Dart

Create an Opaque (Attached) S/MIME Signature

Demonstrates the Chilkat Mime.ConvertToSigned method, which creates an opaque (attached) S/MIME signature. The only argument is the signing Cert. On success the entity is replaced by a CMS/PKCS #7 signed-data structure that wraps the content inside the signature.

Note: The file paths are relative to the application's current working directory. Absolute paths may also be used. Supply the paths appropriate to your own environment.

Background: An opaque signature bundles the content inside the CMS structure, so the message is a single application/pkcs7-mime blob that only S/MIME-aware software can read. It is more tamper-resistant in transit — nothing can alter the content without breaking the wrapper — at the cost of the backward compatibility a detached signature offers. Use it when every recipient is known to support S/MIME.

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Dart

import 'package:chilkat/chilkat.dart';

void main() {
  final mime = CkMime();
  try {
    mime.setBodyFromPlainText('This message will be signed.');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Load the signing certificate (which must have access to its private key) from a PFX file.
  // The PFX password should come from a secure source rather than being hard-coded.
  final pfxPassword = 'myPfxPassword';
  final cert = CkCert();
  try {
    cert.loadPfxFile('qa_data/signer.pfx', pfxPassword);
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Create an OPAQUE (attached) S/MIME signature.  The entity is replaced by a CMS/PKCS #7
  // signed-data structure that wraps the content inside the signature.
  try {
    mime.convertToSigned(cert);
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  try {
    mime.saveMime('qa_output/signed_opaque.eml');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  print('Message converted to an opaque signed message.');
}