mitre-cyber-academy / 2012-crypto-b

Crypto B challenge from the June 2012 MITRE CTF

Crypto Challenge, 300 points:

Summary: Flag appears as text in an image, which is then encrypted using Blowfish in a weak mode of operation. The encrypted image and the encryption algorithm is available; the image encrypted was in .bmp format which is hinted at in the file name. The image is encrypted in ECB mode, which is not secure. I think this challenge is made a lot easier by the fact that a google search for "encrypted bitmap wikipedia" returns a page "Block cipher modes of operation" as its second result which gives an example of exactly the trick they need to use.

Challenge files:

index.html -- The challenge entry point.
/files -- this should be a readable sub-folder.
  /files/flag.bmp.encrypted -- pointed to by index.html

Solution walkthrough:

The problem with the encryption method used to encrypt flag.bmp is 

that it uses ECB, an insecure mode of operation.

flag.bmp is the actual file that was encrypted.  This is just for 

reference, students should not be able to easily recover it.

  From flag.bmp.encrypted, students have to work to understand the 

.bmp file format. They'll need to make some guesses; I would expect it's likely students will try using Windows paint to export into .bmp file, which (with the 24-bit color setting) produces the right kind of headers.
They'll have to figure out what the size is; from the filesize of flag.bmp.encrypted, given the correct color settings and the 64-bit IV, the file would contain 1024000 2/3 pixels. Naturally this suggests 1024000 pixels with 2 bytes of padding. There are a number of likely image sizes (1024x1000 springs to mind, but it turns out to be 1280x800). A .bmp file header can be created by hand; Wikipedia has a good explanation of the format with an example. Students will find that overwriting the first N bytes with a basic .bmp header doesn't work, because the file was padded by 2 bytes; this can be learned from the algorithm and observing the file sizes of flag_original.bmp and flag.bmp.encrypted.

  Walkthrough continues: open both flag.bmp and flag.bmp.encrypted

in a hex editor such as the one included. Copy the first 54 bytes from flag.bmp to flag.bmp.encrypted.ecb. Delete the last byte also, to make the pixel array the correct size. Save this as a .bmp file. flag.bmp.ecb.bmp is the result of this process.

Open flag.bmp.ecb.bmp in an image viewer.  The image is garbled but 

the flag string pops out clearly.