(C#) Create PKCS7 Attached/Opaque Signature (S/MIME)

Create an S/MIME message with a PKCS7 attached/opaque signature.

Chilkat .NET Downloads Chilkat .NET Framework Chilkat for .NET Core

// 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";
bool 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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// 

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