What kind of cipher is this?
I didn’t recognize the format of the encrypted message that was provided, so I started with google. I figured I could google the name of the challenge and that the Google Gods would steer me in the right direction….
Yep, google did its thing for me!
How do we crack it?
I’ll be honest, I didn’t really do a lot of research here trying to understand how to manually crack it. More googling showed that it’s known to be pretty easily brute-forced. I love the decode.fr site for these types of things. They do have an affine cipher brute-forcing tool. I plugged the input into that, and it presented a wide variety of results based on the values of A/B. The first one it found looked to be the flag:
Conclusion
I didn’t do myself any favors by plugging this into a tool without learning more about it, but it does seem that like this is a very “CTF” type of challenge, as I haven’t ever seen AFFine Ciphers used in the wild. While it’s possible, it’s extremely uncommon and trivial to crack.
