using System;
using System.Security.Cryptography;
using System.Security.Cryptography.Xml;
using System.Windows.Forms;
using System.Xml;

namespace DigitalSignCheck
{
    internal class Program
    {
        [STAThread]
        static void Main(string[] args)
        {
            var openResult = GetFileName();

            if (openResult.Item1)
            {
                var xmlDoc = new XmlDocument();

                xmlDoc.PreserveWhitespace = true;

                xmlDoc.Load(openResult.Item2);

                var result = CheckSign(xmlDoc);

                Console.WriteLine(result);
            }
        }

        static (bool, string) GetFileName()
        {
            using (var dialog = new OpenFileDialog())
            {
                dialog.Title = "Open Excel Document";

                dialog.Filter = "XML Document|*.xml";

                if (dialog.ShowDialog() == DialogResult.OK)
                {
                    return (true, dialog.FileName);
                }
            }
            return (false, null);
        }

        static bool CheckSign(XmlDocument xmlDoc)
        {
            var signedXml = new SignedXml(xmlDoc);

            var nodeList = xmlDoc.GetElementsByTagName("Signature", SignedXml.XmlDsigNamespaceUrl);

            if (nodeList.Count == 0)
            {
                throw new CryptographicException("Verification failed: No Signature was found in the document.");
            }

            signedXml.LoadXml((XmlElement)nodeList[0]);

            return signedXml.CheckSignature();
        }
    }
}