protected string CheckSig()
        {
            var formData = Request.Form;
            var text = formData["txtSign"];
            var sig = formData["txtSig"];

            string output = "INVALID!";

            if (!string.IsNullOrEmpty(sig))
            {
                try
                {
                    ContentInfo contentInfo = new ContentInfo(Encoding.UTF8.GetBytes(text));

                    SignedCms signedCms = new SignedCms(contentInfo, true);

                    signedCms.Decode(Convert.FromBase64String(sig));

                    // This checks if the signature is valid, but doensn't actually verify the cert (TODO)
                    signedCms.CheckSignature(true);

                    output = "Signature valid.";

                    signedCms.CheckSignature(false);

                    output += "<br>Cert valid";
                }
                catch (Exception e)
                {
                    output += "<br>" + e.ToString();
                }
            }

            return output;
        }

        public sealed override byte[] Encrypt(CmsRecipientCollection recipients, ContentInfo contentInfo, AlgorithmIdentifier contentEncryptionAlgorithm, X509Certificate2Collection originatorCerts, CryptographicAttributeObjectCollection unprotectedAttributes)
        {
            using (SafeCryptMsgHandle hCryptMsg = EncodeHelpers.CreateCryptMsgHandleToEncode(recipients, contentInfo.ContentType, contentEncryptionAlgorithm, originatorCerts, unprotectedAttributes))
            {
                byte[] encodedContent;
                if (contentInfo.ContentType.Value.Equals(Oids.Pkcs7Data, StringComparison.OrdinalIgnoreCase))
                {
                    unsafe
                    {
                        byte[] content = contentInfo.Content;
                        fixed (byte* pContent = content)
                        {
                            DATA_BLOB blob = new DATA_BLOB((IntPtr)pContent, (uint)(content.Length));
                            encodedContent = Interop.Crypt32.CryptEncodeObjectToByteArray(CryptDecodeObjectStructType.X509_OCTET_STRING, &blob);
                        }
                    }
                }
                else
                {
                    encodedContent = contentInfo.Content;
                }

                if (encodedContent.Length > 0)
                {
                    if (!Interop.Crypt32.CryptMsgUpdate(hCryptMsg, encodedContent, encodedContent.Length, fFinal: true))
                        throw Marshal.GetLastWin32Error().ToCryptographicException();
                }

                byte[] encodedMessage = hCryptMsg.GetMsgParamAsByteArray(CryptMsgParamType.CMSG_CONTENT_PARAM);
                return encodedMessage;
            }
        }

        /// <summary> 
        /// Firma mensaje 
        /// </summary> 
        /// <param name="argBytesMsg">Bytes del mensaje</param> 
        /// <param name="argCertFirmante">Certificado usado para firmar</param> 
        /// <returns>Bytes del mensaje firmado</returns> 
        /// <remarks></remarks> 
        public static byte[] FirmaBytesMensaje(byte[] argBytesMsg, X509Certificate2 argCertFirmante)
        {
            try
            {
                // Pongo el mensaje en un objeto ContentInfo (requerido para construir el obj SignedCms)
                ContentInfo infoContenido = new ContentInfo(argBytesMsg);
                SignedCms cmsFirmado = new SignedCms(infoContenido);

                // Creo objeto CmsSigner que tiene las caracteristicas del firmante
                CmsSigner cmsFirmante = new CmsSigner(argCertFirmante);
                cmsFirmante.IncludeOption = X509IncludeOption.EndCertOnly;

                if (VerboseMode)
                {
                    Console.WriteLine("***Firmando bytes del mensaje...");
                }
                // Firmo el mensaje PKCS #7
                cmsFirmado.ComputeSignature(cmsFirmante);

                if (VerboseMode)
                {
                    Console.WriteLine("***OK mensaje firmado");
                }

