POV: You made :nerd: challs for bwctf
I authored some challenges to the first instance of Blue Water CTF, mostly nerdy math challs, but not all of them entirely crypto.
Very happy to get my name there! (I am @ah_p_uh, come follow me)
All my challs are uploaded on my GitHub and you should be able to find brief solutions in English and solve codes, so I’ll pass them this time. The reason is totally not because I am too lazy for it :))
Instead, I will share my journey for bwctf this time, including what I went through and memorable things.
An anonymous saint (***beo) once said the following phrase, and I was deeply touched by it.
@genni, when are you going to post about how to spin up 100 cores for Coppersmith?
1. Are you even a member of Blue Water?
No :(
However at the end of June, RBTree asked me if I wanted to author a challenge for pbctf(Before the name changed to bwctf). If I remember correctly, I was on a family trip at that time, so it was totally unexpected and made my day. Thanks to perfect blue and Water Paddler, and special thanks to RBTree for the opportunity!
I remember last year’s pbctf when I played solo with soon_haari solo play with beer, see 2023 pbctf. rkm0959 authored one challenge back then, and I thought, “Oh I want to guest author pbctf too one day lol”.
I immediately chose what challenge to reveal in pbctf. I had multiple challenges in my secret folder and chose the hardest and proudest one. That challenge was Collider.
2. So what did you do during the gap, only author collider?
Not at all, RBTree, joseph, and I tried to cook up some more awesome crypto challenges! RBTree suggested multiple ideas including {redacted} and {redacted}, but we couldn’t make all of them into a cool CTF challenge. Gladly, he eventually cooked up super interesting challenges: MD5.01 & MD5.1 and Fruit Math.
Also, I finalized a GCM challenge thanks to RBTree’s idea.
It translates to:
- “The gcm challenge title is Greatest Common Multiple”
3. Wait wait, Greatest Common Multiple? That’s your weird chall from codegate finals!
Yeah, the thing is, I already planned a challenge for codegate, and was going to call it Greatest Common Multiple II because codegate was happening after pbctf. But pbctf got delayed, and codegate came first, so I had to make the codegate chall Greatest Common Multiple.
I asked ChatGPT to make a new name for the bwctf GCM challenge. And that is how Counter Strike was born.
I still think Counter Strike is a great challenge, kinda different from casual GCM challs. Although codegate one is my masterpiece.
4. How hard was collider? You said it’s the hardest one you’ve ever made.
I expected 0~1 solves within the CTF time which is 36 hours, and (fortunately?) ended with 0 solve. We made a first-blood bounty and didn’t reveal the solution even after the CTF ended, and @user202729 blooded it after around 2 days, congratulations!
His solution has some similarities with mine, but different parts too. It should be worth reading both the solutions, please join the bwctf discord and check #crypto for detailed discussions!
Oh also, the previous code was implemented with my handmade GF.py for field elements and polynomials which was 5kb.
I later found a thing called sympy and changed to that, so the source code was much shorter =)
4. I also saw you drawing a cool art for Fruit Math!
Cool drawing isn’t it?
Also, I’m proud to say I contributed to finding an additional idea to make the root calculation much quicker!
5. And what about RSAjail challs?
This, I give the credit to D1N0 who authored a similar pyjail challenge for internal CyKor CTF.
It was a challenge to get a shell with repeated max 4 characters where exec, eval is blocked, and the intended solution was using vars.
D1N0’s solution uses _ since it stores the latest evaluated data.
Python 3.11.1 (v3.11.1:a7a450f84a, Dec 6 2022, 15:24:06) [Clang 13.0.0 (clang-1300.0.29.30)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> ("soon_haari", "babo")
('soon_haari', 'babo')
>>> _
('soon_haari', 'babo')
And should be something like this:
vars
_()
...
However, my unintended solution was using that it allow multiple lines when a bracket is open.
(
vars
())
...
I found the concept very interesting and created a challenge that requires both of the interesting properties for Python interpreter.
I thought the solution to RSAjail-1 was clever and hard and thought not many people would solve this, but 8 teams solved the final version of the RSAjail series. People are just too good at jails with a hacker mindset :/
Lyndon came up with a really clever idea to do a big integer division with a 1-byte limit, making his(her) solution super shorter than others! :brain:
Also, my bro Teddy wrote a cool write-up for all 3 RSAjail series step-by-step, thanks to teddy as well!
6. Aaaaaaand now here we are, back in the present, the CTF is over!
Hope everyone who played bwctf had a great time, it always feels great watching people attempting or eventually solving my challenges.
Lastly, it was a great time organizing bwctf, with super talented people, as everyone knows pb and wp include some of the best hackers around the world. And thanks Sampriti for handling everything related to infra.. :pray:
This post is rather written freely than written in a write-up format, but I wanted to share my journey with you. Thanks for reading everyone!
- Btw dear crypto addicts, RBTree’s MD5.1 is still not blooded and has a 100 bounty, please try it!!
