Basic RCE L13

문제

정답은 무엇인가

실행 화면

IDA 분석

입력한 문자열과 복호화한 암호문을 비교해서 성공 메시지를 보여주는 동작을 하는 것 같다.

RijndaelSimple 은 AES 암호화 알고리즘이라고 한다.
계속 찾아보니 C#으로 짜여진 .Net 프로그램이라서 ida에서 분석이 제대로 안된 것이라고 한다.
dotpeek이라는 .net decompiler를 사용해서 분석을 해보도록 하겠다.

Main

RijndaelSimple

IDA로 분석한 동작과 같은 동작을 한다.
decompile된 코드를 가져와서 정답문자열을 볼 수 있게 해보자
Main 사이에 Console.WriteLine(str);를 넣어서 정답 문자열이 출력되게 하였다.

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

public class RijndaelSimple
{
    public static string Encrypt(
      string plainText,
      string passPhrase,
      string saltValue,
      string hashAlgorithm,
      int passwordIterations,
      string initVector,
      int keySize)
    {
        byte[] bytes1 = Encoding.ASCII.GetBytes(initVector);
        byte[] bytes2 = Encoding.ASCII.GetBytes(saltValue);
        byte[] bytes3 = Encoding.UTF8.GetBytes(plainText);
        byte[] bytes4 = new PasswordDeriveBytes(passPhrase, bytes2, hashAlgorithm, passwordIterations).GetBytes(keySize / 8);
        RijndaelManaged rijndaelManaged = new RijndaelManaged();
        rijndaelManaged.Mode = CipherMode.CBC;
        ICryptoTransform encryptor = rijndaelManaged.CreateEncryptor(bytes4, bytes1);
        MemoryStream memoryStream = new MemoryStream();
        CryptoStream cryptoStream = new CryptoStream((Stream)memoryStream, encryptor, CryptoStreamMode.Write);
        cryptoStream.Write(bytes3, 0, bytes3.Length);
        cryptoStream.FlushFinalBlock();
        byte[] array = memoryStream.ToArray();
        memoryStream.Close();
        cryptoStream.Close();
        return Convert.ToBase64String(array);
    }

    public static string Decrypt(
      string cipherText,
      string passPhrase,
      string saltValue,
      string hashAlgorithm,
      int passwordIterations,
      string initVector,
      int keySize)
    {
        byte[] bytes1 = Encoding.ASCII.GetBytes(initVector);
        byte[] bytes2 = Encoding.ASCII.GetBytes(saltValue);
        byte[] buffer = Convert.FromBase64String(cipherText);
        byte[] bytes3 = new PasswordDeriveBytes(passPhrase, bytes2, hashAlgorithm, passwordIterations).GetBytes(keySize / 8);
        RijndaelManaged rijndaelManaged = new RijndaelManaged();
        rijndaelManaged.Mode = CipherMode.CBC;
        ICryptoTransform decryptor = rijndaelManaged.CreateDecryptor(bytes3, bytes1);
        MemoryStream memoryStream = new MemoryStream(buffer);
        CryptoStream cryptoStream = new CryptoStream((Stream)memoryStream, decryptor, CryptoStreamMode.Read);
        byte[] numArray = new byte[buffer.Length];
        int count = cryptoStream.Read(numArray, 0, numArray.Length);
        memoryStream.Close();
        cryptoStream.Close();
        return Encoding.UTF8.GetString(numArray, 0, count);
    }
}


public class RijndaelSimpleTest
{
    [STAThread]
    private static void Main(string[] args)
    {
        string plainText = "";
        string cipherText = "BnCxGiN4aJDE+qUe2yIm8Q==";
        string passPhrase = "^F79ejk56$£";
        string saltValue = "DHj47&*)$h";
        string hashAlgorithm = "MD5";
        int passwordIterations = 1024;
        string initVector = "&!£$%^&*()CvHgE!";
        int keySize = 256;
        RijndaelSimple.Encrypt(plainText, passPhrase, saltValue, hashAlgorithm, passwordIterations, initVector, keySize);
        string str = RijndaelSimple.Decrypt(cipherText, passPhrase, saltValue, hashAlgorithm, passwordIterations, initVector, keySize);
        Console.WriteLine(str);
        while (true)
        {
            Console.WriteLine("Please enter the password: ");
            if (!(Console.ReadLine() == str))
                Console.WriteLine("Bad Luck! Try again!");
            else
                break;
        }
        Console.WriteLine("Well Done! You cracked it!");
        Console.ReadLine();
    }
}

정답