MRCTF My secret

My secret

知识点：wireshark基本操作，shadowsocks3.0源码利用，拼图（os脚本编写能力），

根据这里的信息可以知道，tcp所传输的源数据是在target address后面的数据。那么我们现在去寻找target address(即wireshark中的Destination)

那么我们已经找到了，即第一张图中的2f 5e ca a8。我们取走后面的数据存储为文件。

通过导出字节流文件可以得到一下两个文件

通过010拿走16进制编码数然后进行解密。

D1 DF BC 46 8E 33 B2 AE A4 E7 D7 4F 35 B2 0D 0A
CE EC 19 F7 2B FC B7 64 1B E8 55 12 5C 53 F1 08
C7 B9 FD EE B1 16 B5 79 C2 EE 6F F6 3D 74 8D 54
D6 4D DC 6B A2 50 C7 0C 32 5F 8E 38 9D 78 73 AA
88 1E 40 2C BF 32 54 E8 6C 6C 33 E2 B7 8E CA 89
9F 51 BB 36 87 8F AF CF EA 75 3A 10 CE 0D A7 C7
9F F7 84 91 AF 1D 53 81 87 97 33 5E 2D 7C DF

那么我的思路在这里就停滞了。经过师傅的提醒和帮助下，我下载了shadowsocks3.0版本的源码（python版），然后很容易找到一个显眼的cryptor.py文件。（这里有一个误区，如果你下载到的源码中没有题目所示的aes-256-gcm加密方法，后续是无法进行解密的。因此需要下载到相应的版本的shadowsocks3.0）

我们仔细看一下这个加密的算法。

加密算法如下。当设置了iv_sent时进行iv的设置，false时不设置iv

然后发现这里是自带decrypt函数的。那么数据包解密就简单多了。

这个函数对数据进行了iv的判断和提取。我们只需要传入完整的数据包进行解密就可以了。

with open('tcp.txt','rb') as f:
    encrypted_data = f.read()

c = Cryptor('db6c73af3d8585c', 'aes-256-gcm')

data = (c.decrypt(encrypted_data))
with open('tcp2.txt', 'wb') as f:
    f.write(data)

对数据进行解密，然后就可以在tcp2.txt中看到我们想要看到的数据包了！

那么数据该提取哪一部分呢？红色部分或者蓝色部分。也就是请求部分和响应部分。

拿到压缩包后就是一个2500个图片的拼图题。hint为gps。

我们在exiftool中可以找到图片的gps值。编写脚本进行提取并修改名字。（我写的脚本）

import os
import re

path = r"/root/Desktop/jigsaw on earch"  # 文件夹目录
files = os.listdir(path)  # 得到文件夹下的所有文件名称

for file in files:  # 遍历文件夹
	cmd=r"exiftool {0} | grep GPS\ Position".format(re.escape(file))
	#print(cmd)
	#print(file)
	gpsTemp = os.popen(cmd).read()

	for i in range(len(gpsTemp)):
		if gpsTemp[i]==':':
			num1 = int(gpsTemp[i+2:i+4].strip())
		if gpsTemp[i] == ',':
			num2 = int(gpsTemp[i+2:i+4].strip())
	fileName = num1*50+num2
	cmdNext = "mv {0} {1}\.png".format(re.escape(file),fileName)
	#print(cmdNext)
	os.system(cmdNext)
	#print(num1*50+num2)
	#print(gpsTemp)

后面拼图脚本不会写了。

师傅放了wp出来。看了一下，真复杂。看不懂。先进行rename

import os
import piexif
from PIL import Image

file = os.listdir('jigsaw')

def calc_file(x, y):
    num = str(y * 50 + x).zfill(4)
    return f'jigsaw/{num}.png'

for i in file:
    img = Image.open(f'jigsaw/{i}')
    exif_dict = piexif.load(img.info['exif'])
    img.close()
    latitude = exif_dict['GPS'][piexif.GPSIFD.GPSLatitude]
    longtitude = exif_dict['GPS'][piexif.GPSIFD.GPSLongitude]
    y = latitude[0][0]
    x = longtitude[0][0]
    file = calc_file(x, y)
    os.rename(f'jigsaw/{i}', file)

