Introduction
Uhmmm… It’s been a long time since I last touched any CTF stuff. The last time I played was back in THEMCTF v2.
I also contributed some challenges to it (thanks for the opportunity, BTW! XD) including a bunch of, well, sh1t challenges
Apart from those sh1t challenges, I really want to talk about Rev/Old Cassette. (Sorry for taking forever to release the author’s writeup! QAQ)
Lastly, I also saw some GOATED writeups from other players!!! E.g. (in random order):
- https://advaybagaria.github.io/Old_Cassette/
- https://advaybagaria.github.io/old_cassette_/
- https://github.com/AdvayBagaria/Old_Cassette
- https://github.com/Abdelkad3r/themectf-2026/tree/main/rev/old-cassette (If I missed yours, please contact [email protected] to get it added!)
Annnd, I noticed that many players actually chose Rev/Old Cassette for their writeups and submitted them to the writeup contest. (I’m so happy sob) That’s another major reason why I really wanted to write this blog lol
Uhmmm… I think this challenge is just an entry-level reverse engineering challenge, and honestly, some of the players’ writeups are even better than I expected. So, if you are looking for a sophisticated or perfect writeup, please check out the references above.
Challenge
OK, OK. I know you guys also use AI to solve challenges and write writeups, but hey, I’d say it’s totally fair game (cuz I also use AI to author challenges :P). Now, lez reveal the first myth.
The “K7” in The Flag
The flag is THEMCTF?!{0LD_T4P3_N3V3R_D1E5K7}.
I bet most of you guys were wondering, “What the hell does K7 even mean???” right?
Like, OLD TAPE NEVER DIES makes total sense, but K7 <- ???
I noticed that most writeups claimed K7 is French slang for “cassette” (ka-sept $\rightarrow$ cassette).
However guys, I think you over-thought this one. It was literally just two randomly typed characters to fulfill the 32-byte flag length requirement lmfao
About the Challenge
This challenge adopted CHIP-8 as the target architecture because:
CHIP-8 is an interpreted programming language, developed by Joseph Weisbecker on his 1802 microprocessor.
Which 100% fits the background story. (So guys, remember to read the challenge description :)
Another reason I chose this language is that it’s widely known. You know how Modern Rev is often heavily combined with OSINT, right? Especially for this kind of customized VM challenge.
The Catch (The Math)
Now, here comes the craziest part of the challenge: a grand total of 17,524,898,219,365 iterations. If you run it at a speed of 100,000,000 iterations/sec, it would take about 2 days to recover the whole flag by brute force. But I believe that under the assistance of AI (or some smart optimization), you guys recovered it much faster :)
To be completely honest, I didn’t know what else to write in this specific blog post. This challenge is built on quite a few clichés like unoptimized VM, PRNG, custom encryption ts yk, the typical rev challenge
Actually, I came up with the idea for this challenge a long time ago but never deployed it in any public CTF. THEMCTF was its world premiere. Hope you enjoyed it!