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Solving a CTF challenge

atan profile image atan ・3 min read

In my quest to promote CTFs within the dev.to community, here's a writeup that demonstrates what solving a challenge may look like.

The Task

This challenge was published by user RedK on the CTFLearn platform. (Link here login required) The details are as follows!


F1l3 M1X3R

I think my amazing photo was hit by a mixer and now it is not working. Help me fix it? https://mega.nz/#!Ds0mWaCJ!4uKfJeJwhupG7Tvx8ReTBP1reFgdzRLE3YrN0l-5Jrg hint: visit: https://en.wikipedia.org/wiki/List_of_file_signatures Programming might be useful in this challenge.


Feel free to download and attempt this challenge out before reading how I solved it :)

First steps

After downloading the file fl4g.jpeg, the first thing I did was try to open it.

img attempting to open file

Obviously that didn't work out. The hint in the challenge description lead me to assume that the image's file signature must have been tampered with. Let's take a look at the first few bytes of fl4g.jpeg and compare it to the expected file signature for .jpeg files.

Here's the expected file signature.

FF D8 FF E0 00 10 4A 46 49 46 00 01

I use the xxd command and fl4g.jpeg as an argument to get a hex dump of the first 12 bytes.

$ xxd -l 12 fl4g.jpeg
00000000: e0ff d8ff 464a 1000 0100 4649  ....FJ....FI

At first glance, I saw that the hex values were present, but just in the wrong order.
Using a hex editor (I used 0xed), I deleted the first twelve bytes and replaced it with the correct signature.

$ xxd -l 12 modified_fl4g.jpeg                                                                                                                            
00000000: ffd8 ffe0 0010 4a46 4946 0001            ......JFIF..

Thinking I was done, I opened the file again expecting to be rewarded only to find that the file still wouldn't open....😞

Brainstorming

Fixing the signature didn't work, BUT because the values were all present only scrambled, I decided to take a look at the original file again and noticed a pattern. Every four bytes was reversed in order! FF D8 FF E0 had been reversed to read E0 FF D8 FF and so on for every four bytes of the signature. Fixing just the signature wouldn't get anywhere because it's possible this reversing had happened to the entire file! I wrote a short script to reverse every four bytes of the image in order to test my hypothesis.

The script!

with open("fl4g.jpeg", "rb") as file:
    BUF = 4    
    bytes_rev = b""
    bytes_read  = bytearray(file.read(BUF))

    while bytes_read:
        bytes_rev += bytes_read[::-1]
        bytes_read = file.read(BUF)
    with open("modified_fl4g.jpeg", "wb") as newfile:
        newfile.write(bytes_rev)

To break this down:

with open("fl4g.jpeg", "rb") as file:

Here we open fl4g.jpeg with the rb mode to indicate that we are reading a file in binary mode.

BUF = 4
bytes_rev = b""
bytes_read = bytearray(file.read(BUF))

BUF is set to 4 to indicate that the buffer for each time we read from the file will be four bytes. bytes_rev is set to an empty bytestring so we have a place to store the reversed bytes. The file is then read from and stored as a bytearray into bytes_read.

while bytes_read:
        bytes_rev += bytes_read[::-1]
        bytes_read = file.read(BUF)

Next up we loop as long as bytes_read is True. bytes_rev is appended the reversed bytearray of 4 bytes using slice notation. bytes_read then reads the next set of four bytes from file

    with open("modified_fl4g.jpeg", "wb") as newfile:
        newfile.write(bytes_rev)

Finally, we open a new file and write our bytes to it

Outcome?

Running the script produced a modified_flag.jpeg file with every four bytes reversed.

I opened the file and....

We did it!

The flag is revealed! I took the liberty to censor out the flag text so that you can try it yourself if you'd like!

Posted on by:

atan profile

atan

@atan

Application Security Engineer based in Japan. Penetration Testing and general hackery fun.

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