Send encrypted and signed email using C# -

i want send encrypted , signed mail without using third-party api. if send alternate view signature, windows mail can validate it. if send alternate view encrypted data, windows mail can decipher it. if send both, windows mail gets 2 attachements. if sign encryptedbytes , add signed bytes alternative view validates signature , message empty. idea?

mailmessage message = new mailmessage(); message.from = new mailaddress(lblmail.text); message.subject = txtsubject.text;  string body = "content-type: text/plain\r\ncontent-transfer-encoding: 7bit\r\n\r\n" + structform();  byte[] messagedata = encoding.ascii.getbytes(body); contentinfo content = new contentinfo(messagedata); envelopedcms envelopedcms = new envelopedcms(content);  message.to.add(new mailaddress(provmail));  cmsrecipient recipient = new cmsrecipient(subjectidentifiertype.subjectkeyidentifier, this.certificate); envelopedcms.encrypt(recipient);  byte[] encryptedbytes = envelopedcms.encode();  signedcms cms = new signedcms(new contentinfo(encryptedbytes)); cmssigner signer = new cmssigner(subjectidentifiertype.issuerandserialnumber, new x509certificate2(@"c:\serv.pfx","123"));  cms.computesignature(signer); byte[] signedbytes = cms.encode();  memorystream encryptedstream = new memorystream(encryptedbytes); alternateview encryptedview = new alternateview(encryptedstream, "application/pkcs7-mime; smime-type=signed--data;name=smime.p7m"); message.alternateviews.add(encryptedview); memorystream signedstream = new memorystream(signedbytes); alternateview signedview = new alternateview(signedstream, "application/pkcs7-mime; smime-type=signed-data;name=sig.p7m"); message.alternateviews.add(signedview);   system.net.networkcredential smtpuserinfo = new system.net.networkcredential("emailaddress@xpto.com", "xxxxxx"); smtpclient client = new smtpclient("smtp.xpto.com");  client.usedefaultcredentials = false; client.credentials = smtpuserinfo;  client.send(message);  label2.text = "assinado e cifrado!"; 

you should sign first, encrypt.

while original cms , s/mime specifications allow operations in either order, later work pointed out signing document can't read bad idea. signature should on plain-text.

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the resulting mime message should have single part, should s/mime enveloped-data. message has 2 parts, , encrypted part mis-labeled signed-data content-type. create , sign signedcms object. encode it, , use encoded value content of envelopedcms object. encrypt that, , use encoded value content of mailmessage, content type of "application/pkcs7-mime; smime-type=enveloped-data".