Create PKCS7 Attached/Opaque Signature (S/MIME)

Create an S/MIME message with a PKCS7 attached/opaque signature.
Chilkat C# Downloads

bool success = false;

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

Chilkat.Mime mime = new Chilkat.Mime();

//  Load a certificate w/ private key for signing.
//  There are many ways of getting a certificate object:
//  from a pfx file, from .cer and private-key files, from the
//  Windows registry-based certificate stores, etc.
//  Load a PFX file into a certificate object.
Chilkat.Cert cert = new Chilkat.Cert();
string pfxFilepath = "pfxFiles/something.pfx";
string pfxPassword = "secret";
success = cert.LoadPfxFile(pfxFilepath,pfxPassword);
if (success == false) {
    Debug.WriteLine(cert.LastErrorText);
    return;
}

//  Create a very simple MIME message to be signed:
success = mime.AddHeaderField("Subject","This is a test");
success = mime.AddHeaderField("MyCustomHeader","abc123");
mime.ContentType = "text/plain";
mime.SetBody("This is a plain-text body.");

//  Examine the MIME prior to signing:
Debug.WriteLine("--- MIME prior to signing ---");
Debug.WriteLine(mime.GetMime());

//  The MIME prior to signing looks like this:
//  -------------------------------------------------------------------------------

//  Subject: This is a test
//  MyCustomHeader: abc123
//  Content-Type: text/plain
//  
//  This is a plain-text body.

//  -------------------------------------------------------------------------------

//  The default hash algorithm used in the signing process is "SHA1".
//  To change it, set the SigningHashAlg property:
mime.SigningHashAlg = "SHA256";

//  Convert the MIME to PKCS7 signed-data.
success = mime.ConvertToSigned(cert);
if (success == false) {
    Debug.WriteLine(mime.LastErrorText);
    return;
}

//  Examine the signed MIME
Debug.WriteLine("--- MIME converted to PKCS7 signed-data ---");
Debug.WriteLine(mime.GetMime());

//  The MIME after signing looks like this:
//  -------------------------------------------------------------------------------

//  Subject: This is a test
//  MyCustomHeader: abc123
//  Content-Disposition: attachment; filename="smime.p7m";
//  Content-Transfer-Encoding: base64
//  Content-Type: application/x-pkcs7-mime;
//   name="smime.p7m"; smime-type="signed-data";
//  
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//  

//  -------------------------------------------------------------------------------