Coinslot
25
#Hope #Change #Obama2008
nc misc.chal.csaw.io 8000
問題サーバに接続すると、金額が提示されるので、それを1万ドル紙幣～1ドル紙幣、および50セント貨幣～1セント貨幣の枚数で答える問題です。
$15180.83
$10,000 bills: 1
$5,000 bills: 1
(略) 
金額の大きい紙幣、貨幣の順に枚数を聞いてくるので、聞かれた紙幣、貨幣の金額で割った商、余りを計算していけば良いです。商が聞かれた紙幣、貨幣の枚数になり、余りを残りの紙幣、貨幣の計算に引き継ぎいでいきます。Pythonプログラムを下記に載せます。
import itertools
import sys,socket

host = 'misc.chal.csaw.io'
if len(sys.argv) > 1:
    host = sys.argv[1]
port = 8000
if len(sys.argv) > 2:
    host = int(sys.argv[2])

client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client.connect((host, port))

client_file = client.makefile('b')

while True:
buf = client.recv(100)
ary = buf.split('\n')
for tmp in ary:
print tmp
if tmp.startswith('$10,000 bills:'):
c = money // (10000 * 100)
money = money - c * (10000 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$5,000 bills:'):
c = money // (5000 * 100)
money = money - c * (5000 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$1,000 bills:'):
c = money // (1000 * 100)
money = money - c * (1000 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$500 bills:'):
c = money // (500 * 100)
money = money - c * (500 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$100 bills:'):
c = money // (100 * 100)
money = money - c * (100 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$50 bills:'):
c = money // (50 * 100)
money = money - c * (50 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$20 bills:'):
c = money // (20 * 100)
money = money - c * (20 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$10 bills:'):
c = money // (10 * 100)
money = money - c * (10 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$5 bills:'):
c = money // (5 * 100)
money = money - c * (5 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('$1 bills:'):
c = money // (1 * 100)
money = money - c * (1 * 100)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('half-dollars (50c):'):
c = money // (50)
money = money - c * (50)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('quarters (25c):'):
c = money // (25)
money = money - c * (25)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('dimes (10c):'):
c = money // (10)
money = money - c * (10)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('nickels (5c):'):
c = money // (5)
money = money - c * (5)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('pennies (1c):'):
c = money // (1)
money = money - c * (1)
client_file.write(str(c) + "\n")
client_file.flush()
print str(c)
elif tmp.startswith('correct!'):
pass
elif tmp.find('flag') > -1:
sys.exit(1)
elif tmp.startswith('$'):
money = int(tmp.strip().translate(None, '$.'))
else:
sys.exit(1)
実行すると、次のようになります。
(略)
$1681.90
$10,000 bills:
0
$5,000 bills:
0
$1,000 bills:
1
$500 bills:
1
$100 bills:
1
$50 bills:
1
$20 bills:
1
$10 bills:
1
$5 bills:
0
$1 bills:
1
half-dollars (50c):
1
quarters (25c):
1
dimes (10c):
1
nickels (5c):
1
pennies (1c):
0
correct!
flag{started-from-the-bottom-now-my-whole-team-fucking-here}
フラグは、
flag{started-from-the-bottom-now-my-whole-team-fucking-here}