Windows Forms: Login

Form1.cs

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.IO;

namespace Login
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private string encrypt(string password)
        {
            char[] c = new char[password.Length];

            for (int i = 0; i < c.Length; i++)
                c[i] = (char)(password[i] + 1);

            return new string(c);
        }
        
        private string decrypt(string password)
        {
            char[] c = new char[password.Length];

            for (int i = 0; i < c.Length; i++)
                c[i] = (char)(password[i] - 1);

            return new string(c);
        }

        private bool userExists(string user)
        {
            bool ret = false;

            FileStream fs = null;
            StreamReader sr = null;

            try
            {
                fs = new FileStream("passwd.txt", FileMode.Open, FileAccess.Read);
                sr = new StreamReader(fs);

                while (sr.Peek() != -1)
                {
                    var p = sr.ReadLine().Split('\t');
                    if (p[0].Equals(user))
                    {
                        ret = true;
                        break;
                    }
                }
            }
            catch (FileNotFoundException fnfe)
            {
                MessageBox.Show("Error: passwd file missing");
            }
            finally
            {
                if (sr != null)
                    sr.Close();
                if (fs != null)
                    fs.Close();
            }

            return ret;
        }

        private bool createEntry(string user, string password)
        {
            bool ret = false;

            FileStream fs = null;
            StreamWriter sw = null;

            try
            {
                fs = new FileStream("passwd.txt", FileMode.Append, FileAccess.Write);
                sw= new StreamWriter(fs);
                sw.WriteLine(user + "\t" + password);
                ret = true;
            }
            catch (FileNotFoundException fnfe)
            {
                MessageBox.Show("Error: passwd file missing");
            }
            finally
            {
                if (sw != null)
                    sw.Close();
                if (fs != null)
                    fs.Close();
            }

            return ret;
        }

        private bool findEntry(string user, string password)
        {
            bool ret = false;
            int entries = 0;

            FileStream fs = null;
            StreamReader sr = null;

            try
            {
                fs = new FileStream("passwd.txt", FileMode.Open, FileAccess.Read);
                sr = new StreamReader(fs);

                while (sr.Peek() != -1)
                {
                    entries++;

                    if (sr.ReadLine().Equals(user + "\t" + password))
                    {
                        ret = true;
                        break;
                    }
                }
            }
            catch(FileNotFoundException fnfe)
            {
                MessageBox.Show("Error: passwd file missing");
            }
            finally
            {
                if (sr != null )
                    sr.Close();
                if (fs != null)
                    fs.Close();
                if (entries == 0)
                    MessageBox.Show("Error: passwd file is empty");
            }

            return ret;
        }

        private void label2_Click(object sender, EventArgs e)
        {

        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBoxUser.Text == "" || textBoxPassword.Text == "")
            {
                MessageBox.Show("Error: user or password missing");
                return;
            }

            if (!userExists(textBoxUser.Text))
            {
                MessageBox.Show("Error: unknown user");
                return;
            }

            if (findEntry(textBoxUser.Text, encrypt(textBoxPassword.Text)))
            {
                Form2 ok = new Form2();
                ok.ShowDialog();
            }
            else
            {
                MessageBox.Show("Error: unknown password");
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (textBoxUser.Text == "" || textBoxPassword.Text == "")
            {
                MessageBox.Show("Error: user or password missing");
                return;
            }

            if (textBoxUser.Text.Contains("\t") || textBoxPassword.Text.Contains("\t"))
            {
                MessageBox.Show("Error: tab character is not allowed");
                return;
            }

            if (userExists(textBoxUser.Text))
            {
                MessageBox.Show("Error: user already registered");
                return;
            }

            if (createEntry(textBoxUser.Text, encrypt(textBoxPassword.Text)))
            {
                Form2 ok = new Form2();
                ok.ShowDialog();
            }
            else
            {
                MessageBox.Show("Error: couldn't register new user");
            }
        }
    }
}

TODO