Offline Chat — a multi-user chat with no internet, no installs, no accounts

Build spec — Offline Chat (LAN, zero-internet multi-user chat)

Goal

A multi-user chat with file sharing that runs with no internet at all: no tunnel, no relay, no external service. One host starts it; everyone else joins from a browser on the same local network. Python standard library only — no pip install, no virtualenv, no build step. Ship it as a new file beside the existing app, never editing the original.

Explicit constraint from the request: "make sure its seperate file dont delete the old ne".

Hard requirements

Protocols

• Push: GET /events?session_id=… → text/event-stream, held open per client.
• Actions: POST /send with {action: message | set_name | react | delete | claim_host | ping}.
• Files: POST /upload (multipart) → stored in uploads/, served at /files/<name>.
• Discovery: GET /link → {link, host} where host carries a one-time admin token.

Security features (v2)

1. --tls — self-signed cert, generated on first run, cached in temp.

• Prefer openssl req -x509 -newkey rsa:2048 -addext subjectAltName=IP:<lan>,DNS:localhost.
• Fallback when openssl is missing (stock Windows): pure-Python RSA-2048 keygen

(Miller–Rabin primes) + hand-built X.509 DER, signed sha256WithRSAEncryption (OID 1.2.840.113549.1.1.11), PKCS#1 v1.5 padding, PEM at 64 columns.

1. --password P — room password; /auth returns a token, /events and /send require it.
2. --stealth — port 443 + TLS + password, so flows look like ordinary HTTPS.
3. --bind-ip IP — manual override when LAN IP detection picks the wrong adapter.
4. Panic close — beforeunload + pagehide blank document.body and clear stored state.

Cross-platform rules (learned the hard way)

• Target Python 3.9+: Optional[str], never str | None.
• Do not assume openssl exists; do not assume a POSIX shell.
• Do not assume the host can create a hotspot or an ad-hoc network — many managed Macs cannot.

start_chat.py must explain the realistic options (existing LAN, phone hotspot, direct cable) instead of pretending to create a radio.

Traps that cost real time

• Do not build the HTML with an f-string. JavaScript object literals and CSS braces collide with

Python interpolation. Use a plain triple-quoted string plus __MARKER__ tokens and .replace().

• If the template was migrated from an f-string, every brace is still doubled ({{/}}).

Collapse them once at render time — 204 of them leaked into a shipped page and produced an unstyled page whose JS never ran.

• In a non-raw Python string, \' is consumed by Python. Never generate nested

onclick="window.open('…')" inside a JS single-quoted string; use window.open(this.src).

• Verify the client with node --check on the extracted inline script and a real headless

browser, not just curl returning 200. A page can serve fine and still be inert.

Verification checklist

• [ ] python3 -m py_compile on both files
• [ ] /health returns {status, users, tls, stealth, password_protected}
• [ ] / returns the full shell; {{ count is 0; extracted script passes node --check
• [ ] two concurrent SSE clients: second join emits system + user_list; a message from one is

received by the other and replayed in history for a later session

• [ ] wrong password rejected; correct password yields a token that /events accepts
• [ ] --tls → /health over HTTPS shows tls: true
• [ ] the pure-Python cert path exercised with PATH stripped of openssl, under system Python 3.9
• [ ] headless Chrome on the static demo: message bubbles render, share link fills, 0 console errors
• [ ] original files untouched (git status / checksums)

Deliverable shape

• offline_chat_v2.py — server + client in one file
• start_chat.py — guided launcher
• demos/offline-chat.html — UI demo (real markup, fetch/EventSource stubbed, labelled as demo)
• demos/offline-chat-analysis.html — build analysis with measured numbers and honest limits
• demos/offline-chat-mirror/ — byte-identical copies of both .py files + hashes + downloads