage = new PdfSignatureImage("assets/visual-signature.png", 0, signatureRectangle);

// Option 2: Use the LoadSignatureImageFromFile method.
// This method provides the same functionality with a different syntax
signature.LoadSignatureImageFromFile("assets/visual-signature.png", 0, signatureRectangle);

// Option 3: Load an image from a stream. This is useful for images generated in memory.
// Perfect for scenarios where images are retrieved from databases or web services
AnyBitmap image = AnyBitmap.FromFile("assets/visual-signature.png");
using (var imageStream = image.ToStream())
{
    signature.LoadSignatureImageFromStream(imageStream, 0, signatureRectangle);
}

// After configuring the signature image, apply it to a PDF.
var pdf = PdfDocument.FromFile("invoice.pdf");
pdf.Sign(signature);
pdf.SaveAs("VisualSignature.pdf");

using IronPdf.Signing;
using IronSoftware.Drawing;

// This example demonstrates various ways to add a visual image to a PDF signature.

// Create a PdfSignature object.
var signature = new PdfSignature("IronSoftware.pfx", "123456");

// Define the position and size for the signature image on the first page (index 0).
// Rectangle parameters: x position, y position, width, height
var signatureRectangle = new Rectangle(350, 750, 200, 100);

// Option 1: Set the SignatureImage property directly.
// This is the most straightforward approach
signature.SignatureImage = new PdfSignatureImage("assets/visual-signature.png", 0, signatureRectangle);

// Option 2: Use the LoadSignatureImageFromFile method.
// This method provides the same functionality with a different syntax
signature.LoadSignatureImageFromFile("assets/visual-signature.png", 0, signatureRectangle);

// Option 3: Load an image from a stream. This is useful for images generated in memory.
// Perfect for scenarios where images are retrieved from databases or web services
AnyBitmap image = AnyBitmap.FromFile("assets/visual-signature.png");
using (var imageStream = image.ToStream())
{
    signature.LoadSignatureImageFromStream(imageStream, 0, signatureRectangle);
}

// After configuring the signature image, apply it to a PDF.
var pdf = PdfDocument.FromFile("invoice.pdf");
pdf.Sign(signature);
pdf.SaveAs("VisualSignature.pdf");

Dieser Code zeigt drei gleichwertige Möglichkeiten, eine visuelle Komponente zu einer digitalen Signatur hinzuzufügen. Egal, ob Sie ein Bild auf der Festplatte oder im Speicher haben, Sie können es einfach auf das PDF als Teil des Signaturprozesses stempeln. Dies schließt die Lücke zwischen unsichtbarer kryptografischer Sicherheit und sichtbarer Dokumentgenehmigung.

Wie kann ich die Dokumentberechtigungen nach der Signatur steuern?

Wenn Sie ein Dokument signieren, möchten Sie möglicherweise angeben, welche Änderungen, wenn überhaupt, danach erlaubt sind. Zum Beispiel möchten Sie das Dokument möglicherweise vollständig sperren oder nur Benutzern erlauben, Formulare auszufüllen. IronPDF lässt Sie diese Berechtigungen mithilfe der SignaturePermissions-Enumeration festlegen. Weitere Informationen zur Steuerung von Berechtigungen finden Sie in unserem Leitfaden zum Einstellen von PDF-Passwörtern und Berechtigungen.

Das Festlegen von Signaturberechtigungen ist ein wichtiger Teil der Verwaltung des Lebenszyklus eines Dokuments. Es stellt sicher, dass die Integrität des Dokuments nach Ihrer Signatur gewahrt bleibt. Wenn ein Benutzer eine nicht erlaubte Aktion durchführt, wird die Signatur ungültig.

using IronPdf.Signing;

// Load a PDF file with change tracking enabled.
// This enables incremental save functionality for revision management
var pdf = PdfDocument.FromFile("annual_census.pdf", ChangeTrackingModes.EnableChangeTracking);

// Placeholder for edits: You might add text, fill forms, or add annotations here.
// For example: pdf.Annotations.Add(new TextAnnotation(...));
// Or: pdf.Form["fieldName"].Value = "New Value";

// Sign the current state of the document using SignWithFile for convenience.
// We set permissions to allow further signatures and form filling.
pdf.SignWithFile(
    "assets/IronSignature.p12", 
    "password", 
    SignaturePermissions.AdditionalSignaturesAndFormFillingAllowed);

// Save the current state as a distinct revision within the PDF's history.
// This creates a snapshot that can be referenced later
PdfDocument pdfWithRevision = pdf.SaveAsRevision();

// Save the final PDF with its full revision history to a new file.
pdfWithRevision.SaveAs("annual_census_signed.pdf");

using IronPdf.Signing;