                // Encodeo el mensaje PKCS #7.
                return cmsFirmado.Encode();
            }
            catch (Exception excepcionAlFirmar)
            {
                throw new Exception("***Error al firmar: " + excepcionAlFirmar.Message);
            }
        }

		public void ConstructorContent () 
		{
			ContentInfo ci = new ContentInfo (asnNull);
			Assert.AreEqual (asnNull, ci.Content, "Content");
			Assert.AreEqual (defaultName, ci.ContentType.FriendlyName, "ContentType.FriendlyName");
			Assert.AreEqual (defaultOid, ci.ContentType.Value, "ContentType.Value");
		}

		public void ConstructorNonPkcs7Oid ()
		{
			Oid o = new Oid ("1.2.3.4");
			ContentInfo ci = new ContentInfo (o, asnNull);
			Assert.AreEqual (asnNull, ci.Content, "Content");
			Assert.AreEqual ("1.2.3.4", ci.ContentType.Value, "ContentType.Value");
		}

        /// <summary> 
        /// Firma el mensaje PKCS #7 con el certificado del firmante
        /// </summary> 
        /// <param name="pMensaje">Mensaje (como cadena de bytes)</param> 
        /// <param name="pCertificadoFirmante">Certificado usado para firmar</param> 
        /// <returns>Mensaje Firmado (como cadena de bytes)</returns> 
        /// <remarks></remarks> 
        public static byte[] FirmarMensaje(byte[] pMensaje, X509Certificate2 pCertificadoFirmante)
        {
            byte[] msjFirmado;

            try
            {
                // Se pone el Mensaje recibido en un objeto ContentInfo                 
                ContentInfo infoContenidoMsj    = new ContentInfo(pMensaje);
                // Se instancia el CMS Firmado con el ContentInfo
                SignedCms cmsFirmado            = new SignedCms(infoContenidoMsj);
                // Se instancia el objeto CmsSigner con las caracteristicas del firmante 
                CmsSigner cmsFirmante = new CmsSigner(pCertificadoFirmante);

                cmsFirmante.IncludeOption = X509IncludeOption.EndCertOnly;

                // Se firma el mensaje PKCS #7 con el certificado
                cmsFirmado.ComputeSignature(cmsFirmante);

                msjFirmado = cmsFirmado.Encode();

                // Retorno el mensaje PKCS #7 firmado . 
                return msjFirmado;
            }
            catch (Exception excepcionAlFirmar)
            {
                throw new Exception("ERROR: Procedimiento: FirmarMensaje. Al intentar firmar el mensaje con el certificado del firmante: " + excepcionAlFirmar.Message);
            }
        }

		public EnvelopedCms (SubjectIdentifierType recipientIdentifierType, ContentInfo contentInfo) 
			: this (contentInfo) 
		{
			_idType = recipientIdentifierType;
			if (_idType == SubjectIdentifierType.SubjectKeyIdentifier)
				_version = 2;
		}

 public byte[] Sign(byte[] data)
 {
     ContentInfo contentInfo = new ContentInfo(_md5.ComputeHash(data));
     SignedCms signedCms = new SignedCms(contentInfo);
     CmsSigner cmsSigner = new CmsSigner(_cert);
     cmsSigner.IncludeOption = X509IncludeOption.WholeChain;
     signedCms.ComputeSignature(cmsSigner);
     return signedCms.Encode();
 }

 private byte[] EncryptedBytes(byte[] bytes)
 {
     var contentInfo = new ContentInfo(bytes);
     var encryptAlgoOid = new Oid("2.16.840.1.101.3.4.1.42"); // AES-256-CBC            
     var envelopedCms = new EnvelopedCms(contentInfo, new AlgorithmIdentifier(encryptAlgoOid));
     var recipient = new CmsRecipient(CryptographicCertificate);
     envelopedCms.Encrypt(recipient);
     return envelopedCms.Encode();
 }

		public SignedCms (ContentInfo content, bool detached) 
			: this ()
		{
			if (content == null)
				throw new ArgumentNullException ("content");

