Found Key Hidden in JPEG File

Today, solved the picoGym ethical hacking challenge titled Information, which challenged users to find a picoCTF key hidden in a JPEG image file. But decided to respectfully tech-splain or summarize instead of blatantly giving away detailed spoilers.

The JPEG image features a kitten which appeared to have commandeered a laptop computer, but failed to hack it using redundant code.

On My Local Machine

• used the xxd hex dump tool, which created a dump of cat.jpg. I found an interesting looking CTF string segment in that dump, but there was no picoCTF key.
• installed exiftool locally using the tar.gz archive found at exiftool.org, but, for some reason, it failed to return enough metadata from the cat.jpg file which was downloaded to this local machine. In short, the metadata returned by ./exiftool ~/Desktop/cat.jpg failed to list the key/value needed to help solve the challenge. Am able to state this now from a hind-sight-is-twenty-twenty point of view.

Next time: may try to git-clone exiftool/exiftool from github.com, desiring to use it locally in the future.

Using the picoCTF Webshell

1. used GNU wget tool to download cat.jpg to a picoCTF Webshell instance (see header image).
2. used exiftool to print a list of metadata keys/values to the picoCTF Webshell display. The Current IPTC Digest key's hexadecimal-based hash looked interesting. Scrolling down further, found the License key which contained a promising base64 hash value.
3. used Bash's built-in echo command to stdout the base64 hash string, which then was piped to base64 -d or base64 --decode, and which became my solution. Success.

exiftool (update)

Above I mentioned cloning the exiftool repo from github.com. Well, I tried it and it worked on my machine locally, so I may follow up with a short script that returns the picoCTF key solution—perhaps a gist link.

nick3499/find-hidden-key-cat-jpg.sh (update)

I wrote a three-line script that returns the picoCTF key. After first using wget to download cat.jpg to the local machine, then grepping the line with that base64 hash, then isolating the hash and passing it to base64 --decode.

The new shell script is linked below:

find-hidden-key-cat-jpg.sh