// Load a PDF file with change tracking enabled.
// This enables incremental save functionality for revision management
var pdf = PdfDocument.FromFile("annual_census.pdf", ChangeTrackingModes.EnableChangeTracking);

// Placeholder for edits: You might add text, fill forms, or add annotations here.
// For example: pdf.Annotations.Add(new TextAnnotation(...));
// Or: pdf.Form["fieldName"].Value = "New Value";

// Sign the current state of the document using SignWithFile for convenience.
// We set permissions to allow further signatures and form filling.
pdf.SignWithFile(
    "assets/IronSignature.p12", 
    "password", 
    SignaturePermissions.AdditionalSignaturesAndFormFillingAllowed);

// Save the current state as a distinct revision within the PDF's history.
// This creates a snapshot that can be referenced later
PdfDocument pdfWithRevision = pdf.SaveAsRevision();

// Save the final PDF with its full revision history to a new file.
pdfWithRevision.SaveAs("annual_census_signed.pdf");

// Load a PDF with a complex signature history.
var pdf = PdfDocument.FromFile("multi_signed_report.pdf");

// Verify all signatures across all revisions.
// This ensures document integrity throughout its entire history
bool allSignaturesValid = pdf.VerifySignatures();
Console.WriteLine($"All signatures are valid: {allSignaturesValid}");

// Roll back to the first revision (index 0).
// Useful for reviewing the original document state
if (pdf.RevisionCount > 1)
{
    PdfDocument firstRevision = pdf.GetRevision(0);
    firstRevision.SaveAs("report_first_revision.pdf");
}

// Create a completely unsigned version of the document.
// This removes all digital signatures while preserving content
pdf.RemoveSignatures();
pdf.SaveAs("report_unsigned.pdf");

// Load a PDF with a complex signature history.
var pdf = PdfDocument.FromFile("multi_signed_report.pdf");

// Verify all signatures across all revisions.
// This ensures document integrity throughout its entire history
bool allSignaturesValid = pdf.VerifySignatures();
Console.WriteLine($"All signatures are valid: {allSignaturesValid}");

// Roll back to the first revision (index 0).
// Useful for reviewing the original document state
if (pdf.RevisionCount > 1)
{
    PdfDocument firstRevision = pdf.GetRevision(0);
    firstRevision.SaveAs("report_first_revision.pdf");
}

// Create a completely unsigned version of the document.
// This removes all digital signatures while preserving content
pdf.RemoveSignatures();
pdf.SaveAs("report_unsigned.pdf");

How Do I Stamp a Handwritten Signature onto a PDF?

Sometimes, you don't need the cryptographic security of a digital signature but simply want to apply a visual, electronic signature, like a scanned handwritten signature. This is often referred to as stamping. IronPDF can do this using its Watermark or Stamp features. For more advanced watermarking options, explore our custom watermarks guide.

Let's start with a sample invoice PDF and a .png image of a handwritten signature.

The original invoice PDF before stamping a signature.

Here is the signature image we will apply:

A sample handwritten signature image.

The following code uses the Watermark property to stamp this image onto the bottom-right corner of the PDF.

using IronPdf.Editing;

// Load the existing PDF document.
var pdf = PdfDocument.FromFile("invoice.pdf");

// Create an HtmlStamp containing our signature image.
// HtmlStamp allows us to position HTML content precisely on the page
var signatureStamp = new HtmlStamp("<img src='assets/signature.png'/>")
{
    // Configure the stamp's position and appearance.
    VerticalAlignment = VerticalAlignment.Bottom,
    HorizontalAlignment = HorizontalAlignment.Right,
    Margin = 10,  // Add some space from the edge.
    Opacity = 90  // Make it slightly transparent for a more authentic look.
};

// Apply the stamp to all pages of the PDF.
// You can also specify specific page numbers if needed
pdf.ApplyStamp(signatureStamp);

// Save the modified PDF document.
pdf.SaveAs("official_invoice.pdf");

using IronPdf.Editing;

// Load the existing PDF document.
var pdf = PdfDocument.FromFile("invoice.pdf");

// Create an HtmlStamp containing our signature image.
// HtmlStamp allows us to position HTML content precisely on the page
var signatureStamp = new HtmlStamp("<img src='assets/signature.png'/>")
{
    // Configure the stamp's position and appearance.
    VerticalAlignment = VerticalAlignment.Bottom,
    HorizontalAlignment = HorizontalAlignment.Right,
    Margin = 10,  // Add some space from the edge.
    Opacity = 90  // Make it slightly transparent for a more authentic look.
};

// Apply the stamp to all pages of the PDF.
// You can also specify specific page numbers if needed
pdf.ApplyStamp(signatureStamp);

// Save the modified PDF document.
pdf.SaveAs("official_invoice.pdf");

How Can I Add an Interactive Signature Field to a PDF?

