Rating:
# Angstrom FREE FLAGS Write Up
## Details:
Points: 50
Jeopardy style CTF
Category: Reverse Engineering
Comments:
Clam was browsing armstrongctf.com when suddenly a popup appeared saying "GET YOUR FREE FLAGS HERE!!!" along with a download. Can you fill out the survey for free flags?
Find it on the shell server at /problems/2021/free_flags or over netcat at nc shell.actf.co 21703.
## Write up:
Decompiling the main function we get:
```c
int __cdecl main(int argc, const char **argv, const char **envp)
{
int v4;
int v5;
int v6;
int v7;
char s[264];
unsigned __int64 v9;
v9 = __readfsqword(0x28u);
puts("Congratulations! You are the 1000th CTFer!!! Fill out this short survey to get FREE FLAGS!!!");
puts("What number am I thinking of???");
__isoc99_scanf("%d", &v7;;
if ( v7 == 31337 )
{
puts("What two numbers am I thinking of???");
__isoc99_scanf("%d %d", &v6, &v5;;
if ( v5 + v6 == 1142
&& v5 * v6 == 302937
&& (puts("What animal am I thinking of???"),
__isoc99_scanf(" %256s", s),
s[strcspn(s, "\n")] = 0,
!strcmp(s, "banana")) )
{
puts("Wow!!! Now I can sell your information to the Russian government!!!");
puts("Oh yeah, here's the FREE FLAG:");
print_flag();
v4 = 0;
}
else
{
puts("Wrong >:((((");
v4 = 1;
}
}
else
{
puts("Wrong >:((((");
v4 = 1;
}
return v4;
}
```
We can see that we need to answer a few questions. The first one we need to answer 31337, the second one we need to find two numbers that match the equation, I chose 723 and 419, and for the last one we need to answer banana. When I ran the server and entered the answers I got:
```
Congratulations! You are the 1000th CTFer!!! Fill out this short survey to get FREE FLAGS!!!
What number am I thinking of???
31337
What two numbers am I thinking of???
723 419
What animal am I thinking of???
banana
Wow!!! Now I can sell your information to the Russian government!!!
Oh yeah, here's the FREE FLAG:
actf{what_do_you_mean_bananas_arent_animals}
```
