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

class Program
{
    static void Main()
    {
        PasswordDeriveBytes pdb = new PasswordDeriveBytes("Kanxue2019", null);
        Byte[] Key = pdb.GetBytes(0x20);
        
        Encoding unicode = Encoding.UTF8;
        Byte[] IV = unicode.GetBytes("Kanxue2019CTF-Q1");
       
        string plaintext = null;
        byte[] cipherText = Convert.FromBase64String("4RTlF9Ca2+oqExJwx68FiA==");
        using (RijndaelManaged rijAlg = new RijndaelManaged())
        {
            rijAlg.Key = Key;
            rijAlg.IV = IV;

            ICryptoTransform decryptor = rijAlg.CreateDecryptor(rijAlg.Key, rijAlg.IV);

            using (MemoryStream msDecrypt = new MemoryStream(cipherText))
            {
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                    {
                            plaintext = srDecrypt.ReadToEnd();
                    }
                }
            }

        }
        Console.WriteLine(plaintext);
    }
}
结果为：Kanxue2019Q1CTF