Scratch ERCode Writeup

题目描述

I made a ERCode generator! Featuring red-blue encoding, looks more colorful than QRCode, right?

Flag is in the ERCodes.

Scratch: https://scratch.mit.edu/projects/1086251314/

Chilli Level:

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观察 Scratch 源码，从用户输入的文本通过一堆算法，用点阵的方式生成了一个 ERCodes，然后通过一个 pen 画出来，所以我们只需要把附件中的 500 张图片数据通过逆向算法解码就可以得到用户输入的内容

对 1.png 做了下测试，数据是二进制，每个像素代表一位，从内圈到外圈，从菱形的底端开始读取，左边读一个右边读一个

那么我们以此类推，一次性用顺时针把二进制数据读取出来，再反向把后面的数据插入回去，就可以读到数据

from PIL import Image

def get_pixel_hex_color(image_path, x, y):
    # #0000ff 蓝色, #ff0000 红色
    with Image.open(image_path) as img:
        r, g, b = img.getpixel((x, y))
        color = "#{:02x}{:02x}{:02x}".format(r, g, b)
        return "0" if color == "#0000ff" else "1" if color == "#ff0000" else "?"


for img in range(1,501):
    r = 25
    text_bin = ""
    for _ in range(7):
        bin = ""
        x = 240 + 1
        y = 203 + 1 + _
        for i in range(r):
            x -= 1
            y -= 1
            bin += get_pixel_hex_color(f"images/{img}.png", x, y)
        for i in range(r-1):
            x += 1
            y -= 1
            bin += get_pixel_hex_color(f"images/{img}.png", x, y)
        for i in range(r-1):
            x += 1
            y += 1
            bin += get_pixel_hex_color(f"images/{img}.png", x, y)
        for i in range(r-2):
            x -= 1
            y += 1
            bin += get_pixel_hex_color(f"images/{img}.png", x, y)
        r += 1
        for i in range(len(bin)//2+1):
            text_bin += bin[i]
            if i != 0 and i != len(bin)//2:
                text_bin += bin[len(bin)-i]

    text = ""
    for i in range(0, len(text_bin), 8):
        text += chr(int(text_bin[i:i+8], 2))

    print(f"{text}", end="")

运行代码后使用搜索找到 flag