OffByOne

i hid a qr inside a qr

points: 497

solves: 44

author: Connor Chang

Solution

Honestly very simple, i can’t believe we were the 6th solve

We were given hidden.png which contains a qr that rick rolls you (i’m so proud i didn’t fall for this)

hidden.png

OffByOne implies lsb stego, opening up in gimp and turning on high contrast showed some stuff going on in the top row of pixels

The description immediately made me think this reshapes into a qr, so i checked if there was a perfect square, and yes, this weird yellow row continues for 841 pixels which is \(29^2\)

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from PIL import Image

im = Image.open("./hidden.png")
width, height = im.size
pixels = im.load()

for x in range(0, 841):
    if x % 29 == 0:
        print()
    r, g, b = pixels[x, 0]
    if b == 255:
        print(1, end='')
    else:
        print(0, end='')

python solve.py > qr.txt

Then opening up in QRazybox

New -> Import from Text, then Tools -> Extract QR Information gives the flag

scriptCTF{qrqrqrc0d3s}

someone put a fake flag in the comments lol