# HKCERT CTF 2021 Postmortem (III): The Reverse Challenges

As the third part of the series, three reversing challenges will be included: The Hardest Path, A Junior Mathematician and Let’s Chill.

# HKCERT CTF 2021 Postmortem (II): Harder Crypto Challenges

In this part, three harder crypto challenges will be covered: Tenet: The Plagarism, Sratslla SEA and Sign in Please, Again.

# HKCERT CTF 2021 Postmortem (I): Easier Crypto Challenges

Black Bauhinia coorganized HKCERT CTF 2021 and helped 95% of the technical stuffs, including challenge setting, platform development, infrastructure and etc. I will be writing a series of blog posts talking about the contest, and the first four would be the writeups of the challenges those I wrote.

In the first blog post, we will be going through four easier crypto challenges: A Joke Cipher, Cipher Mode Picker, Key Backup Service 1 and Key Backup Service 2.

# BSides Ahmedabad CTF 2021 Writeup

No, we did not win. I see that coming and took a screenshot during the game.

BSides Ahmedabad CTF 2021 is organized by @zer0pts. That implies that players could spend a day happily working on the challenges. I enjoyed working on the challenges a lot. I’ll go through the harder crypto challenges and the reversing challenge called Collapsed License in this blog post.

# Factoring a special RSA modulus from ASIS CTF 2021 Quals

LagLeg is a crypto challenge in ASIS CTF Quals 2021. We are asked to factor a given $n$ with $n = (r^5 + s)(r + s)$ as part of the challenge. Hereby $r$ and $s$ are respectively 256 and 64 bits long. I think my approach used is very unique and creative - and definitely worth mentioning. It does not mean that it is a good approach, however.

I will record the exact steps I solved the challenge. However, I will update the bounds and prove the whole thing so that it looked more rigorous. The full code will be attached at the end of the blog post, too.