Codigo De Encriptacion En C#

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;
usingSystem.IO;

namespace Web_II_31_enero_2013
{
public partial class Form1 : Form
{
int key=0;

FileStream archivo;
StreamWriter salida;
StreamReaderentrada;

public Form1()
{
InitializeComponent();

}

string encription(string frase)
{
int valorentero;string fraseencriptada;
fraseencriptada = "";
for (int m = 0; m < frase.Length; m++)
{
valorentero = (int)char.Parse(frase.Substring(m, 1)) +key;
fraseencriptada = fraseencriptada + (char)valorentero;

}
return fraseencriptada;
}
string desencription(string frase)
{int valorentero;
string frasedesencriptada;
frasedesencriptada = "";
for (int m = 0; m < frase.Length; m++)
{valorentero=(int)char.Parse(frase.Substring(m, 1))- key;
frasedesencriptada = frasedesencriptada+(char)valorentero ;

}
return frasedesencriptada;
}private void salir_Click(object sender, EventArgs e)
{
ActiveForm.Close();
}

private void encriptar_Click(object sender, EventArgs e)
{CajaMultiple.Text=encription(CajaMultiple.Text);
}

private void Desencriptar_Click(object sender, EventArgs e)
{
CajaMultiple.Text =desencription(CajaMultiple.Text);
}

private void borrar_Click(object sender, EventArgs e)
{
CajaMultiple.Clear();
CajaMultiple.Focus();
}

private void...