Program creates a buffer with 32 random bytes followed by flag data and does a special string comparison agains a user input string. This comparison uses SSE instructions and leaks information about non-matching characters. You can perform a binary search attack character by character to leak the flag.
Full writeup here: [https://ctf.harrisongreen.me/2019/bsidessf-ctf/fastflag/](https://ctf.harrisongreen.me/2019/bsidessf-ctf/fastflag/)