Merge pull request #896 from syncfusion-content/893607_ug

893607: UG documentation for LTV and Timestamp - Flutter PDF.

Author: chinnumuniyappan

Flutter/pdf/working-with-digital-signature.md

@@ -255,3 +255,173 @@ File('Output.pdf').writeAsBytes(await document.save());
 document.dispose();
 
 {% endhighlight %}
+
+## Long Term Validation (LTV) PDF signature
+
+The Syncfusion Flutter PDF supports creating long term signature validation for the signed PDF document. The LTV signature allows you to check the validity of a signature long after the document has been signed. To achieve long term validation, all the required elements for signature validation must be embedded in the signed PDF.
+
+N> The resulting PDF document size will be substantial because all the necessary signature information, Certificate Revocation List (CRL), and Online Certificate Status Protocol (OCSP) are embedded.
+
+The following code example shows how to enable LTV for a signed PDF document using createLongTermValidity method in [PdfSignature](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfSignature-class.html) class.
+
+{% highlight dart %}
+
+    // Load the existing PDF document.
+    PdfDocument document =
+        PdfDocument(inputBytes: File('input.pdf').readAsBytesSync());
+    // Load the existing signature field.
+    PdfSignatureField field = document.form.fields[0] as PdfSignatureField;
+    // Create LTV for loaded signed signature.
+    bool isLTVAdded = await field.signature!.createLongTermValidity();
+    // Save the document.
+    File('output.pdf').writeAsBytesSync(await document.save());
+    // Dispose the document.
+    document.dispose();
+
+{% endhighlight %}
+
+## Create Long Term Validation (LTV) with public certificates data
+
+The following code example shows how to create an LTV for a signed PDF document using public certificates with the createLongTermValidity method in [PdfSignature](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfSignature-class.html) class.
+
+{% highlight dart %}
+
+    // Load the existing PDF document.
+    PdfDocument document =
+        PdfDocument(inputBytes: File('input.pdf').readAsBytesSync());
+    // Load the existing signature field.
+    PdfSignatureField field = document.form.fields[0] as PdfSignatureField;
+    // Load the certificate from the PFX file.
+    PdfCertificate certificate =
+        PdfCertificate(File('PDF.pfx').readAsBytesSync(), 'syncfusion');
+    // Get the public certificates data.
+    List<List<int>>? publicCertificatesData = certificate.getCertificateChain();
+    // Create LTV with your public certificates.
+    await field.signature!.createLongTermValidity(
+        publicCertificatesData: publicCertificatesData,
+        includePublicCertificates: true);
+    // Save the document.
+    File('output.pdf').writeAsBytesSync(await document.save());
+    // Dispose the document.
+    document.dispose();
+
+{% endhighlight %}
+
+## Adding a timestamp in digital signature
+
+The Syncfusion Flutter PDF allows you to add timestamp in the digital signature of the PDF document using timestampServer property in [PdfSignature](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfSignature-class.html) class. The following code example explains the same.
+
+N> Signing using TimestampServer only works when the document is saved using asynchronous [save](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfDocument/save.html). It is not supported in synchronous [saveSync](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfDocument/saveSync.html).
+
+{% highlight dart %}
+
+    // Create a new PDF document.
+    PdfDocument document = PdfDocument();
+    // Add a new page to the document.
+    PdfPage page = document.pages.add();
+    // Create a new timestamp server.
+    TimestampServer server = TimestampServer(
+        Uri.parse('http://syncfusion.digistamp.com'),
+        userName: 'user',
+        password: '123456',
+        timeOut: const Duration(milliseconds: 5000));
+    // Check whether the timestamp server is valid.
+    bool isValid = await server.isValid;
+    if (isValid) {
+        // Add a new signature field to the page.
+        PdfSignatureField field = PdfSignatureField(page, 'signature',
+            bounds: const Rect.fromLTWH(0, 0, 200, 100),
