ångstromCTF 2021: Free Flags!!1!!

Free Flags!!1!!

Category: Rev

50 points

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.

Author: aplet123

Solution

The program asks some questions and then printing the flag.txt file if the answers were correct.

Congratulations! You are the 1000th CTFer!!! Fill out this short survey to get FREE FLAGS!!!
What number am I thinking of???
1234
Wrong >:((((

Let’s take a look into Ghidra…

Ok, so there are three questions.

Answer for the first one is 31337, for the second one we need to find two numbers which fits the condition number1 + number2 == 0x476 && number1 * number2 == 0x49f59, answer for the last question is banana (lol).

I’m a bit lazy, so I used python to find 2nd answer.

number1 = 0x476
number2 = 0

while not (number1 + number2 == 0x476 and number1 * number2 == 0x49f59):
    number1 -= 1
    number2 += 1

print(number1, number2)

Output:

723 419

Let’s try the answers and get the flag.

team7784@actf:/problems/2021/free_flags$ ./free_flags
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}

Flag

actf{what_do_you_mean_bananas_arent_animals}