Tags: python programming
Rating: 5.0
# CSAW 2016 Quals
# coinslot
##### Brad Daniels -- USF Whitehatter's Computer Security Club
##### misc -- 25 points
## Description
\#Hope \#Change \#Obama2008
`nc misc.chal.csaw.io 8000`
## Solution
The server gives us the following.
~~~
sh$ nc misc.chal.csaw.io 8000
$0.03
$10,000 bills: 0
$5,000 bills: 0
$1,000 bills: 0
$500 bills: 0
$100 bills: 0
0$50 bills: 0
$20 bills: 0
$10 bills: 0
$5 bills: 0
$1 bills: 0
half-dollars (50c): 0
quarters (25c): 0
dimes (10c): 0
nickels (5c): 0
pennies (1c): 3
correct!
$0.01
$10,000 bills: 1
...
~~~
It's a classic [change-making problem](https://en.wikipedia.org/wiki/Change-making_problem)!
This was relatively straightforward to code a solution to, however I did run into some issues with Python occasionally casting the change string to an incorrect float, so I opted to convert everything to integers (# of pennies).
~~~python
import socket
import re
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("misc.chal.csaw.io", 8000))
resp = s.recv(1024)
print resp,
while len(resp) > 0:
m = re.match(".*\$(\d{1,50})\.(\d\d).*", resp, re.DOTALL)
if m:
change = int(m.group(1)) * 100 + int(m.group(2))
else:
resp = s.recv(1024)
print resp
quit()
denoms = [1000000, 500000, 100000, 50000, 10000, 5000, 2000,
1000, 500, 100, 50, 25, 10, 5, 1]
for denom in denoms:
num = 0
num = change/denom
print num
if num > 0:
change = change - (denom * num)
s.send(str(num) + '\n')
resp = s.recv(1024)
print resp,
~~~
15 minutes later, we get the flag:
~~~
...
$81667.36
$10,000 bills: 8
$5,000 bills: 0
$1,000 bills: 1
$500 bills: 1
$100 bills: 1
$50 bills: 1
$20 bills: 0
$10 bills: 1
$5 bills: 1
$1 bills: 2
half-dollars (50c): 0
quarters (25c): 1
dimes (10c): 1
nickels (5c): 0
pennies (1c): 1
correct!
flag{started-from-the-bottom-now-my-whole-team-fucking-here}
~~~