			_content = content;
			_detached = detached;
		}

        /// <summary>
        /// Encrypts the specified string.
        /// </summary>
        /// <param name="plaintext">The plaintext to be encrypted.</param>
        /// <param name="certificate">The certificate to be used for encryption.</param>
        /// <returns>The encrypted text.</returns>
        public static string Encrypt(this string plaintext, X509Certificate2 certificate)
        {
            var contentInfo = new ContentInfo(Encoding.UTF8.GetBytes(plaintext));
            var envelopedCms = new EnvelopedCms(contentInfo);

            var cmsRecipient = new CmsRecipient(certificate);
            envelopedCms.Encrypt(cmsRecipient);

            return Convert.ToBase64String(envelopedCms.Encode());
        }

 public static void DecodeAlgorithmDes_RoundTrip()
 {
     AlgorithmIdentifier algorithm = new AlgorithmIdentifier(new Oid(Oids.Des));
     ContentInfo contentInfo = new ContentInfo(new byte[] { 1, 2, 3 });
     EnvelopedCms ecms = new EnvelopedCms(contentInfo, algorithm);
     using (X509Certificate2 cert = Certificates.RSAKeyTransfer1.GetCertificate())
     {
         CmsRecipient cmsRecipient = new CmsRecipient(cert);
         ecms.Encrypt(cmsRecipient);
     }
     byte[] encodedMessage = ecms.Encode();
     VerifyAlgorithmDes(encodedMessage);
 }

        public static void DecodeCertificates0_RoundTrip()
        {
            ContentInfo contentInfo = new ContentInfo(new byte[] { 1, 2, 3 });
            EnvelopedCms ecms = new EnvelopedCms(contentInfo);
            using (X509Certificate2 cert = Certificates.RSAKeyTransfer1.GetCertificate())
            {
                CmsRecipient cmsRecipient = new CmsRecipient(cert);
                ecms.Encrypt(cmsRecipient);
            }
            byte[] encodedMessage = ecms.Encode();

            VerifyCertificates0(encodedMessage);
        }

        public static void CheckSig(byte[] sig, byte[] data)
        {
            ContentInfo contentInfo = new ContentInfo(data);

            SignedCms signedCms = new SignedCms(contentInfo, true);

            signedCms.Decode(sig);

            // This checks if the signature is valid, but doensn't actually verify the cert (TODO)
            signedCms.CheckSignature(true);

            signedCms.CheckSignature(false);
        }

        public static void DecodeRecipients3_RoundTrip()
        {
            ContentInfo contentInfo = new ContentInfo(new byte[] { 1, 2, 3 });
            EnvelopedCms ecms = new EnvelopedCms(contentInfo);
            CmsRecipientCollection recipients = new CmsRecipientCollection();
            foreach (X509Certificate2 cert in s_certs)
            {
                recipients.Add(new CmsRecipient(cert));
            }
            ecms.Encrypt(recipients);
            byte[] encodedMessage = ecms.Encode();

            VerifyRecipients3(encodedMessage);
        }

        public EnvelopedCms(ContentInfo contentInfo, AlgorithmIdentifier encryptionAlgorithm)
        {
            if (contentInfo == null)
                throw new ArgumentNullException(nameof(contentInfo));
            if (encryptionAlgorithm == null)
                throw new ArgumentNullException(nameof(encryptionAlgorithm));

