c# 替换密码解密

问：

所以我是一名老师，希望明年能经营一个小型黑客/密码学俱乐部。 因此，我正在为一些加密方法编写解决方案。我已经编写了一个随机替换密码，但无法计算出随机的解密方法。

我知道我需要使用字母频率和字母对，但不确定如何完美地编程。

我使用的句子是“这是对 alans 密码学技能的测试”

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Enter the sentence to encrypt");
        string plainText = Console.ReadLine();

        string cipherText = encrypt(plainText.ToUpper());
        Console.WriteLine(cipherText);
        Console.ReadLine();


    }
    public static string encrypt(string phrase)
    {
        List<string> originalLetters = new List<string>();
        //enter the original phrase to the list
        for (int i = 0; i < phrase.Length; i++ )
        {
            originalLetters.Add(phrase[i].ToString());
        }

        //randomise the subsitutions
        //all
        List<string> allLetters = new List<string>{"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"};

        //shuffled
        List<string> shuffledLetters = new List<string>();
        int times = allLetters.Count;
        Random rng = new Random();
        for (int i = 0; i < times;i++ )
        {
            int position = rng.Next(0, allLetters.Count);
            shuffledLetters.Add(allLetters[position]);

            Console.Write(allLetters[position] + ", ");
            allLetters.RemoveAt(position);

        }
        Console.WriteLine();
        string cipherText = "";
        //now change letters into shuffled letters
        string SortingLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

        for (int i = 0; i < originalLetters.Count;i++)
        {
            //take the original letter
            //find its position in the sorting letters
            int index = SortingLetters.IndexOf(originalLetters[i]);

            //use position to replace to the new string
            if (index != -1)
            {
                //swap the letter with the index plus the key
                cipherText += shuffledLetters[index];

            }
            else
            {
                cipherText += originalLetters[i];
            }
        }





            //return the encrypted message
            return cipherText;
    }

    public static string decrypt(string phrase)
    {
       //take the encrypted message

        //find most frequent letter as E, focus on single letter words as either I or a



        return "finalAnswer";
    }
}
}

我还包含了示例输出。我想让我的学生很难破解它，但只要使用一个固定的密钥，基本上就可以把它变成一个凯撒密码，我已经有了解决方案。我对程序显示一系列答案没有问题，用户必须从中找到正确的答案

任何帮助都是值得赞赏的

谢谢