Create an Opaque (Attached) S/MIME Signature

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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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bool success = false;

Chilkat.Mime mime = new Chilkat.Mime();
success = mime.SetBodyFromPlainText("This message will be signed.");
if (success == false) {
    Debug.WriteLine(mime.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.
string pfxPassword = "myPfxPassword";
Chilkat.Cert cert = new Chilkat.Cert();
success = cert.LoadPfxFile("qa_data/signer.pfx",pfxPassword);
if (success == false) {
    Debug.WriteLine(cert.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.
success = mime.ConvertToSigned(cert);
if (success == false) {
    Debug.WriteLine(mime.LastErrorText);
    return;
}

success = mime.SaveMime("qa_output/signed_opaque.eml");
if (success == false) {
    Debug.WriteLine(mime.LastErrorText);
    return;
}

Debug.WriteLine("Message converted to an opaque signed message.");