            Version = 0;  // It makes little sense to ask for a version before you've decoded, but since the desktop returns 0 in that case, we will too.
            ContentInfo = contentInfo;
            ContentEncryptionAlgorithm = encryptionAlgorithm;
            Certificates = new X509Certificate2Collection();
            UnprotectedAttributes = new CryptographicAttributeObjectCollection();
            _decryptorPal = null;
            _lastCall = LastCall.Ctor;
        }

        public static String CheckFileSignature(ContentInfo content, byte[] signature)
        {
            var verifyCms = new SignedCms(content, true);
            verifyCms.Decode(signature);

            var cert = verifyCms.SignerInfos[0].Certificate;

            try
            {
                verifyCms.CheckSignature(new X509Certificate2Collection(cert), false);
                return @"Signature is valid";
            }
            catch (CryptographicException)
            {
                return @"Signature is not valid for content";
            }
        }

        private byte[] FirmaBytesMensaje( byte[] argBytesMsg, X509Certificate2 argCertFirmante )
        {
            ContentInfo infoContenido = new ContentInfo( argBytesMsg );
            SignedCms cmsFirmado = new SignedCms( infoContenido );
            CmsSigner cmsFirmante = new CmsSigner( argCertFirmante );

            try
            {
                cmsFirmante.IncludeOption = X509IncludeOption.EndCertOnly;
                cmsFirmado.ComputeSignature( cmsFirmante );

            }
            catch ( Exception error )
            {
                this.manejadorErrores.ManejarError( error, "FirmaBytesMensaje", error.Message );
            }
            return cmsFirmado.Encode();
        }

        private MailMessage GenerateHtmlMessage(string from, string to, string subject, string content, string[] attachmentFilepaths)
        {
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress(from);
            mail.To.Add(to);
            mail.Subject = subject;
            string body = null;
            if (attachmentFilepaths != null && attachmentFilepaths.Length > 0)
            {
                StringBuilder sb = new StringBuilder();
                sb.Append("MIME-Version: 1.0\r\n");
                sb.Append("Content-Type: multipart/mixed; boundary=unique-boundary-1\r\n");
                sb.Append("\r\n");
                sb.Append("This is a multi-part message in MIME format.\r\n");
                sb.Append("--unique-boundary-1\r\n");
                sb.Append("Content-Type: text/html\r\n");  //could use text/plain as well here if you want a plaintext message
                sb.Append("Content-Transfer-Encoding: 7Bit\r\n\r\n");
                sb.Append(content);
                if (!content.EndsWith("\r\n"))
                    sb.Append("\r\n");
                sb.Append("\r\n\r\n");
                foreach (string filepath in attachmentFilepaths)
                {
                    sb.Append(GenerateRawAttachement(filepath));
                }
                body = sb.ToString();
            }
            else
            {
                body = "Content-Type: text/html\r\nContent-Transfer-Encoding: 7Bit\r\n\r\n" + content;
            }
            //input your certification and private key.
            X509Certificate2 cert = new X509Certificate2("emailcertification.pfx", "6522626", X509KeyStorageFlags.Exportable | X509KeyStorageFlags.PersistKeySet);
            ContentInfo contentInfo = new ContentInfo(Encoding.UTF8.GetBytes(body));
            SignedCms signedCms = new SignedCms(contentInfo, false);
            CmsSigner Signer = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, cert);
            signedCms.ComputeSignature(Signer);
            byte[] signedBytes = signedCms.Encode();
            MemoryStream stream = new MemoryStream(signedBytes);
            AlternateView view = new AlternateView(stream, "application/pkcs7-mime; smime-type=signed-data;name=smime.p7m");
            mail.AlternateViews.Add(view);

            return mail;
        }

 public static void ZeroLengthContent_RoundTrip()
 {
     ContentInfo contentInfo = new ContentInfo(Array.Empty<byte>());
     EnvelopedCms ecms = new EnvelopedCms(contentInfo);
     using (X509Certificate2 cert = Certificates.RSAKeyTransfer1.GetCertificate())
     {
         CmsRecipient cmsRecipient = new CmsRecipient(cert);
         try
         {
             ecms.Encrypt(cmsRecipient);
         }
         catch (CryptographicException e)
         {
             throw new Exception("ecms.Encrypt() threw " + e.Message + ".\nIf you're running on the desktop CLR, this is actually an expected result.");
         }
     }
     byte[] encodedMessage = ecms.Encode();
     ValidateZeroLengthContent(encodedMessage);
 }