> For the complete documentation index, see [llms.txt](https://kos0ng.gitbook.io/ctfs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://kos0ng.gitbook.io/ctfs/write-up/2022/compfest-final/forensic.md).
# Forensic
## Nintendo (484 pts)
### Description
\-
### Solution
Diberikan image dan script disini kami lakukan analisis terhadap script tersebut. Intinya dari base image (nintendo.jpg) dibuat beberapa file dan masing-masing file di ubah nilai pixel i,i dimana i adalah index. Nilai pixel i diubah dengan format pixel\[0]\*flag dan hasilnya disimpan pada pixel dengan struktur (pixel\[1],pixel\[0],0) . Kemudian semua gambar digabung menjadi apng dengan library apng. Untuk extract apng menjadi file kami gunakan referensi berikut namun kami ubah beberapa kode hingga menjadi berikut
```python
import apng
from struct import pack, unpack
im = apng.APNG.open("nintendo.apng")
i = 0
for png, control in im.frames:
print(png)
w, h = unpack(">I", png.chunks[0][1][8:12])[0], unpack(">I", png.chunks[0][1][12:16])[0]
png.chunks[0] = ('IHDR', apng.make_chunk("IHDR", pack(">I", w) + pack(">I", h) + b'\x08\x06\x00\x00\x00'))
png.save("%02d.png" % i)
i += 1
```
Sebelumnya disini file nintendo.apng juga diubah headernya namun bisa kita kembalikan dengan menyesuaikan dengan header png asli
Selanjutnya setelah setiap image terextract(dimana setiap image terdapat flag) kita bisa langsung lakukan extract value flag dari setiap image. Berikut script yang kami gunakan
```python
from PIL import Image
from Crypto.Util.number import long_to_bytes, bytes_to_long
length = 19
my_png = Image.open("nintendo.png")
pixels = my_png.load()
filename = "{}.png"
flag = ""
for i in range(length):
fn = str(i).rjust(2,"0")
tmp = filename.format(fn)
extracted_png = Image.open(tmp)
pixel1 = list(extracted_png.getpixel((i, i)))
pixel2 = list(my_png.getpixel((i, i)))
tmp2 = bytes([pixel1[1],pixel1[0]])
value = bytes_to_long(tmp2)
res = value//pixel2[0]
flag += chr(res)
print(flag)
```
Flag : COMPFEST14{1ts\_aN\_4n1Mat3d\_PNG}
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