public class SharedUtility
    {
        public static String Encrypt(string strData)
        {
            if (strData != "")
            {
                strData = string.Format("{0}|{1}", HttpContext.Current.Session.SessionID, strData);
                SHA1Managed shaM = new SHA1Managed();
                Convert.ToBase64String(shaM.ComputeHash(Encoding.ASCII.GetBytes(strData)));
                Byte[] encByteData;
                encByteData = ASCIIEncoding.ASCII.GetBytes(strData);
                String encStrData = Convert.ToBase64String(encByteData);
                return encStrData;
            }
            else
            {
                return "";
            }

        }

        public static String Decrypt(string strData)
        {
            if (string.IsNullOrEmpty(strData) == false)
            {
                Byte[] decByteData;
                decByteData = Convert.FromBase64String(strData);
                String decStrData = ASCIIEncoding.ASCII.GetString(decByteData);

                String[] SplitValue = decStrData.Split('|');

                String ReturnValue = SplitValue[1];
                return ReturnValue;
            }
            else
            {
                return "";
            }

        }

protected void btnSign_Click(object sender, EventArgs e)
    {
        try
        {
            string UserName = TextBoxUserName.Text.Trim().Replace("'", " ");
            string Password = SharedUtility.Encrypt(TextBoxPassword.Text.Trim().Replace("'", " "));
            string Result = WebUsers.AuthenticateUser(UserName, Password);
            if (Result.Contains("Success"))
                Response.Redirect("~/Home/home.aspx", false);
            else
            {

                divResult.Visible = true;
                ResultLabel.Text = Result;
                ResultLabel.Visible = true;
            }
        }
        catch (Exception ex)
        {
            ResultLabel.Text = ex.Message;
        }
    }


}

 Invalid length for a Base-64 char array or string.