# STEM CTF Cyber Challenge 2019 – Journey to the Center of the File

* **Category:** Grab Bag
* **Points:** 100

## Challenge

> W(e( (h(a(v(e( (t(o( (g(o( (d(e(e(p(e(r)))))))))))))))))))
>
> [download](https://github.com/m3ssap0/CTF-Writeups/raw/master/STEM%20CTF%20Cyber%20Challenge%202019/Journey%20to%20the%20Center%20of%20the%20File/flag)

## Solution

There are different types of files nested in each other:
* zip;
* bzip2;
* gzip;
* base64 encoded data.

Nested files are hundreds, so a [Python script](https://github.com/m3ssap0/CTF-Writeups/raw/master/STEM%20CTF%20Cyber%20Challenge%202019/Journey%20to%20the%20Center%20of%20the%20File/journey.py) can be written to open them all.

```python
import os, base64
import bz2, zipfile, gzip
import magic # pip install python-magic

i = 0
current_file = "flag"

while True:
new_file = "decompressed-" + str(i)

# Analyzing file type.
file_type = magic.from_file(current_file)
print "[*] File '{}' is '{}'.".format(current_file, file_type)

# Found the flag.
with open(current_file, "r") as cf:
read_data = cf.read()
if "MCA{" in read_data:
print read_data
break

# Analyzing archives.

if "bzip2" in file_type:

with open(new_file, 'wb') as nf, open(current_file, 'rb') as cf:
decompressor = bz2.BZ2Decompressor()
for data in iter(lambda : cf.read(100 * 1024), b''):
nf.write(decompressor.decompress(data))

elif "Zip" in file_type:

with zipfile.ZipFile(current_file) as cf:
if len(cf.namelist()) == 1:
file_to_be_extracted = cf.namelist()[0]
else:
print "[!] Too much files into the archive!"
break
cf.extractall()
os.rename(file_to_be_extracted, new_file)

elif "ASCII text" in file_type:

with open(current_file, "r") as cf:
encoded_data = cf.read()
decoded_data = base64.b64decode(encoded_data)
with open(new_file, "wb") as nf:
nf.write(decoded_data)

elif "gzip" in file_type:

with gzip.open(current_file, "r") as cf:
read_data = cf.read()
with open(new_file, "wb") as nf:
nf.write(read_data)

else:
print "[!] Unknown archive, exiting."
break

# Removing old file and going on with analysis.
os.remove(current_file)
current_file = new_file
i += 1
```

The flag is the following.

```
MCA{Wh0_Needz_File_Extensions?}
```

Original writeup (https://github.com/m3ssap0/CTF-Writeups/blob/master/STEM%20CTF%20Cyber%20Challenge%202019/Journey%20to%20the%20Center%20of%20the%20File/README.md).