code-snippet.cs

// first we need to base64 decode the content of the existing pkcs7-mime part
var content = new MemoryStream ();
pkcs7.Content.DecodeTo (content);
content.Position = 0;

// now we create a new pkcs7-mime part with the proper smime-type parameter
pkcs7 = new ApplicationPkcs7Mime (SecureMimeType.EnvelopedData, content);

// now decrypt it:
var decrypted = pkcs7.Decrypt (ctx);