impossible stego — sekai2026

Description

i rolled my own stego and made sure to make it wayyyyy too hard for someone to guess how it works...

er, i mean claude did. i totally read the code tho.

The provided archive contains only two files:

misc_impossible-stego/flag.png
misc_impossible-stego/messages.log

flag.png is an RGBA PNG image. messages.log is much more important: it is a JSONL log of API requests and responses to an Anthropic/Claude-compatible gateway.

Analysis

The challenge text says Claude wrote the stego code, so the right first step is not to guess the pixel algorithm. Instead, inspect the log and recover what Claude generated.

Each line of messages.log is a JSON object. The interesting fields are req_body and resp_body; both are base64-encoded. Decoding request bodies reveals Claude Code conversation state, including tool calls such as Write, Edit, Read, and Bash.

A quick parser confirms the late conversation contains the whole source tree creation history:

import json, base64
from pathlib import Path

p = Path("misc_impossible-stego/messages.log")
last = None
for line in p.open():
    rec = json.loads(line)
    s = rec.get("req_body") or ""
    if not s:
        continue
    try:
        body = json.loads(base64.b64decode(s))
    except Exception:
        continue
    if "messages" in body:
        last = body

for msg in last["messages"]:
    for c in msg.get("content") or []:
        if isinstance(c, dict) and c.get("type") == "tool_use":
            print(c.get("name"), c.get("input", {}).get("file_path"))

The log shows writes for files like:

stego/secret.py
stego/crypto/kdf.py
stego/crypto/chacha20.py
stego/crypto/csprng.py
stego/coding/frame.py
stego/image/scatter.py
stego/image/embed.py
stego/pipeline.py
stego/__main__.py

So messages.log effectively leaks the exact steganography implementation.

Solution

Reconstruct the final source tree by replaying logged Write and Edit operations. The following script extracts the last full Claude request body, then applies all file writes/edits whose paths are under /home/claude/projects/impossible-stego/:

#!/usr/bin/env python3
import base64
import json
import shutil
from pathlib import Path

...