53-kilo-med-data

Random Flag Generator

The challenge provides us with a jar file, RandomFlagGenerator.jar, which we have to reverse.

Loading the file into JDec reveals the following code:

import java.util.Base64;
import java.util.Random;
import java.util.Base64.Decoder;
import java.util.Base64.Encoder;

public class RandomFlagGenerator {
   public static void main(String[] var0) {
      String var1 = "ATU6Wy9BFBMqCCsXMVQNRT0lRABlGA==";
      String var2 = "Den e brun";
      String var3 = xorString(base64Decode(var1), var2);
      String var4 = generateRandomString(20, var3);
      System.out.println("Random Flag: " + var4);
   }

   public static String generateRandomString(int var0, String var1) {
      String var2 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz" + var1;
      Random var3 = new Random();
      StringBuilder var4 = new StringBuilder(var0);

      for(int var5 = 0; var5 < var0; ++var5) {
         var4.append(var2.charAt(var3.nextInt(var2.length())));
      }

      return var4.toString();
   }

   public static String xorString(String var0, String var1) {
      StringBuilder var2 = new StringBuilder();

      for(int var3 = 0; var3 < var0.length(); ++var3) {
         var2.append((char)(var0.charAt(var3) ^ var1.charAt(var3 % var1.length())));
      }

      return var2.toString();
   }

   public static String base64Encode(String var0) {
      Encoder var1 = Base64.getEncoder();
      return var1.encodeToString(var0.getBytes());
   }

   public static String base64Decode(String var0) {
      Decoder var1 = Base64.getDecoder();
      return new String(var1.decode(var0));
   }
}

The program base64 decodes var1 and xor it with the string Den e brun. It then uses the result of the xor to generate and print a random flag. Because var4 is not the correct flag, it is reasonable to think that var3 might be the decrypted flag, so we base64 decode var1 and xor it with var2 with CyberChef.