Generate cryptographically unique, less predictable flags per lab

Goal

Make every lab instance use flags that are effectively unique and harder to predict, while keeping the learner-facing CTF{...} format.

Current observations

• setup/flags.py uses one random secrets.token_hex(4) suffix for most challenge flags. That is 32 bits of randomness shared across challenges.
• Challenge 12 currently uses only the first 4 hex characters, which is 16 bits of randomness.
• Flag bases such as hidden_files, file_search, and log_analysis reveal the challenge identity.
• MASTER_SECRET is hard-coded in the repo and is used for completion-token signing through derive_verification_secret.

Proposed direction

Use a per-instance random secret and derive each challenge flag with HMAC-SHA256 or another standard keyed derivation. For example, derive each flag from instance_secret, challenge number, and purpose string, then encode a sufficiently long prefix into CTF{...}. This gives each challenge its own value and avoids a shared suffix pattern.

Questions to answer

1. Should flags include challenge numbers for supportability, for example CTF{01_<random>}, or should they contain no challenge hint at all?
2. What entropy target should each flag have? A practical target could be at least 96 or 128 bits per flag.
3. Should the completion-token signing secret be generated per lab instead of derived from a hard-coded repository constant?
4. How should test tooling retrieve expected answers without reintroducing predictable flags?

Acceptance criteria

• Every non-example challenge flag has independent cryptographic randomness.
• No challenge uses a very short suffix such as 16 bits.
• Flag values no longer expose challenge names unless we intentionally keep a small challenge number prefix.
• Verify still stores only hashes for flag checking.
• Completion-token signing does not rely on a repository hard-coded production secret.
• The ctf-testing suite still solves flags from the VM artifacts, not from checked-in answer strings.

Links

• HMAC background and secret handling guidance: https://blog.gitguardian.com/hmac-secrets-explained-authentication
• OWASP Juice Shop CTF mode notes using generated flag codes: https://pwning.owasp-juice.shop/companion-guide/latest/part4/ctf.html