public string GetMD5(string txt)
        {
            System.Security.Cryptography.MD5 md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
            byte[] frmData = System.Text.Encoding.UTF8.GetBytes(txt);
            byte[] targetData = md5.ComputeHash(frmData);
            string str = null;
            for (int i = 0; i < targetData.Length; i++)
            {
                str = str + targetData[i].ToString("x2");
            }
            return str;
        }