using (var contentStream = new MemoryStream())
{
    var namespaces = new XmlSerializerNamespaces();
    namespaces.Add("", "");

    var serializer = new XmlSerializer(requestEntity.GetType(), string.Empty);
    var settings = new XmlWriterSettings
    {
        Encoding = Encoding.GetEncoding(1251),
        NamespaceHandling = NamespaceHandling.OmitDuplicates,
        Indent = true
    };

    using (var xmlTextWriter = XmlWriter.Create(contentStream, settings))
    {
        serializer.Serialize(xmlTextWriter, requestEntity, namespaces);
    }

    // Rewind the Stream.
    contentStream.Seek(0, SeekOrigin.Begin);

    return SecurityHelper.SignRequestContent(contentStream, personalCertificateSerialNumber);
}

// Sign the stream and save it to another stream.
private static Stream SignRequestContent(MemoryStream streamToSign, X509Certificate2 personalCertificate)
{
    var contentInfo = new ContentInfo(streamToSign.ToArray());
    var signedCms = new SignedCms(contentInfo);
    var cmsSigner = new CmsSigner(personalCertificate);
    signedCms.ComputeSignature(cmsSigner);
    return new MemoryStream(signedCms.Encode());
}