Introduction #
The annual main event of CTF in Asia Pacific University (APU): International Battle of Hackers, hosted by FSEC-CC, with amazing CTF players from not only Malaysia, but other countries such as Singapore and Indonesia participating.
The challenges were difficult in my opinion, and most of them were removed after the event, therefore I did not manage to screenshot the solution for most of them.
Therefore, here is my humble writeup for one of the crypto challenge: I’ll Kill Yall. Enjoy.
It's honest work
Crypto: I’ll Kill Yall #
The challenge provides an attachment of a hand-written message, looks like it’s ciphered.
Creepy Message
After searching in the web, the cipher is identified as the ‘Zodiac Killer Cipher’. One of the biggest mystery till this day.
At first the cipher text was unreadable, however, the symbol with a circle and cross should be at the bottom, therefore the image is rotated 180 degrees.
Found this online
Readable now..
To crack the cipher, head on to this webpage and enter the code manually.
This is the returned message, which doesn’t mean a thing.
What does it mean?
To reveal the full ciphered message, increase the height byte value using Hexeditor, and the full message is revealed.
Increase height byte
Full encrypted message
Note that the original image cannot be rotated before changing the height byte, else the note will not reveal.
Failed to reveal message
Rotate the revealed cipher and redo the deciphering process , a plaintext message will show 😨.
Deciphered Message
The flag should start after ‘THIS IS THE FLAG’, which is ‘KILLINGPEOPLEE?AFSTOPFED’. But the flag was rejected when entered.
After sanity checking with the challenge creator, turns out the question mark should be replaced by ‘z’, as there is a bug within the calculation of the deciphering process, causing the question mark to appear.
Sanity Check Message
I was dumb enough to try the new flag ‘KILLINGPEOPLEEZAFSTOPFED’, which was rejected again btw.
After reading the message a few times, it doesn’t make sense to include everything after EZAF.
FLAG: IBOH24{KILLINGPEOPLEEZAF}
Conclusion #
Our team have won 4th place for local category! 🥳
Could not have done it without my teammates:
ZD & Yvonne 🥰🥰🥰
Till next time! 😬
