Flutter Guides Create digital signatures

Create Digital Signatures

Starting with 3.2.0, ComPDF Flutter SDK supports certificate-based signing of PDF signature fields. A digital signature provides document integrity and signer information; an image signature only adds a signature image.

Prepare the certificate path, password, and a writable output path, then open the target PDF. In these examples, document is the current CPDFDocument.

import 'package:compdfkit_flutter/compdfkit.dart';
import 'package:compdfkit_flutter/annotation/form/cpdf_signature_widget.dart';

Get an Unsigned Signature Field

final widgets = await document.pageAtIndex(0).getWidgets();
final unsignedFields = widgets
    .whereType<CPDFSignatureWidget>()
    .where((field) => !field.isSigned)
    .toList();

isSigned indicates whether the field already contains a digital signature. If the document has no signature field, create a form field first and read it again.

The field passed to the signing API must belong to the current document and have a valid native uuid. Use a field returned by getWidgets() or a reader callback. A newly constructed Dart object alone cannot be used directly for signing.

Sign and Write a Copy

if (unsignedFields.isNotEmpty) {
  final success = await document.writeSignature(
    unsignedFields.first,
    '/path/to/signed.pdf',
    CPDFDigitalSignatureConfig(
      certificatePath: certificatePath,
      password: certificatePassword,
      reason: 'Approved',
      location: 'Office',
      permissions: CPDFSignaturePermission.fillAndSign,
      appearance: const CPDFDigitalSignatureAppearance(
        text: 'Signed by Document Signer',
        content: 'Approved',
        drawLogo: false,
      ),
    ),
  );
  if (success) {
    await document.open('/path/to/signed.pdf');
  }
}

The signature is written to a new file. Use an output path different from the source, with an existing parent directory. After success, open the output file to view or verify the signature.

Configure Signing

Setting	Purpose
certificatePath, password	PKCS#12 certificate and password
name, reason, location	Signer name, signing reason, and location
permissions	Document changes allowed by the signature
appearance	Signature text, colors, images, and layout
timestampServerUrl	Signature timestamp service; see Timestamps and Long-Term Validation

CPDFSignaturePermission defines the changes allowed after signing:

Value	Description
forbidChange	Disallow changes to the document
fillAndSign	Allow form filling and signing
annotationFillAndSign	Allow annotation, form filling, and signing

permissions defaults to fillAndSign. It describes modification permissions associated with the signature, separately from document opening passwords and encryption permissions.

Configure the appearance with text, content, textColor, contentColor, imagePath, logoPath, drawLogo, drawOnlyContent, and contentAlignLeft. Image settings use local file paths.

Invalid arguments may throw ArgumentError. Native signing failures may return false or throw a platform exception. Open the output only after confirming success. See Read and Verify Digital Signatures.