For documents that need to be signed by an end-user in a PDF viewer like Adobe Acrobat, you can add an interactive signature form field. This creates an empty, clickable area that prompts the user to apply their own digital signature. For a complete guide on PDF forms, see our creating PDF forms tutorial.

You can create a SignatureFormField and add it to the PDF's form collection. You have precise control over its location and size on the page. This is particularly useful for documents that require multiple signatures or when you need to collect signatures from external parties.

using IronPdf.Forms;
using IronSoftware.Drawing;

// Create a new PDF to add the signature field to.
var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Bitte unten unterschreiben</h1>");

// Define the properties for the signature form field.
string fieldName = "ClientSignature";  // Unique identifier for the field
int pageIndex = 0;  // Add to the first page (zero-indexed)
var fieldRect = new Rectangle(50, 200, 300, 100);  // Position: (x, y), Size: (width, height)

// Create the SignatureFormField object.
// This creates an interactive field that users can click to sign
var signatureField = new SignatureFormField(fieldName, pageIndex, fieldRect);

// Add the signature field to the PDF's form.
pdf.Form.Add(signatureField);

// Save the PDF with the new interactive signature field.
pdf.SaveAs("interactive_signature.pdf");

using IronPdf.Forms;
using IronSoftware.Drawing;

// Create a new PDF to add the signature field to.
var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Bitte unten unterschreiben</h1>");

// Define the properties for the signature form field.
string fieldName = "ClientSignature";  // Unique identifier for the field
int pageIndex = 0;  // Add to the first page (zero-indexed)
var fieldRect = new Rectangle(50, 200, 300, 100);  // Position: (x, y), Size: (width, height)

// Create the SignatureFormField object.
// This creates an interactive field that users can click to sign
var signatureField = new SignatureFormField(fieldName, pageIndex, fieldRect);

// Add the signature field to the PDF's form.
pdf.Form.Add(signatureField);

// Save the PDF with the new interactive signature field.
pdf.SaveAs("interactive_signature.pdf");

When a user opens this PDF, they will see a clickable field, allowing them to complete the signing process using their own digital ID. You can learn more about creating and managing interactive forms in our How-To guide on creating PDF forms.

An unsigned, interactive signature field added programmatically to a PDF document.

How Do I Retrieve the Signer Name from Verified Signatures?

To obtain the common name of the certificate owner who signed a signature, we can use the VerifiedSignature class to access the SignerName property. Below is a code snippet demonstrating how to accomplish this.

:path=/static-assets/pdf/content-code-examples/how-to/signing-find-signer-name.cs

using IronPdf;
using System;

// Import the Signed PDF report
var pdf = PdfDocument.FromFile("multi_signed_report.pdf");

// Using GetVerifiedSignatures() obtain a list of `VerifiedSignature` objects from the PDF
pdf.GetVerifiedSignatures().ForEach(signature =>
{
    // Print out the SignerName of each `VerifiedSignature` object
    Console.WriteLine($"SignatureName: {signature.SignerName}");
});

Nach dem Importieren der signierten PDF-Datei verwenden wir die Methode GetVerifiedSignatures, um eine Liste von VerifiedSignature-Objekten innerhalb des Berichts abzurufen und den SignerName für jede Signatur zu drucken.

Bitte beachten Sie, dass dieser Wert aus dem Subject Distinguished Name (SubjectDN) des Zertifikats extrahiert wird und null zurückgibt, wenn das CN-Feld nicht vorhanden ist.

Was sind die nächsten Schritte für das PDF-Signieren mit IronPDF?

Diese Anleitung hat die leistungsstarken und flexiblen PDF-Signierfunktionen von IronPDF gezeigt. Ganz gleich, ob Sie sichere digitale Signaturen mit detaillierten Metadaten anwenden, Dokumentrevisionen verwalten, visuelle Signaturen stempeln oder interaktive Formulare erstellen möchten, IronPDF bietet eine umfassende und entwicklerfreundliche API, um diese Aufgabe zu bewältigen.

Um weitere Informationen zu erhalten, können Sie die IronPDF-Bibliothek für .NET herunterladen und eine kostenlose Testlizenz erhalten, um alle Funktionen in Ihren Projekten zu testen. Weitere fortgeschrittene Techniken zur Bearbeitung von Dokumenten, einschließlich Hinzufügen von Anmerkungen und Arbeiten mit Formularfeldern, finden Sie in unserer umfangreichen Dokumentation und in den Tutorials.

Möchten Sie sehen, was Sie sonst noch tun können? Sehen Sie sich hier unsere Tutorial-Seite an: PDFs signieren und sichern