接下来就是编写解密函数了，代码如下：

 public static string a(string A_0, string A_1)
 {
     byte[] bytes = Encoding.UTF8.GetBytes("Kanxue2019CTF-Q1");
     byte[] bytes2 = Encoding.UTF8.GetBytes(A_0);
     byte[] bytes3 = new PasswordDeriveBytes(A_1, null).GetBytes(32);
     ICryptoTransform transform = new RijndaelManaged
     {
         Mode = CipherMode.CBC
     }.CreateEncryptor(bytes3, bytes);
     MemoryStream memoryStream = new MemoryStream();
     CryptoStream cryptoStream = new CryptoStream(memoryStream, transform, CryptoStreamMode.Write);
     cryptoStream.Write(bytes2, 0, bytes2.Length);
     cryptoStream.FlushFinalBlock();
     byte[] inArray = memoryStream.ToArray();
     memoryStream.Close();
     cryptoStream.Close();
     return Convert.ToBase64String(inArray);
 }

using System;
using System.Text;
using System.Security.Cryptography;
using System.IO;
public class Test
{
 public static void Main()
 {    Console.WriteLine(Test.Decode("4RTlF9Ca2+oqExJwx68FiA=="));
 }

 public static string Decode(string data)
{
 byte[] byIV = Encoding.UTF8.GetBytes("Kanxue2019CTF-Q1");
 byte[] byKey = new PasswordDeriveBytes("Kanxue2019", null).GetBytes(32);

 byte[] byEnc;
 try
 {
     byEnc = Convert.FromBase64String(data);
 }
 catch
 {
     return null;
 }

 ICryptoTransform cryptoProvider = new RijndaelManaged{
         Mode = CipherMode.CBC
     }.CreateDecryptor(byKey, byIV);

 byte[] resultArray = cryptoProvider.TransformFinalBlock(byEnc,0,byEnc.Length);
 return UTF8Encoding.UTF8.GetString(resultArray);
}
}

1. 先打开，是console程序，通过EXEInfo查到是.NET程序，载入dnSpy
2. 代码不多，做了混淆，但是基本能看明白，也可以通过de4dot脱之
3. 主要流程就是将输入的串进行一系列操作，然后和4RTlF9Ca2+oqExJwx68FiA==比较，一致则pass
4. 由于对c#不是很熟，搜索了代码中用到的几个函数，大概明白了是加解密，主要函数在这：

    public static string a(string A_0, string A_1)
    {
        byte[] bytes = Encoding.UTF8.GetBytes("Kanxue2019CTF-Q1");
        byte[] bytes2 = Encoding.UTF8.GetBytes(A_0);
        byte[] bytes3 = new PasswordDeriveBytes(A_1, null).GetBytes(32);
        ICryptoTransform transform = new RijndaelManaged
        {
            Mode = CipherMode.CBC
        }.CreateEncryptor(bytes3, bytes);
        MemoryStream memoryStream = new MemoryStream();
        CryptoStream cryptoStream = new CryptoStream(memoryStream, transform, CryptoStreamMode.Write);
        cryptoStream.Write(bytes2, 0, bytes2.Length);
        cryptoStream.FlushFinalBlock();
        byte[] inArray = memoryStream.ToArray();
        memoryStream.Close();
        cryptoStream.Close();
        return Convert.ToBase64String(inArray);
    }
   

5. 搜索了一下，就是通过DES加密，key和iv就是bytes3和bytes

6. 接下来就是编写解密函数了，代码如下：

   using System;
   using System.Text;
   using System.Security.Cryptography;
   using System.IO;
   public class Test
   {
    public static void Main()
    {    Console.WriteLine(Test.Decode("4RTlF9Ca2+oqExJwx68FiA=="));
    }
   
    public static string Decode(string data)
   {
    byte[] byIV = Encoding.UTF8.GetBytes("Kanxue2019CTF-Q1");
    byte[] byKey = new PasswordDeriveBytes("Kanxue2019", null).GetBytes(32);
   
    byte[] byEnc;
    try
    {
        byEnc = Convert.FromBase64String(data);
    }
    catch
    {
        return null;
    }
   
    ICryptoTransform cryptoProvider = new RijndaelManaged{
            Mode = CipherMode.CBC
        }.CreateDecryptor(byKey, byIV);
   
    byte[] resultArray = cryptoProvider.TransformFinalBlock(byEnc,0,byEnc.Length);
    return UTF8Encoding.UTF8.GetString(resultArray);
   }
   }
   

7. 最后结果为：Kanxue2019Q1CTF

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