Stream Reuse
A04 · ≈ 40 min · 0 solves
Explore the application as a real target: map the exposed functionality, inspect requests and responses, build a working exploit, and recover the rotating flag. If you get stuck, the verified official solution is available below as an optional spoiler.
Rules of engagement
Stay inside the generated lab URL. Use only the synthetic accounts and data in this instance. The main SecOne4all site and external systems are out of scope.
Challenge brief
What counts as a valid solve
Recover two messages encrypted under the same stream-cipher keystream.
GET /api/relay.
recover a protected value or produce an accepted synthetic forgery through the application service; the successful workflow returns the current rotating flag.
Work only inside your generated lab URL and submit the flag within 40 minutes.
Official solution
Contains the complete exploitation path. Open it only when you want the spoiler.
Reveal official solutionStep-by-step · verified against the live lab
Official solution · A04 · Stream Reuse
Objective
Recover two messages encrypted under the same stream-cipher keystream.
Steps
- Start the isolated CipherBank workspace and inspect its service operation and requests.
- Capture the synthetic cryptographic artifact, token, oracle response, or protocol negotiation output.
- Exploit the cryptographic design or implementation failure: GET
/api/relay; XOR the supplied crib with the first capture to recover the repeated keystream, then decrypt the second capture. - Confirm the recovered secret, accepted forgery, or bypass returns the rotating instance flag.
- Submit the exact flag before stopping or expiring the instance.
Expected proof
A successful response for stream-cipher-keystream-reuse returns the instance-specific rotating flag. The flag is valid only for the current player and running instance.
Safety boundary
Use only the generated lab URL. No request to the main site, another user, or an external service is part of this solution.