Septem Attack

Work on attacking the Septem cryptocurrency puzzle as described on this Reddit thread.

Puzzle Overview

Septem is a cryptocurrency puzzle described in /r/bitcoinpuzzles, but allegedly originally published on 4Chan. The PNG image uses steganography to hide the puzzle description and the ciphertext. In broad strokes, it is a 7-layer deep nesting doll of encryption (each layer is XOR then Vigenere), each layer with a different key.

Approach

Each nested layer has (English?) plaintext preceeding binary ciphertext. This plaintext is likely how one can reduce the search space for the XOR key + Vigenere key.

In general, the approach will be to find likely candidates for keys (look for patterns to guess the key period), limit our search space to BIP-39 wordlist matches, then try to parallelize the remaining search.

Run the Code

Step 1

Decode the image steganography:

stegify decode --carrier ./wh8hlbzrsfx61.png --result ./stegify-result.png

Step 2

Separate the description/hint from the ciphertext:

cat ./stegify-result.png | tail -c 7519 | head -c 6685

(these numbers were derived from guess-and-check to find the boundaries of the English text / ciphertext)

Step 3

Run the Python analysis script with chapter1 as STDIN:

cat ./chapters/1/ciphertext.data | python3 python/main.py --hint="****face****"

Note: --hint="****face***" tells the script that the XOR decryption key is 12 characters long with 4 unknown characters, 4 known characters (face) and 4 unknown characters. This is useful for restricting the search space.

Data Files

Chapter 1

chapter 1 ciphertext. This was obtained from "Step 2" of "Run the Code".

Initial Steganography results

stegify-result.png. This was obtained from "Step 1" of "Run the Code".

BIP-39 words

English BIP-39 words gathered from Bitcoin BIP-39

wh8hlbzrsfx61.png

ihKnTsY.png

Tools and Reference

Steganography

• Stegify

Decryption

• XOR Brute Force
• Vigenere Decryption

Statistical Analysis

• Incidence of Coincidence
• Vigenere Cryptanalysis