samberry.dev
guest@samberry.dev:/writeup# cat Writeup:_Aarhus_CTF_2019_‑_Repeated_XOR_2:_Electric_Boogaloo_(Crypto)

Writeup: Aarhus CTF 2019 - Repeated XOR 2: Electric Boogaloo (Crypto)

For the first time ever, the Department of Computer Science at Aarhus University hosted a Capture the Flag competition, where contestants participated in 24 hours of trying to break Cyber Security.

Aarhus CTF 2019 - Repeated XOR 2: Electric Boogaloo

(Crypto, 499 Points, 2 Solves)

By Team "! leftovers" (Sam Beresford & Emilie Bjerg)

Challenge Description:

We are being asked to decrypt a flag file that was encrypted using repeated XOR. A hint, about the length of the key being 10 bytes and the file being a 500x281 image, encourages us to start using image file signatures (The first few bytes of a file that help determine it's type) to obtain the key.

challenge

Solution

First we tried XORing the first few bytes of the file with known file signatures of common image file types.

Bitmap	- 0x42 0x4D ...
Jpeg	- 0xFF 0xD8 0xFF ...
Png	- 0x89 0x50 0x4E 0x47 0x0D ...
Gif87	- 0x47 0x49 0x46 0x38 0x37 0x61 ...
Gif89	- 0x47 0x49 0x46 0x38 0x39 0x61 ...

This gives us the following results:

From this we can see a Gif87 or Gif89 header gives an interesting result of key= something.

If we try extending on these Gif headers by adding the known height(500, 0x1F4) and width (281, 0x119) of the image, in little-endian format:

Gif87	- 0x47 0x49 0x46 0x38 0x37 0x61 0xF4, 0x01, 0x19, 0x01
Gif89	- '                 ' 0x39 0x61 0xF4, 0x01, 0x19, 0x01

After XOR these two we have a choice between these two keys:

Using the nicer looking key of key=Banana we can decrypt the file and get:

Flag:

CTF{B3w4RE_PTXT_knOwL3dg3}

Code used

headers = [
  [0x42, 0x4D], # Bitmap
  [0xFF, 0xD8, 0xFF], # Jpeg
  [0x89, 0x50, 0x4E, 0x47], # Png
  [0x47, 0x49, 0x46, 0x38, 0x37, 0x61], #Gif87
  [0x47, 0x49, 0x46, 0x38, 0x39, 0x61] #Gif89
]
with open("flag.bin") as file:
  head = file.read(10)
  for i in range(0, len(headers)):
    output = ""
    for j in range(0, len(headers[i])):
      output += chr(ord(head[j]) ^ headers[i][j])
    print output

#Gif87 Header + Height (LSB) + Width (LSB)
header = [0x47, 0x49, 0x46, 0x38, 0x37, 0x61, 0xF4, 0x01, 0x19, 0x01]
key = []
with open("flag.bin") as file:
  head = file.read(10)
  for i in range(0, len(header)):
    key.append(chr(ord(head[i]) ^ header[i]))

print key

#Gif89 Header + Height (LSB) + Width (LSB)
header = [0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0xF4, 0x01, 0x19, 0x01]
key = []
with open("flag.bin") as file:
  head = file.read(10)
  for i in range(0, len(header)):
    key.append(chr(ord(head[i]) ^ header[i]))

print key

with open("flag.bin") as file:
 content = file.read()
 output = ""
 for i in range(0, len(content)):
  output += chr(ord(content[i]) ^ ord(key[i%len(key)]))

print output[:10]

with open("flag.gif", "w") as out:
  out.write(output)

Challenge Solved!