Merge pull request #1501 from bryanthaboi/dev

docs/link-security.md

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+# Link play: threat model and what the code actually guarantees
+
+Link play is the only part of this game that reads bytes written by
+somebody else. This is what it defends against, what it does not, and
+where each guarantee lives.
+
+## The boundary
+
+Everything a peer or the relay sends arrives as one JSON object per line.
+There is exactly one place it becomes a message:
+
+    src/link/Net.lua        reads bytes, frames lines, decodes JSON
+    src/link/Wire.lua       rebuilds each line as a typed message
+    src/link/Session.lua    the only path from a transport into a mode
+
+`Session:update` runs `Wire.sanitize` on every message before anything
+else sees it. A schema returns a **new** table holding only the fields it
+names, at the Lua types it names, so the rest of `src/link/` can read
+`msg.slot`, `msg.parts.actives` or `msg.mons[i].dvs.hp` directly and be
+right by construction. A message with no schema (a mod's, or a future
+build's) keeps a bounded, scalar-only copy of its payload instead of
+being dropped.
+
+A message that fails its schema is **dropped and logged**, never fatal.
+Latching a terminal failure would hand a hostile peer a cheaper
+disconnect than sending nothing at all.
+
+### Why the bounds are loose
+
+Wire's numeric bounds are deliberately wider than the game's own clamps in
+`Protocol.unpackMon`. Both peers run identical clamps over identical
+packets; a bound that bit an honest value would change one side's copy of
+a mon and desync the lockstep. Wire's job is types and sizes. Rules are
+`Protocol`'s job, and it keeps its own clamps for the callers that reach
+it without a Session (the mod API, `tests/`).
+
+### Containment behind it
+
+Assume something still gets through:
+
+- `Game:step` pcalls the link pump, and pcalls `stack:update` **only
+  while a link session is active**. On a throw, `Game:breakLink` closes
+  the connection, unwinds to the overworld and says "The link was
+  broken." Outside link play the stack is unguarded on purpose: a blanket
+  pcall would swallow real engine bugs and leave the game silently wrong
+  instead of loudly broken.
+- `Net` caps `rxBuf` at 256KB and its per-frame read at 512KB, so a peer
+  that never sends a newline ends as a clean disconnect.
+- `Json.decode` refuses documents nested past 64 levels, and takes an
+  optional length cap that the link path passes and the mod-manifest path
+  does not.
+
+## The relay (`../pokeserver`)
+
+- A line that is not a JSON **object** with a string `type` is dropped
+  before any handler runs, and `onLine` is wrapped in try/catch.
+  `server.js` installs `uncaughtException`/`unhandledRejection` handlers:
+  one bad packet must never take every live match down with the process.
+- Line buffers are capped, lines per second are capped, connections per
+  IP and in total are capped, and an unbound connection that never hosts
+  or joins is swept after 30s.
+- `SERVER_ONLY` is the set of message types the server is the only
+  legitimate author of (`peer_gone`, `bracket_update`, `match_start`,
+  `tournament_over`, `spectate`, ...). A peer that sends one has them
+  dropped rather than forwarded, so a bracket opponent cannot forge a
+  tournament result or fake "your opponent left".
+- Trainer names are reduced to a printable subset and capped at the same
+  10 characters the game enforces, on the way in, because they are
+  rendered by the dashboard and broadcast to every participant.
+
+`pokeserver/test/hostile.js` is the regression net for all of that.
+
+## What is NOT defended
+
+**Party legality is trust-the-client.** Online play meets strangers, and
+`Handshake.onlineAllowed` is a Lua function in the same VM the mods load
+into. It cannot be made tamper-proof in-process, and pretending otherwise
+would only cost honest mod authors. What lockstep and
+`Protocol.unpackMon`'s recompute-from-species-data *do* guarantee is that
+a cheater cannot invent stats, moves, or a shiny: every derived value is
+rebuilt locally from real species data. They can send a legal party they
+farmed or edited. That is the honest boundary.
+
+What the relay does instead is **observe and record**. It already sees
+every `hello`, so it keeps each connection's self-reported
+`engineVersion`, `fingerprint` and `linkModified`, compares the two sides
+of a room or a live tournament match, and logs and surfaces a
+`modded` / `fingerprint_mismatch` / `version_skew` flag on the dashboard.
+A patched client can still lie; what it cannot do is lie without the
+tournament organizer having a record of it.
+
+Client-side attestation is deliberately not built. This is an
+open-source Lua game: it would be theater, and it would break honest
+mods.
+
+**The relay has no TLS.** Port 7778 is plaintext, so party contents,
+trades and trainer names are visible to anyone on the network path. There
+is nothing secret in a Pokemon party, but it is a real property of the
+system and not an oversight. Fixing it means a TLS terminator in front of
+the relay and a client that speaks it, which is a version break for every
+shipped build.
+
+**The dashboard has no default password.** `DASHBOARD_PASSWORD` is
+required; with it unset the relay runs and the dashboard simply does not
+start. It is still Basic Auth over plain HTTP, so it belongs behind an
+IP restriction or an SSH tunnel (`pokeserver/DEPLOY.md`).
+
+## Tests
+
+    luajit tests/link_hostile.lua        every message type x every wrong type
+    luajit tests/link_desync_fuzz.lua    lockstep fuzz, plus a mutation mode
+    luajit tests/run_link_tests.lua      both of the above, plus the rest
+    cd ../pokeserver && npm test         relay smoke, 16-player bracket, hostile
+
+`tests/link_hostile.lua` builds its corpus from a template per message
+type, replaces each field (and several nested ones) with every wrong Lua
+type, and drives the survivors through the real trade session, a real
+lockstep battle, a real spectator battle, and the tournament screen
+**including its draw** -- because the two nastiest payloads are
+delayed-fuse ones that crash on render rather than on receipt.