Task:
There must be a hidden message somewhere.
Hints:
* 2 bpp. That’s incredible.
* 2x3 matrix embedding.
* Jessica Fridrich would solve this task easily.
download file (2.5 MB) 

import Image
def tlsb():
    allData =''
    data = ''
    im = Image.open('stego.png')
    im.convert("RGB")
    (w, h ) = im.size
    bi =0
    k = 3121
    for j in range(h):
        for i in range(w):
            if k==0:
                break
            bi+=2
            (r, g, b) = im.getpixel((i, j))
            c0 = r % 2
            c1 = g % 2
            c2 = b % 2
            a = (c0+c1) %2
            b = (c1+c2) %2
            data+=str(a)
            data+=str(b)
            if (bi == 8):
                k-=1
                allData+=chr(int(data, 2))
                data=''
                bi=0
    print allData
    with open('flag.png', 'wb') as f:
        f.write(allData)
tlsb()


Result: