This was a pretty simple challenge, each character was sent in a function and compared with a hardcoded value, always the comparison was like `cmp rdi, 0x7a`.
So i wrote a quick bash command to get all such instructions and parse their hex values. Luckily there weren't any other `cmp rdi` instructions.
Then we just had to base64 encode the output and send to the server.

objdump crackme -d -M intel| grep 'cmp rdi' | python3 -c 'while 1: print(chr(int(input()[-2:],16)),end="")' 2>/dev/null