private static void SignXml(XmlDocument xmlDoc, X509Certificate2 uidCert)
{
    RSACryptoServiceProvider rsaKey = (RSACryptoServiceProvider)uidCert.PrivateKey;

    // Chequea argumentos.
    if (xmlDoc == null)
        throw new ArgumentException("xmlDoc");
    if (rsaKey == null)
        throw new ArgumentException("Key");

    // Crea un objeto SignedXml.
    SignedXml signedXml = new SignedXml(xmlDoc);

    // Agrega la clave al documento SignedXml .
    signedXml.SigningKey = rsaKey;

    // Crea una referencia a ser firmada.
    Reference reference = new Reference();
    reference.Uri = "";

    // Agrega una ensobrada transformación a la referencia.
    XmlDsigEnvelopedSignatureTransform env = new XmlDsigEnvelopedSignatureTransform();
    reference.AddTransform(env);

    // Agrega la referencia al objeto SignedXml.
    signedXml.AddReference(reference);


    // Agrega una RSAKeyValue KeyInfo
    KeyInfo keyInfo = new KeyInfo();

    KeyInfoX509Data clause = new KeyInfoX509Data();
    clause.AddSubjectName(uidCert.Subject);
    clause.AddCertificate(uidCert);
    keyInfo.AddClause(clause);
    signedXml.KeyInfo = keyInfo;

    // Computa la firma.
    signedXml.ComputeSignature();

    // Obtiene la representación XML de la firma y la salva
    //  en un objeto  XmlElement .
    XmlElement xmlDigitalSignature = signedXml.GetXml();
    xmlDigitalSignature.Prefix = "ds";
    signedXml.ComputeSignature();

    System.Console.WriteLine(signedXml.GetXml().InnerXml);

    // Agrega el elemento al documento XML .
    xmlDoc.DocumentElement.AppendChild(xmlDoc.ImportNode(xmlDigitalSignature, true));
    //check signature
    XmlNodeList nodeList = xmlDoc.GetElementsByTagName("ds:Signature");
    if (nodeList.Count <= 0)
    {
        MessageBox.Show("Verification failed: No Signature was found in the document.");
    }
    else if (nodeList.Count >= 2)
    {
        MessageBox.Show("Verification failed: More that one signature was found for the document.");
    }
    else
    {
        signedXml.LoadXml((XmlElement)nodeList[0]);
        if (signedXml.CheckSignature())
        {
            MessageBox.Show("signature ok");
        }
        else
        {
            MessageBox.Show("signature failed");
        }
    }
}   // Fin SignXml

<CFE:EnvioCFE version="1.0" xmlns:CFE="http://cfe.dgi.gub.uy">
      <CFE:Caratula version="1.0"
.....

      </CFE:Caratula>
          <CFE xmlns:xd="http://www.w3.org/2000/09/xmldsig#" version="1.0" xmlns="http://cfe.dgi.gub.uy">
          <eTck>
    .............
      </eTck>
        <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
    ........
        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
        <DigestValue>eFoM+HEMPH+Ya+D1ljOm1yxEqsg=</DigestValue>
            </ds:Signature>
      </CFE>
    </CFE:EnvioCFE>