Div

Author: NoobMaster

I love division

the python server code was given in chall.py

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22

import os
import decimal
decimal.getcontext().prec = 50

secret = int(os.urandom(16).hex(),16)
num = input('Enter a number: ')

if 'e' in num.lower():
    print("Nice try...")
    exit(0)

if len(num) >= 10:
    print('Number too long...')
    exit(0)

fl_num = decimal.Decimal(num)
div = secret / fl_num

if div == 0:
    print(open('flag.txt').read().strip())
else:
    print('Try again...')

basically you want to input a number large enough that it divides the 16bit secret to give zero, but you have certain checks preventing you from inputting large numbers

lets have a look at the decimal python package being used

and immediately we see this

A decimal number is immutable. It has a sign, coefficient digits, and an exponent. To preserve significance, the coefficient digits do not truncate trailing zeros. Decimals also include special values such as Infinity, -Infinity, and NaN. The standard also differentiates -0 from +0.

Infinity is only 8 characters and bypasses the checks to give us the flag

❯ nc play.scriptsorcerers.xyz 10166
Enter a number: Inf
scriptCTF{70_1nf1n17y_4nd_b3y0nd_75409d50f3a7}