然后利用拼图脚本进行拼图

from PIL import Image
from tqdm import tqdm

x_sum = 50
y_sum = 50
ori_width = 60
ori_height = 60
jigsaw_width = 20

width = ori_width + jigsaw_width * 2
height = ori_height + jigsaw_width * 2

def calc_file(x, y):
    num = str(y * x_sum + x).zfill(4)
    return f'jigsaw/{num}.png'

def check_info(file):
    img_info = [0, 0, 0, 0]
    img = Image.open(file)
    pix_out1 = img.getpixel((width // 2, 0))[3]
    pix_out2 = img.getpixel((width - 1, height // 2))[3]
    pix_out3 = img.getpixel((width // 2, height - 1))[3]
    pix_out4 = img.getpixel((0, height // 2))[3]
    pix_out = [pix_out1, pix_out2, pix_out3, pix_out4]
    pix_in1 = img.getpixel((width // 2, jigsaw_width))[3]
    pix_in2 = img.getpixel((width - jigsaw_width - 1, height // 2))[3]
    pix_in3 = img.getpixel((width // 2, height - jigsaw_width - 1))[3]
    pix_in4 = img.getpixel((jigsaw_width, height // 2))[3]
    pix_in = [pix_in1, pix_in2, pix_in3, pix_in4]
    for i in range(4):
        if pix_out[i] == 0 and pix_in[i] == 0:
            img_info[i] = -1
        elif pix_out[i] != 0 and pix_in[i] != 0:
            img_info[i] = 1
        elif pix_out[i] == 0 and pix_in[i] != 0:
            img_info[i] = 0
        else:
            raise Exception("Invalid jigsaw!", file)
    return img_info

def init_table():
    info_table = []
    for y in range(y_sum):
        row_info = []
        for x in range(x_sum):
            file = calc_file(x, y)
            img_info = check_info(file)
            row_info.append(img_info)
        info_table.append(row_info)
    return info_table

def cut(direction, file):
    if direction == 0:
        left_top_x = (ori_width - jigsaw_width) // 2 + jigsaw_width
        left_top_y = 0
    elif direction == 1:
        left_top_x = ori_width + jigsaw_width
        left_top_y = (ori_height - jigsaw_width) // 2 + jigsaw_width
    elif direction == 2:
        left_top_x = (ori_width - jigsaw_width) // 2 + jigsaw_width
        left_top_y = ori_height + jigsaw_width
    elif direction == 3:
        left_top_x = 0
        left_top_y = (ori_height - jigsaw_width) // 2 + jigsaw_width

    right_bottom_x = left_top_x + jigsaw_width
    right_bottom_y = left_top_y + jigsaw_width
    img = Image.open(file)
    cut_img = img.crop((left_top_x, left_top_y, right_bottom_x, right_bottom_y))
    blank_img = Image.new('RGBA', (jigsaw_width, jigsaw_width), (0, 0, 0, 0))
    img.paste(blank_img, (left_top_x, left_top_y))
    img.save(file)
    return cut_img

def paste(direction, file, cut_img):
    if direction == 0:
        left_top_x = (ori_width - jigsaw_width) // 2 + jigsaw_width
        left_top_y = jigsaw_width
    elif direction == 1:
        left_top_x = ori_width
        left_top_y = (ori_height - jigsaw_width) // 2 + jigsaw_width
    elif direction == 2:
        left_top_x = (ori_width - jigsaw_width) // 2 + jigsaw_width
        left_top_y = ori_height
    elif direction == 3:
        left_top_x = jigsaw_width
        left_top_y = (ori_height - jigsaw_width) // 2 + jigsaw_width

    img = Image.open(file)
    img.paste(cut_img, (left_top_x, left_top_y))
    img.save(file)

def recover_jigsaw(info_table):
    for y in tqdm(range(y_sum)):
        for x in range(x_sum):
            img_info = info_table[y][x]
            for direction in range(4):
                if img_info[direction] != 'free':
                    file = calc_file(x, y)

                    if direction == 0:
                        x2 = x
                        y2 = y - 1
                        file2 = calc_file(x2, y2)
                        direction2 = 2
                    elif direction == 1:
                        x2 = x + 1
                        y2 = y
                        file2 = calc_file(x2, y2)
                        direction2 = 3
                    elif direction == 2:
                        x2 = x
                        y2 = y + 1
                        file2 = calc_file(x2, y2)
                        direction2 = 0
                    elif direction == 3:
                        x2 = x - 1
                        y2 = y
                        file2 = calc_file(x2, y2)
                        direction2 = 1

                    if img_info[direction] == 1:
                        cut_img = cut(direction, file)
                        paste(direction2, file2, cut_img)
                    elif img_info[direction] == -1:
                        cut_img = cut(direction2, file2)
                        paste(direction, file, cut_img)
                    info_table[y2][x2][direction2] = 'free'
                    img_info[direction] = 'free'

def remove_border(file):
    img = Image.open(file)
    new_img = img.crop((jigsaw_width, jigsaw_width, width - jigsaw_width, height - jigsaw_width))
    new_img.save(file)

if __name__ == '__main__':
    info_table = init_table()
    recover_jigsaw(info_table)
    for i in range(x_sum * y_sum):
        file = 'jigsaw/' + str(i).zfill(4) + '.png'
        remove_border(file)

扫描后得到flag

MRCTF{795c666e-6244-4768-981d-3b******2c}