gistfile1.cs

namespace EncryptionWithCertificate
{
    using System;
    using System.Security.Cryptography;
    using System.Security.Cryptography.X509Certificates;
    using System.Text;

    class Program
    {
        ////  makecert -pe MyCryptCert.cer -ss my -n "CN=Frans2" -sky exchange -r

        static void Main(string[] args)
        {
            Console.WriteLine("***********************");
            string input = "Hello there";
            Console.WriteLine("Input: {0}", input);
            var encrypted = Encrypt(input);
            Console.WriteLine("Encrypted: {0}", encrypted);
            var decrypted = Decrypt(encrypted);
            Console.WriteLine("Decrypted: {0}", decrypted);

            Console.WriteLine("***********************");
            Console.ReadLine();
        }

        private static string Encrypt(string input)
        {
            var cert = new X509Certificate2(@"c:\temp\MyCryptCert.cer");

            RSACryptoServiceProvider rsaEncryptor = (RSACryptoServiceProvider)cert.PublicKey.Key;
            
            byte[] cipherData = rsaEncryptor.Encrypt(Encoding.UTF8.GetBytes(input), true);
            return Convert.ToBase64String(cipherData);
        }

        private static string Decrypt(string input)
        {
            X509Store store = new X509Store("MY", StoreLocation.CurrentUser);
            store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);

            var matches = store.Certificates.Find(X509FindType.FindBySubjectName, "Frans2", false); // You should use a thumbprint instead of name here
            X509Certificate2 cert = null;
            foreach (var c in matches) // Not sure if you need this with thumbprints...
            {
                cert = c;
                Console.WriteLine("Found a cert"); // Should test for finding more than one
            }

            RSACryptoServiceProvider encryptor = (RSACryptoServiceProvider)cert.PrivateKey;
            var inputBytes = Convert.FromBase64String(input);
            var decoded = encryptor.Decrypt(inputBytes, true);
            var decodedString = Encoding.UTF8.GetString(decoded);
            return decodedString;
        }
    }
}