Ouija
You've made contact with a spirit from beyond the grave! Unfortunately, they speak in an ancient tongue of flags, so you can't understand a word. You've enlisted a medium who can translate it, but they like to take their time...
Exploitation
Executing the binary takes a long time and outputs the following:
$ ./ouija
Retrieving key.
..... done!
Hmm, I don't like that one. Let's pick a new one.
..... done!
Yes, 18 will do nicely.
..... done!
Let's get ready to start. This might take a while!
..... done!
This one's an uppercase letter!
..... done!
Okay, let's write down this letter! This is a pretty complex operation, you might want to check back later.
..... done!
H
...
And on and on and on, which nobody has time to wait for.
Running strings ouija gives us a lot of output, but the most interesting part is the following:
ZLT{Svvafy_kdwwhk_lg_qgmj_ugvw_escwk_al_wskq_lg_ghlaearw_dslwj!}
This is obviously the encoded flag, since it follows the same format as the other flags. We can try to use a Caesar cipher to decode it. We can use CyberChef to do this. We can use the ROT13 recipe to decode the flag.
After experimenting with the ROT recipe, we find the key that decodes the flag to the proper format, 8. That gives us the following flag:
HTB{Adding_sleeps_to_your_code_makes_it_easy_to_optimize_later!}