Tags: ppc
Rating:
# Magic Brackets
## Task
## Solution
First, I looked at the source code of the application:
```Python
import random
print("Welcome, traveller!")
print("I have a task for you.")
print("I will give you some numbers.")
print("The only thing you have to do is send them back to me.")
print("If you do everything correctly, I will give you the flag.")
print("Sounds easy, right?")
print("However, to prove that you are the true bracket master, you can only use the following symbols: ()[]<>|")
print("Well here we go:")
for _ in range(100):
num = random.randint(100, 100000)
print(num)
try:
if num != eval("".join([c for c in input() if c in "()[]<>|"])):
print("Sorry, that's wrong :c")
exit(0)
except:
print("Ouch, that hurts :c")
exit(0)
print("Good job!")
print("Your flag is [REDACTED]")
```
So, we need to enter the string, which contains only symbols from this list: `()[]<>|`. Okay. I was confused for a while. But after about an hour and a half I came up with this code:
```Python
>>> [()] > []
True
```
Ha, I made 1. But I need numbers in range 100..100000. Now I remembered about bit shifts. And here we go:
```Python
>>> ([()] > []) << ([()] > [])
2
```
Yeah! Now, shifting this, I can get any number 2<sup>n</sup>. And las thing left is to get any number `2 % n != 0`. Luckily, I can use `|`. So I can use it to make any uneven number:
```Python
>>> ([()] > []) << ([()] > []) | ([()] > [])
3
```
Using all these things I made this code to solve the task:
```Python
import socket
def to_brackets(number: int):
bint = bin(number)[2:]
one = "([()]>[])"
result = one
for bit in bint[1:]:
if bit == "1":
result = f"(({result}) << ({one})) | {one}"
else:
result = f"(({result}) << ({one}))"
return result
def main():
s = socket.socket()
s.connect(("tasks.realctf.pro", 28392))
print(s.recv(1024).decode())
while 1:
data = s.recv(1024).decode()
print(data)
try:
num = int(data.strip())
res = to_brackets(num) + "\n"
print(res)
s.send(res.encode())
except:
break
print(to_brackets(543))
if __name__ == "__main__":
main()
```
Run it and here we go:
Flag: `HSE{8RrrR4cK3t_P0O0w3rrr}`
