Heavy Blue Writeup

Posted 2022-12-30 末初mochu7

一位师傅发的、来源不知、但是觉得这题可以记录一下

链接：https://pan.baidu.com/s/1GoPtUU4uqa4yYMfetRRZuQ 
提取码：v87q 

flag.bmp

读取RGB数据，只有Blue通道有数据，而且看起来都在32-126的ASCII可显示字符范围

将Blue通道的数据提取出来转换成字符，然后拼接起来，以一行一行的方式读取

from PIL import Image
from base64 import *

img = Image.open('flag.bmp')

width, height = img.size
blue_data = ''
for h in range(height):
	for w in range(width):
		pix = img.getpixel((w, h))
		blue_data += chr(int(pix[2]))
with open('flag', 'wb') as f:
	f.write(b64decode(blue_data))

XZ的压缩文件，接下来就是套娃解压，只不过这里混合了xz、gz、zip三种压缩格式的套娃压缩

通过识别它们的文件头进行进一步解压

.xz      FD 37 7A 58
.gz      1F 8B 08 08
.zip     50 4B 03 04

Python简单处理

import lzma
import zipfile
import gzip
import os

def dec_xz(current_file, count_num):
    with open(current_file, 'rb') as f:
        next_file = 'flag'.format(count_num)
        with open(next_file, 'wb') as f1:
            f1.write(lzma.decompress(f.read()))
    count_num += 1
    return next_file, count_num

def dec_zip(current_file, count_num):
    zf = zipfile.ZipFile(current_file, 'r')
    zf.extractall('./')
    next_file = 'flag'.format(count_num)
    os.rename(zf.namelist()[0], next_file)
    count_num += 1
    return next_file, count_num

def dec_gz(current_file, count_num):
    with open(current_file, 'rb') as f:
        next_file = 'flag'.format(count_num)
        with open(next_file, 'wb') as f1:
            f1.write(gzip.decompress(f.read()))
    count_num += 1
    return next_file, count_num

current_file = 'flag.xz'
count_num = 1
while True:
    try:
        with open(current_file, 'rb') as f:
            file_header = f.read(4).hex().upper()
        if file_header == 'FD377A58':
            current_file, count_num = dec_xz(current_file, count_num)
        elif file_header == '1F8B0808':
            current_file, count_num = dec_gz(current_file, count_num)
        elif file_header == '504B0304':
            current_file, count_num = dec_zip(current_file, count_num)
        else:
            with open(current_file, 'r') as f:
                print(f.read())
            break
    except Exception as ex:
        print(ex)

flag1d3ca261-82e7-4710-a6d5-d6878176aab0