+            signature: PdfSignature(
+            certificate:
+                PdfCertificate(File('PDF.pfx').readAsBytesSync(), 'syncfusion'),
+            contactInfo: 'johndoe@owned.us',
+            locationInfo: 'Honolulu, Hawaii',
+            reason: 'I am author of this document.',
+            ));
+        // Add the timestamp server to the signature.
+        field.signature!.timestampServer = server;
+        // Get the graphics of the signature field.
+        final PdfGraphics? graphics = field.appearance.normal.graphics;
+        // Draw an image to the signature field.
+        graphics!.drawImage(PdfBitmap(File('picture.png').readAsBytesSync()),
+            Rect.fromLTWH(0, 0, field.bounds.width, field.bounds.height));
+        // Add the signature field to the form fields collection.
+        document.form.fields.add(field);
+    }
+    // Save the document.
+    File('output.pdf').writeAsBytesSync(await document.save());
+    // Dispose the document.
+    document.dispose();
+
+{% endhighlight %}
+
+## Adding a timestamp in the PDF document
+
+You can add timestamp to the PDF document using timestampServer property in [PdfSignature](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfSignature-class.html) class. The following code example explains the same.
+
+N> Signing using TimestampServer only works when the document is saved using asynchronous [save](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfDocument/save.html). It is not supported in synchronous [saveSync](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfDocument/saveSync.html).
+
+
+{% highlight dart %}
+
+    // Create a new PDF document.
+    PdfDocument document = PdfDocument();
+    // Add a new page to the document.
+    PdfPage page = document.pages.add();
+    // Create a new timestamp server.
+    TimestampServer server = TimestampServer(
+        Uri.parse('http://syncfusion.digistamp.com'),
+        userName: 'user',
+        password: '123456',
+        timeOut: const Duration(milliseconds: 5000));
+    // Check whether the timestamp server is valid.
+    bool isValid = await server.isValid;
+    if (isValid) {
+        // Add a new signature field to the page.
+        PdfSignatureField field =
+            PdfSignatureField(page, 'signature', signature: PdfSignature());
+        // Add the timestamp server to the signature.
+        field.signature!.timestampServer = server;
+        // Add the signature field to the form fields collection.
+        document.form.fields.add(field);
+    }
+    // Save the document.
+    File('output.pdf').writeAsBytesSync(await document.save());
+    // Dispose the document.
+    document.dispose();
+
+{% endhighlight %}
+
+## Adding a timestamp in an existing PDF document
+
+You can add timestamp to the existing PDF document using timestampServer property in [PdfSignature](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfSignature-class.html) class. The following code example explains the same.
+
+N> Signing using TimestampServer only works when the document is saved using asynchronous [save](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfDocument/save.html). It is not supported in synchronous [saveSync](https://pub.dev/documentation/syncfusion_flutter_pdf/latest/pdf/PdfDocument/saveSync.html).
+
+{% highlight dart %}
+
+    // Create a new PDF document.
+    PdfDocument document =
+        PdfDocument(inputBytes: File('input.pdf').readAsBytesSync());
+    // Load the existing signature field.
+    PdfSignatureField field = document.form.fields[0] as PdfSignatureField;
+    // Create a new timestamp server.
+    TimestampServer server = TimestampServer(
+        Uri.parse('http://syncfusion.digistamp.com'),
+        userName: 'user',
+        password: '123456',
+        timeOut: const Duration(milliseconds: 5000));
+    // Check whether the timestamp server is valid.
+    bool isValid = await server.isValid;
+    if (isValid) {
+        // Add new signature to the existing signature field.
+        field.signature = PdfSignature();
+        // Add the timestamp server to the signature.
+        field.signature!.timestampServer = server;
+    }
+    // Save the document.
+    File('output.pdf').writeAsBytesSync(await document.save());
+    // Dispose the document.
+    document.dispose();
+
+{% endhighlight %}