ASIS CTF Stego/Chessboard

chessboard.py

import Image
import base64

chess = Image.open('chess.png')

b64charset = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
stack = []

for y in range(8):
    for x in range(8):
        index = x + (7-y)*8
        p = (x*32, y*32)
        color = chess.getpixel(p)
        if color != (0, 0, 0):
            stack.append((color[0], index))
stack.sort()

encoded = ''.join([b64charset[i[1]] for i in stack])
flag = base64.b64decode(encoded)
print flag