Dart

PDF Sign Under Existing Signature

See more PDF Signatures Examples

This example explains how sign a previously-signed PDF and place the signature appearance below the existing signature.

Note: This example requires Chilkat v9.5.0.85 or greater.

Dart

import 'package:chilkat/chilkat.dart';

void main() {
  // This example requires the Chilkat API to have been previously unlocked.
  // See Global Unlock Sample for sample code.

  final pdf = CkPdf();

  // Load a PDF to be signed.
  try {
    pdf.loadFile('qa_data/pdf/sign_testing_4/hello_signed.pdf');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // The PDF we'll be signing already has a signature added in Adobe Acrobat DC.
  // It looks like this:
  // (Notice the Adobe background logo, which is commonly what you'll see when a signature
  // is created by Adobe Acrobat.  Chilkat (obviously) does not create signatures using the Adobe
  // logo because it would be a copyright violation.)

  // (image:https://example-code.com/images/signed_once_with_adobe_acrobat.jpg/endImage)

  // Options for signing are specified in JSON.
  final json = CkJsonObject();

  // In most cases, the signingCertificateV2 and signingTime attributes are required.
  json.updateInt('signingCertificateV2', 1);
  json.updateInt('signingTime', 1);

  // To put our new signature underneath the existing signature, we have to specify the page
  // where the existing signature is found, and then we can specify "under" for the "appearance.y"
  json.updateInt('page', 1);
  json.updateString('appearance.y', 'under');

  json.updateString('appearance.fontScale', '10.0');

  json.updateString('appearance.text[0]', 'Reason: Agreement Accepted by John');
  json.updateString('appearance.text[1]', 'Date: current_dt');

  // Load the signing certificate. (Use your own certificate.)
  final cert = CkCert();
  try {
    cert.loadPfxFile('qa_data/pfx/myPdfSigningCert.pfx', 'secret');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Tell the pdf object to use the certificate for signing.
  try {
    pdf.setSigningCert(cert);
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Note: When adding an additional signature to a PDF, the existing signatures
  // are validated, and this includes validating the certificates previously used to 
  // create the existing signatures.  (A signature typically embeds the signing certs.)
  // In some cases, the certificates and/or certs in the chain of authentication for 
  // existing signatures are not present, and are not available on the current system.
  // In this case, you can skip the validation by setting this keyword in UncommonOptions:
  pdf.uncommonOptions = 'NO_VERIFY_CERT_SIGNATURES';

  try {
    pdf.signPdf(json, 'qa_output/hello_signed.pdf');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  print('The PDF has been successfully cryptographically signed.');

  // Here's a screenshot of the 2nd signature positioned under the 1st in Adobe Acrobat:

  // (image:https://example-code.com/images/signed_under_existing_signature.jpg/endImage)
}