简单移位加密法：也是所谓的凯撒移位密码（只是凯撒的移位就是3位），最早追

溯到古罗马时期，《高卢战记》描述到它是使用替代密码，通过将字母按顺序推

后3位起到加密的作用，如将字母A换做D，将字母B换做E。
可以看到确实很简答，要想破解此中类型的密码做个最多25次的推测循环就能得

到移位的位数。
下面我们做个程序例子来进行加密和解密的算法实现。
我们就以英文为例，中文的是比较复杂，以后再讨论。
这个移位算法中a-z是个循环。
下面是用c#语言写的一个加密解密程序：
using System;
using System.Collections.Generic;
using System.Text;

namespace 密码学
{
    class Program
    {
        static void Main(string[] args)
        {
            string plaintext = "";//明文
            string ciphertext = "";//密文
            int n = 0;//位数
            plaintext = Console.ReadLine();
            n = int.Parse(Console.ReadLine());
            ciphertext=getCipherText(plaintext,n);
            Console.WriteLine("密文为："+ciphertext);
            Console.WriteLine("明文："+getPlainText(ciphertext,n));
        }
        #region 加密
        public static string getCipherText(string plaintext, int n)
        {
            StringBuilder ciphertext = new StringBuilder();
            if (plaintext == "")
            {
                Console.WriteLine("Sorry,plaintext cann\'t be null");
            }
            else if (n < 1 || n > 25)
            {
                Console.WriteLine("Sorry,the n is not right!");
            }
            else
            {
                char[] str = plaintext.ToCharArray();
                for (int i = 0; i < str.Length; i++)
                {
                    if (str[i] == ' ')
                    {
                        ciphertext.Append(str[i]);
                    }
                    else if (str[i] >= 'A' && str[i] <= 'Z')
                    {
                       // 这其中有个规律，以小写为例，因为a-z有个循环，
                       //当一个字母加上移位数n大于z时则减去26（英文中总共26个字母），
                       //否则不用减。
                        

                        int inter = (int)str[i] + n;
                        if (inter > (int)('Z'))
                        {
                            str[i] = (char)(inter - 26);
                            ciphertext.Append(str[i]);
                        }
                        else
                        {
                            str[i] = (char)inter;
                            ciphertext.Append(str[i]);
                        }
                    }
                    else if (str[i] >= 'a' && str[i] <= 'z')
                    {
                        int inter = (int)str[i] + n;
                        if (inter > (int)('z'))
                        {
                            str[i] = (char)(inter - 26);
                            ciphertext.Append(str[i]);
                        }
                        else
                        {
                            str[i] = (char)inter;
                            ciphertext.Append(str[i]);
                        }
                    }
                    else
                    {
                        Console.WriteLine("Sorry,plaintext\'s convert not right!");
                    }
                }
            }
            return ciphertext.ToString();
        }
        #endregion
        #region 解密
        public static string getPlainText(string ciphertext, int n)
        {
            StringBuilder plaintext = new StringBuilder();
            if (ciphertext == "")
            {
                Console.WriteLine("Sorry,plaintext cann\'t be null");
            }
            else if (n < 1 || n > 25)
            {
                Console.WriteLine("Sorry,the n is not right!");
            }
            else
            {
                char[] str = ciphertext.ToCharArray();
                for (int i = 0; i < str.Length; i++)
                {
                    if (str[i] == ' ')
                    {
                        plaintext.Append(str[i]);
                    }
                    else if (str[i] >= 'A' && str[i] <= 'Z')
                    {                      
                        //这其中有个规律，以小写为例，因为a-z有个循环，
                        //当一个字母加上移位数-n小于a时则加上26（英文中总共26个字母），
                        //否则不用减。这是加密算法的逆过程
                        int inter = (int)str[i] - n;
                        if (inter < (int)('A'))
                        {
                            str[i] = (char)(inter + 26);
                            plaintext.Append(str[i]);
                        }
                        else
                        {
                            str[i] = (char)inter;
                            plaintext.Append(str[i]);
                        }
                    }
                    else if (str[i] >= 'a' && str[i] <= 'z')
                    {
                        int inter = (int)str[i] - n;
                        if (inter < (int)('a'))
                        {
                            str[i] = (char)(inter + 26);
                            plaintext.Append(str[i]);
                        }
                        else
                        {
                            str[i] = (char)inter;
                            plaintext.Append(str[i]);
                        }
                    }
                    else
                    {
                        Console.WriteLine("Sorry,plaintext\'s convert not right!");
                    }
                }
            }
            return plaintext.ToString();
        }      
        #endregion
       
    }
}