CodeShell.kr - Quiet Frame

Challenge

256×160 的彩色斜条纹图，题面提示画面里有眼睛看不到的东西。难度在于位平面候选极多（通道 × 主序 × 翻转），要靠可输出的可打印性筛出唯一正确的那一个。

The picture remembers what the eye misses. Recover the flag in the form CodeShell{...}

图像记得眼睛漏掉的东西。

https://codeshell.kr/challenges/quiet-frame/

Solution

附件 quiet-frame.png 是 256×160 RGB 图，画面是彩色斜条纹。

Step 1：逐通道取 LSB，按 8 位 MSB-first 组字节，枚举行/列主序与各种翻转组合。

Step 2：只有 B 通道 row-major 在字节偏移 0 处出现明文，第 23 字节以 \x00 结束；其余位置是条纹自身的规律位（0x55 / 0xAA）。

CodeShell{QUIET_PIXELS}\x00UUUUUUUU\xaa\xaa...

Script

#!/usr/bin/env python3
"""CodeShell.kr - Quiet Frame (Stegano, 50p) solver.

The payload is in the blue channel's LSB in row-major order, packed 8 bits
MSB-first, terminated by a NUL byte.  The stripes themselves contribute regular
bits (0x55 / 0xAA), so the payload is the leading run of printable bytes.
"""
import numpy as np
from PIL import Image

PNG = "assets/challenge-images/quiet-frame.png"


def main():
    a = np.array(Image.open(PNG).convert("RGB"))
    flat = (a[:, :, 2] & 1).flatten()          # B channel LSB, row-major
    out = bytearray()
    for i in range(0, len(flat) - 7, 8):
        v = 0
        for bit in flat[i:i + 8]:
            v = (v << 1) | int(bit)
        out.append(v)
    print(out.split(b"\x00")[0].decode())


if __name__ == "__main__":
    main()

CodeShell{QUIET_PIXELS}