PicoCTF_2017: Coffee

Category: Reverse Engineering Points: 115 Description:

You found a suspicious USB drive in a jar of pickles. It contains this file file.

Hint:

Is there a way to get the source of the program?

Write-up

Using something like JD-GUI, we get the source of the class file

import java.io.PrintStream;
import java.util.Arrays;
import java.util.Base64;
import java.util.Base64.Decoder;

public class problem
{
  public static String get_flag()
  {
    String str1 = "Hint: Don't worry about the schematics";
    String str2 = "eux_Z]\\ayiqlog`s^hvnmwr[cpftbkjd";
    String str3 = "Zf91XhR7fa=ZVH2H=QlbvdHJx5omN2xc";
    byte[] arrayOfByte1 = str2.getBytes();
    byte[] arrayOfByte2 = str3.getBytes();
    byte[] arrayOfByte3 = new byte[arrayOfByte2.length];
    for (int i = 0; i < arrayOfByte2.length; i++) {
      arrayOfByte3[i] = arrayOfByte2[(arrayOfByte1[i] - 90)];
    }
    System.out.println(Arrays.toString(Base64.getDecoder().decode(arrayOfByte3)));
    return new String(Base64.getDecoder().decode(arrayOfByte3));
  }

  public static void main(String[] paramArrayOfString)
  {
    System.out.println("Nothing to see here");
  }
}

Like the source says, I'm way too lazy to work out the schematics, so lets try calling the get_flag() method from another file. This is where my solver comes in! Just compile and run!

$ javac solve.java 
$ java solve
[102, 108, 97, 103, 95, 123, 112, 114, 101, 116, 116, 121, 95, 99, 111, 111, 108, 95, 104, 117, 104, 125]
flag_{pretty_cool_huh}

Therefore, the flag is flag_{pretty_cool_huh}.

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