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 onlyhave 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)