= qrgarden =
== 問題 ==
Read a lot, and the flag begins with "ADCTF_".

[[http://adctf2014.katsudon.org/dat/jowoVadKQnmSGupH/qrgarden.png|qrgarden.png]]

== 解法 ==
10000個のQRコードからADCTF_で始まっている物を探しだす問題。

{{{#!highlight python
import Image
import sys
from qr import *

def scan(img, top, left, msize, version):
    w = h = version * 4 + 17
    qr = []
    for y in range(h):
        line = []
        for x in range(w):
            c = img.getpixel((top + msize * x, left + msize * y))
            if c == (0, 0, 0):
                x = 1
            elif c == (255, 255, 255):
                x = 0
            else:
                x = None
            line.append(x)
        qr.append(line)
    return qr

version = 3
qrcode_size = version * 4 + 17
module_size = 3

img = Image.open("qrgarden.png")
img = img.convert('RGB')
for y in range(0, 100):
    for x in range(100):
        qrtxt = scan(img, 29*module_size*x, 29*module_size*y, module_size, version)
        print read_qr(qrtxt)
}}}
{{{
% python scanqr.py| grep ADCTF_
ADCTF_re4d1n9_Qrc0de_15_FuN
}}}

動作が遅い遅いと思っていたら無駄なファイルIOが時間を食っていた……

{{{
% time python scanqr.py | grep ADCTF_
ADCTF_re4d1n9_Qrc0de_15_FuN
python scanqr.py  22.84s user 0.11s system 99% cpu 22.978 total
grep ADCTF_  0.00s user 0.00s system 0% cpu 22.978 total

% workon pypy
% time python scanqr.py| grep ADCTF_
ADCTF_re4d1n9_Qrc0de_15_FuN
python scanqr.py  9.83s user 0.11s system 99% cpu 9.959 total
grep ADCTF_  0.00s user 0.00s system 0% cpu 9.959 total
(pypy)
}}}