C# 如何验证电子邮件中的数字签名？

如果签名有效，我打算从EmailMessage对象（Microsoft.Exchange…）中提取附件。 这是我的测试邮件：

MIME-Version: 1.0
Content-Type: multipart/signed;
protocol="application/x-pkcs7-signature";
micalg=SHA1;
boundary="----=_NextPart_000_0039_01D1C7DA.CFEBF380"

This is a multipart message in MIME format.

------=_NextPart_000_0039_01D1C7DA.CFEBF380
Content-Type: multipart/mixed;
boundary="----=_NextPart_001_003A_01D1C7DA.CFEBF380"


------=_NextPart_001_003A_01D1C7DA.CFEBF380
Content-Type: multipart/alternative;
boundary="----=_NextPart_002_003B_01D1C7DA.CFEBF380"


------=_NextPart_002_003B_01D1C7DA.CFEBF380
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

{body text ...}

------=_NextPart_002_003B_01D1C7DA.CFEBF380
Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

{html formating ...}

------=_NextPart_002_003B_01D1C7DA.CFEBF380--

------=_NextPart_001_003A_01D1C7DA.CFEBF380
Content-Type: application/octet-stream;
name="s15sq07 - clearing & margining.sql"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="s15sq07 - clearing & margining.sql"

{file content in base64 ...}

------=_NextPart_001_003A_01D1C7DA.CFEBF380--

------=_NextPart_000_0039_01D1C7DA.CFEBF380
Content-Type: application/pkcs7-signature;
name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="smime.p7s"

{signature ...}

------=_NextPart_000_0039_01D1C7DA.CFEBF380--

签名验证的原理如下：计算消息（m）的散列（H（m））并使用发送方公钥（pubk）和验证函数（V）将结果与签名（sig）进行比较

表示V_pubk（H（m））=sig

我的代码：

private static bool iDoHandleMail( EmailMessage msg )
  {
    try
    {
      msg.Load( new PropertySet( EmailMessageSchema.Attachments ) );

      foreach ( Attachment att in msg.Attachments )
      {
        FileAttachment fatt = att as FileAttachment;
        if ( fatt == null )
          continue;

        using ( MemoryStream ms = new MemoryStream( ) )
        {
          fatt.Load( ms );
          ms.Position = 0;
          MimeKit.MimeMessage mmsg = MimeKit.MimeMessage.Load( ms );

我试过了，（页面底部）：

然后我尝试了这个，但没有成功：

          MimeEntity sig = mmsg.BodyParts.Where(x => x.IsAttachment && x.ContentType.Name != null && x.ContentType.Name.EndsWith(".p7s")).Select(x => x).ToArray()[0];
          MIME.MimeMessage mSig = new MIME.MimeMessage();
          mSig.LoadBody(sig.ToString());
          string cont = mSig.mContent;
          byte[] siCont = enc.GetBytes(cont);
          byte[ ] siCont64 = Convert.FromBase64String( cont );

          foreach ( MimeEntity me in mmsg.BodyParts )
          {
            if ( !me.IsAttachment ) continue;

            using ( MemoryStream ms2 = new MemoryStream( ) )
            {
              me.WriteTo( ms2 );
              ms2.Position = 0;

              MIME.MimeMessage msg2 = new MIME.MimeMessage();
              msg2.LoadBody(me.ToString());

              byte[] textb = enc.GetBytes(msg2.mContent);
              byte[] hashb = sha.ComputeHash(textb);

              ContentInfo inf = new ContentInfo(hashb);

              SignedCms cms = new SignedCms(inf, true);

              cms.Decode(siCont64);

              try
              {
                // CertificateManager is a custom class and returns a X509Certificate2-Collection based on a config in my app
                cms.CheckSignature( CertificateManager.GetCertificate( cfg ), false );

                MessageBox.Show("luck");
              }
              catch ( Exception ex )
              {
                 // hash is always wrong 
              }
            }
          }

现在我已经到了无法再思考的地步

          MimeEntity sig = mmsg.BodyParts.Where(x => x.IsAttachment && x.ContentType.Name != null && x.ContentType.Name.EndsWith(".p7s")).Select(x => x).ToArray()[0];
          MIME.MimeMessage mSig = new MIME.MimeMessage();
          mSig.LoadBody(sig.ToString());
          string cont = mSig.mContent;
          byte[] siCont = enc.GetBytes(cont);
          byte[ ] siCont64 = Convert.FromBase64String( cont );

          foreach ( MimeEntity me in mmsg.BodyParts )
          {
            if ( !me.IsAttachment ) continue;

            using ( MemoryStream ms2 = new MemoryStream( ) )
            {
              me.WriteTo( ms2 );
              ms2.Position = 0;

              MIME.MimeMessage msg2 = new MIME.MimeMessage();
              msg2.LoadBody(me.ToString());

              byte[] textb = enc.GetBytes(msg2.mContent);
              byte[] hashb = sha.ComputeHash(textb);

              ContentInfo inf = new ContentInfo(hashb);

              SignedCms cms = new SignedCms(inf, true);

              cms.Decode(siCont64);

              try
              {
                // CertificateManager is a custom class and returns a X509Certificate2-Collection based on a config in my app
                cms.CheckSignature( CertificateManager.GetCertificate( cfg ), false );

                MessageBox.Show("luck");
              }
              catch ( Exception ex )
              {
                 // hash is always wrong 
              }
            }
          }