C#的加密解密，请教这个哪有有关问题如何改

C#的加密解密，请问这个哪有问题怎么改？
using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            int Key;
            string p, c;
            while(true)
            {
                Console.WriteLine("请输入密钥:");
                Key = int.Parse(Console.ReadLine());
                Console.WriteLine("加密请输入(1),解密请输入(2)");
                int n=int.Parse(Console.ReadLine());
                if(n==1)
                {
                    Console.WriteLine("请输入明文:");
                    string str = Console.ReadLine();
                    char[] plaintext = str.ToCharArray();
                    for (int i = 0; i <= plaintext.Length; i++)
                    {
                        if (plaintext[i] >= 'A' && plaintext[i] <= 'Z')
                        {
                            int k = (int)plaintext[i];
                            k += Key;
                            if (plaintext[i] > 'Z')
                            {
                                k -=  26;
                            }
                        }
                        if (i >= 'a' && i <= 'z')
                        {
                            k += Key;
                            if (i > 'z')
                            {
                                k += 26;
                            }
                        }
                        if (i >= '0')
                        {
                            k += Key;
                        }
                        char i=(char) plaintext[k];
                    }
                    Console.WriteLine("输出地密文为:{0}",i);
                }
                Console.WriteLine();
                if(n==2)
                {
                    Console.WriteLine("请输入密文:");
                    string str=Console.ReadLine();
                    char[] ciphertext = str.ToCharArray();
                    for(int i=0;i<=ciphertext.Length;i++)
                    {
                        if(i >= 'A' && i <= 'Z')
                        {
                            i=i-Key;
                            if(i>'Z')
                            {
                                i=i+26;
                            }
                        }
                        if(i>='a'&&i<='z')
                        {
                            i=i+Key;
                            if(i>'z')
                            {
                                i=i+26;
                            }
                        }
                        if(i>='0')
                        {
                            i=i-Key;
                        }
                    }
                    Console.WriteLine("输出地明文为:{0}",p);
                }
                Console.WriteLine("* * * * * * * * * * * 感谢使用 * * * * * * * * * * *");
            }
        }
    }
}

------解决思路----------------------
对字符串进行加密和解密
 private void btn_Encrypt_Click(object sender, EventArgs e)
        {
            if (txt_password.Text.Length == 4)//判断加密密钥长度是否正确
            {
                try
                {
                    txt_EncryptStr.Text = //调用实例ToEncrypt方法得到加密后的字符串
                        new Encrypt().ToEncrypt(
                        txt_password.Text, txt_str.Text);
                    //Encrypt P_Encrypt = new Encrypt();
                    //P_Encrypt.ToEncrypt(""
                }
                catch (Exception ex)//捕获异常
                {
                    MessageBox.Show(ex.Message);//输出异常信息
                }
            }
            else
            {
                MessageBox.Show("密钥长度不符！", "提示");//提示用户输入密钥长度不正确
            }
        }
        private void btn_UnEncrypt_Click(object sender, EventArgs e)
        {
            if (txt_password2.Text.Length == 4)//判断加密密钥长度是否正确
            {
                try
                {
                    txt_str2.Text = //调用ToDecrypt方法得到解密后的字符串
                        new Encrypt().ToDecrypt(
                        txt_password2.Text, txt_EncryptStr2.Text);
                }
                catch (Exception ex)//捕获异常
                {
                    MessageBox.Show(ex.Message);//输出异常信息
                }
            }
            else
            {
                MessageBox.Show("密钥长度不符！", "提示");//提示用户输入密钥长度不正确
            }
        }