这是个.net。.net额，不靠谱。
利用dnSpy打开，并在函数入口点断点。

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

// Token: 0x02000003 RID: 3
internal class a
{
    // Token: 0x06000004 RID: 4 RVA: 0x0000209B File Offset: 0x0000029B
    private static void a(string[] A_0)
    {
        Console.WriteLine("Please Input Serial:");
        if (global::a.a(Console.ReadLine(), "Kanxue2019") == "4RTlF9Ca2+oqExJwx68FiA==")
        {
            Console.WriteLine("Congratulations!  : )");
            Console.ReadLine();
        }
    }

    // Token: 0x06000005 RID: 5 RVA: 0x000020D4 File Offset: 0x000002D4
    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);
    }

    // Token: 0x04000003 RID: 3
    private const string a = "Kanxue2019CTF-Q1";

    // Token: 0x04000004 RID: 4
    private const int b = 256;
}

得出加密方式，反转一下即可。
如下代码解决：

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        public static string detest(string A_0, string A_1)
        {

            byte[] bytes = Encoding.UTF8.GetBytes("Kanxue2019CTF-Q1");
            byte[] bytes2 = Convert.FromBase64String(A_0);
            byte[] bytes3 = new PasswordDeriveBytes(A_1, null).GetBytes(32);
            ICryptoTransform transform = new RijndaelManaged
            {
                Mode = CipherMode.CBC
            }.CreateDecryptor(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 System.Text.Encoding.Default.GetString(inArray);
          //  return Convert.ToString(inArray);
        }

        static void Main(string[] args)
        {
            Console.WriteLine("Please Input Serial:");
            Console.WriteLine( detest("4RTlF9Ca2+oqExJwx68FiA==", "Kanxue2019"));
        }
    }
}

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