Cypherpunk Underground 3 Writeup

This challenge come across my feed Saturday morning and thought it would be fun. It appeared to be a steganography challenge with a handful of hints offering a ZEC prize to the first solver.

I'll start with the hints and work up to the main challenge, the same way I solved them.

Hint 1

I recognized these symbols but couldn't remember the name, so I referenced this list of cipher symbols, which led me to this decoder tool where you can click on the matching symbols from the hint and get the english substitution.

Answer : zcash

Hint 2

Lcytl z yojkkksl mqqm ypsjut pr jedwewl

I recognized this to be some sort of cipher text. I quickly landed on working it as a Vigenère cipher. Using this tool I tried brute-forcing first, but eventually used the answer to the previous hint, zcash, as the key, and was rewarded with the answer.

Answer : Maybe a wordlist from github is helpful

Hint 3

xibkglxfiivmxb-dliworhg

This turned out to be a substitution cipher. I used quipqiup to try a few automatic solves. This got me halfway there - It was clear the first word before the hyphen was cryptocurrency but it was unclear what the second word was. My favorite wrong solution I got from the tool was "cryptocurrency-borgslat". I manually worked out the rest and the probability was high that the correct answer was "cryptocurrency-wordlist". I confirmed this with this tool whose default substitution alphabet (Z-A) happened to give the correct answer straight away.

Answer : cryptocurrency-wordlist

The main challenge

I assumed this to be a steganography challenge from the start, where some hidden data would be embedded within the given image. I tried using exiftool, binwalk and foremost to see if there was anything that could be immediately yielded, but these tools turned up nothing. The hint mentioning a wordlist led me to believe that a passphrase would be required to extract the data, and that the correct passphrase would be within a wordlist, available on github, that was related to cryptocurrency.

I used the tool stegseek which can quickly run through a wordlist, using each line in the wordlist as a potential passphrase to extract hidden data within a .jpg.

Initially, I didn't know which wordlist to use. There is a fairly ubiquitous wordlist known as rockyou.txt, that I tried first. But that isn't related to cryptocurrency, so I kept thinking. I next spent a while trying lists of BIP39 mnemonic phrases in all available languages, but that didn't work either. Finally, I searched github.com for the exact phrase "cryptocurrency-wordlist", and found this repo.

Using this wordlist, in combination with stegseek, the hidden data was finally revealed.

Passphrase : Zerocoin

Final answer : CYPHERPUNX! Steganography is cool right? It may be useful in the future...

Thanks to CypherpunkUnderground for the fun challenge.

@blinkingthing