static string CreateRegCode(string mac, DateTime date)
 {
   RSACryptoServiceProvider cryptor = new RSACryptoServiceProvider();
   cryptor.FromXmlString(File.ReadAllText("PrivateKey.xml"));
   string signature = String.Format("[{}][{}]", mac, date.ToString("yyyy-MM-dd"));
   byte[] regCodeBytes = cryptor.SignData(
     Encoding.UTF.GetBytes(signature),
     "SHA");
   return Convert.ToBaseString(regCodeBytes);
 }

static bool Verify(string regCode)
 {
   const string PUBLIC_KEY = "";
   try
   {
     RSACryptoServiceProvider cryptor = new RSACryptoServiceProvider();
     cryptor.FromXmlString(PUBLIC_KEY);
     byte[] signedData = Convert.FromBaseString(regCode);
 
     bool today = cryptor.VerifyData(
       Encoding.UTF.GetBytes(String.Format("[{}][{}]", DateTime.Now.ToString("yyyy-MM-dd"))),
       "SHA", signedData);
     bool machineToday = cryptor.VerifyData(
       Encoding.UTF.GetBytes(String.Format("[{}][{}]", MAC, DateTime.Now.ToString("yyyy-MM-dd"))),
       "SHA", signedData);
     bool forever = cryptor.VerifyData(
       Encoding.UTF.GetBytes(String.Format("[{}][{}]", MAC, Environment.MachineName)),
       "SHA", signedData);
     return today || machineToday || forever;
   }
   catch
   {
     return false;
   }
 }