## Background⌗

I started to collect and document cryptography challenges in CTFs. I'll describe the challenge (or the cryptographic parts for the challenge) briefly and will not be walking through their solutions. In short, the series of the blog posts is served as a directory of crypto challenges happened in CTFs. The objective is to let CTF players on cryptography to identify challenges those they found interesting. In particular, the challenges marked with 🔥 are recommended.

Undeniably, the difficulty is biased and solely determined by the crypto part (i.e., if there are reversing, obfuscation and what-not, they are not included). Look for the solve count if you need an less subjective metric. Also, the difficulty is independent of the challenge quality.

I'll filter out the challenges owing to its nature. I may not include every single cryptography-tagged challenges. Here are some of the challenge types that I would exclude from the list:

• Classical ciphers. I will exclude them unless there are something more than that. Classical ciphers should be well-known for crypto players.
• Unrelated to cryptography. This includes but not limited to:
• Encoding challenges. Encoding is not encryption anyway.
• Cryptocurrency challenges. You knew it.
• For challenges those can be solved without cryptographic elements, I'll see if the intended solution can be solved cryptographically. For example, notbefoooled in DEF CON CTF 20201 and OV in De1CTF 20202 will be included. On the other hand, quaternion-revenge in redpwnCTF 20213 is filtered because the intended solution does not involve cryptographic elements but programming language quirkiness.

Last but not least, the list is nowhere complete since I do not play each and every single CTFs... Feel free to ping me if you think I should document the challenges for the CTFs I didn't play.

## Challenge Directory⌗

1. yyyyyyy (2020) "DEFCON CTF Quals 2020: notbefoooled"
https://hxp.io/blog/72/DEFCON-CTF-Quals-2020-notbefoooled/
2. Mystiz (2020): "De1CTF 2020 Writeup"
https://mystiz.hk/posts/2020-05-05-de1ctf/#ov
3. CTFtime.org (2021) "quaternion-revenge"