Form1

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SQLite;
using System.Security.Cryptography;

namespace Sale_on_credit
{
    public partial class Form1 : MetroFramework.Forms.MetroForm
    {
        Boolean sym;
        string loginDB;
        string passwordDB;
        string login;
        string password;
        string hash_login;
        string hash_password;
        SQLiteCommand cmd;
        SQLiteConnection sqlite_conn;
        SQLiteDataReader reader;
        static int errors=0;

        public Form1()
        {
            InitializeComponent();
            this.StyleManager = metroStyleManager1;
            this.FocusMe();
            sqlite_conn = new SQLiteConnection(getConnectionString());
            sqlite_conn.Open();
        }

        //Метод для получения адреса к БД
        public string getConnectionString()
        {
            string connectionString = @"Data Source=BD_Sales.db;Version=3";
            return connectionString;
        }

       public string MD5(string input)
        {
            MD5 md5 = new MD5CryptoServiceProvider();
            byte[] bytes = md5.ComputeHash(Encoding.Unicode.GetBytes(input));
            string result = BitConverter.ToString(bytes).Replace("-", String.Empty);
            return result.ToLower();
        }

        public string Authorization(string hash_login, string hash_password)
        {
            if (textBox1.Text == "") return "Введите логин пользователя";
            if (textBox2.Text == "") return "Введите пароль пользователя";
            string connectionString = getConnectionString();

            if (textBox1.Text == "admin")
            {


                     cmd = new SQLiteCommand("SELECT * FROM admin WHERE id = 1", sqlite_conn);

                    reader = cmd.ExecuteReader();
                    while (reader.Read())
                    {
                        if (reader["login"].ToString() == hash_login)
                        {
                            if (reader["password"].ToString() == hash_password)
                            {
                                reader.Close();
                                sqlite_conn.Close();
                                return "*";
                            }
                            else return "Введен неверный пароль!!!";
                        }
                        else return "Такого логина не существует!!!";
                    }
                    reader.Close();

                // sqlite_conn.Close();
            }

            cmd = new SQLiteCommand("SELECT * FROM users WHERE login ='" + hash_login + "'", sqlite_conn);
                reader = cmd.ExecuteReader();
                while (reader.Read())
                {
                    if (reader["login"].ToString() == hash_login)
                    {
                        if (reader["password"].ToString() == hash_password)
                        {
                            sqlite_conn.Close();
                            return "*";
                        }
                        else return "Введен неверный пароль!!!";
                    }
                    else return "Такого логина не существует!!!";
                }
                reader.Close();
                return "Введены неверные данные!!!";

        }

        private void button1_Click(object sender, EventArgs e)
        {
            string connectionString = getConnectionString();
            SQLiteCommand sqlite_cmd;

            login = textBox1.Text;
            password = textBox2.Text;
            hash_login = MD5(textBox1.Text);
            hash_password = MD5(textBox2.Text);



                string s = Authorization(hash_login,hash_password);
                if (textBox1.Text == "admin" && s == "*")
                {
                    Form2 form2 = new Form2();
                    form2.Visible = true;
                    Visible = false;

                }
                else if (textBox1.Text == "admin" && s != "*")
                {
                    MessageBox.Show("Ошибка!!!Проверьте правильность введенных данных!!!");
                }else if (textBox1.Text != "admin" && s == "*")
                {
                    Form3 form3 = new Form3();
                    form3.Visible = true;
                    Visible = false;
                }
                else
                {
                    MessageBox.Show("Ошибка!!!Проверьте правильность введенных данных!!!");
                }



        }

        private void Form1_FormClosed(object sender, FormClosedEventArgs e)
        {
            Application.Exit();

        }
    }
}