Hi all,

Today I'm posting a sample which shows how to sign a text with a certificate in my Personal store (this cert will have public and private key associated to it) and how to verify that signature with a .cer file (for i.e. WinForms) applications or a client certificate (for i.e. ASP.NET) (both will only have public key associated to them).

using System;
using System.Collections.Generic;
using System.Text;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

namespace ConsoleApplication1
{
    class Program
    {
        static byte[] Sign(string text, string certSubject)
        {
            // Access Personal (MY) certificate store of current user
            X509Store my = new X509Store(StoreName.My, StoreLocation.CurrentUser);
            my.Open(OpenFlags.ReadOnly);

            // Find the certificate we'll use to sign            
            RSACryptoServiceProvider csp = null;
            foreach (X509Certificate2 cert in my.Certificates)
            {
                if (cert.Subject.Contains(certSubject))
                {
                    // We found it. 
                    // Get its associated CSP and private key
                    csp = (RSACryptoServiceProvider)cert.PrivateKey;
                }
            }
            if (csp == null)
            {
                throw new Exception("No valid cert was found");
            }

            // Hash the data
            SHA1Managed sha1 = new SHA1Managed();
            UnicodeEncoding encoding = new UnicodeEncoding();
            byte[] data = encoding.GetBytes(text);
            byte[] hash = sha1.ComputeHash(data);

            // Sign the hash
            return csp.SignHash(hash, CryptoConfig.MapNameToOID("SHA1"));            
        }

        static bool Verify(string text, byte[] signature, string certPath)
        {
            // Load the certificate we'll use to verify the signature from a file 
            X509Certificate2 cert = new X509Certificate2(certPath);
            // Note: 
            // If we want to use the client cert in an ASP.NET app, we may use something like this instead:
            // X509Certificate2 cert = new X509Certificate2(Request.ClientCertificate.Certificate);

            // Get its associated CSP and public key
            RSACryptoServiceProvider csp = (RSACryptoServiceProvider)cert.PublicKey.Key;

            // Hash the data
            SHA1Managed sha1 = new SHA1Managed();
            UnicodeEncoding encoding = new UnicodeEncoding();
            byte[] data = encoding.GetBytes(text);
            byte[] hash = sha1.ComputeHash(data);

            // Verify the signature with the hash
            return csp.VerifyHash(hash, CryptoConfig.MapNameToOID("SHA1"), signature);
        }

        static void Main(string[] args)
        {
            // Usage sample
            try
            {
                // Sign text
                byte[] signature = Sign("Test", "cn=my cert subject");

                // Verify signature. Testcert.cer corresponds to "cn=my cert subject"
                if (Verify("Test", signature, @"C:\testcert.cer"))
                {
                    Console.WriteLine("Signature verified");
                }
                else
                {
                    Console.WriteLine("ERROR: Signature not valid!");
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("EXCEPTION: " + ex.Message);
            }
            Console.ReadKey();
        }
    }
}

I hope this helps.

Cheers,

Alex (Alejandro Campos Magencio)