Encrypting Editor Notepad Replacement

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

using Messagebox_Class;

namespace CryptPad
{
	public partial class Encryption : Form
	{
		CryptPad Crypt_Pad;
		private string newstring;
		private bool verify;
		const string enter_new = "Enter a new encryption string.";
		const string enter_existing = "Enter the encryption string previously created for this file.";
		const string mismatch = "Mismatch! Try again.";

		public Encryption ()
		{
			InitializeComponent ();
			this.Font = SystemFonts.MessageBoxFont;
		}

		private void Encryption_Load (object sender, EventArgs e)
		{
			Crypt_Pad = (CryptPad) this.Owner;

			// encrypt checksum	action needed

			//	null	0		request new encryption string
			//	null	>0		request existing to match checksum (open)
			//	"--"	0		request old then change
			//	"--"	>0		verify match else request existing (open)

			if (null == Crypt_Pad.encryption_string)
			{
				if (0 == Crypt_Pad.encryption_checksum)
				{
					this.labelTop.Text = enter_new;
					verify = true;
				}
				else
				{
					this.labelTop.Text = enter_existing;
				}
			}
			else
				if (0 == Crypt_Pad.encryption_checksum)
				{
					this.labelTop.Text = "Enter the previous encryption string.";
				}
				else
				{
					if (Crypt_Pad.encryption_checksum == ChecksumXOR (Crypt_Pad.encryption_string))
					{
						Close ();
						return;
					}
					Crypt_Pad.encryption_string = null;
					this.labelTop.Text = enter_existing;
				}
		}

		private void buttonOK_Click (object sender, EventArgs e)
		{
			textBoxEncryption.Focus ();
			// query string for file open
			if (0 != Crypt_Pad.encryption_checksum)
			{
				if (Crypt_Pad.encryption_checksum != ChecksumXOR (textBoxEncryption.Text))
				{
					Messagebox.Show (mismatch);
					textBoxEncryption.Clear ();
					return;
				}
				Crypt_Pad.encryption_string = textBoxEncryption.Text;
				Crypt_Pad.random_seed = ChecksumAdd (Crypt_Pad.encryption_string);
				this.DialogResult = DialogResult.OK;
				return;
			}
			if (null == newstring)
			{
				if (verify)
				{
					int i = textBoxEncryption.Text.Length;
					if (0 != i && 4 > i)
					{
						Messagebox.Show ("Encryption string must be at least 4 characters!");
						return;
					}
					newstring = textBoxEncryption.Text;
					textBoxEncryption.Clear ();
					if (0 == i)
						this.labelTop.Text = "Verify removing encryption string.";
					else
						this.labelTop.Text = "Verify the new encryption string.";
					return;
				}
				if (textBoxEncryption.Text == Crypt_Pad.encryption_string)
				{
					this.labelTop.Text = enter_new;
					verify = true;
					textBoxEncryption.Clear ();
					return;
				}
				Messagebox.Show (mismatch);
				textBoxEncryption.Clear ();
				return;
			}
			if (newstring != textBoxEncryption.Text)
			{
				Messagebox.Show (mismatch);
				textBoxEncryption.Clear ();
				return;
			}
			Crypt_Pad.txtBody.Modified = true;
			if ("" == textBoxEncryption.Text)
			{
				Messagebox.Show ("Encryption string erased!");
				Crypt_Pad.encryption_string = null;
				Crypt_Pad.encryption_checksum = 0;
				Crypt_Pad.random_seed = 0;
			}
			else
			{
				Messagebox.Show ("New encryption string set!");
				Crypt_Pad.encryption_string = textBoxEncryption.Text;
				Crypt_Pad.encryption_checksum = ChecksumXOR (Crypt_Pad.encryption_string);
				Crypt_Pad.random_seed = ChecksumAdd (Crypt_Pad.encryption_string);
			}
			this.DialogResult = DialogResult.OK;
		}

		private void textBoxEncryption_KeyPress (object sender, KeyPressEventArgs e)
		{
			char chr = e.KeyChar;
			if (22!=chr && '\b' != chr && !Crypt_Pad.isASCII (chr))
			{
				e.Handled = true;
				Messagebox.Show ("Only ASCII characters are allowed.");
			}
		}

		// xor checksum for in the file header
		private uint ChecksumXOR (string s)
		{
			if (0 == s.Length)
				return 0;
			uint checksum = 0;
			int j = 32;
			if (32 < s.Length)
				j = s.Length;
			for (int i = 0; i < j; ++i)
			{
				uint t = 0;
				if (0 < (0x80000000 & checksum))
					t = 1;
				checksum <<= 1;
				checksum |= t;
				checksum ^= s[i % s.Length];
			}
			return checksum;
		}

		// plus checksum for randomization
		private int ChecksumAdd (string s)
		{
			int checksum = 0;
			for (int i = 0; i < s.Length; ++i)
			{
				checksum += s[i];
			}
			return checksum;
		}

	}
}