docs: plan for Bitchat geohash chat interop (phase 1 shipped, DM follow-up)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0172JoMccseEKenyWan6txWV

amethyst/plans/2026-07-15-bitchat-geohash-interop.md

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+# Bitchat geohash chat interop
+
+Status: **Phase 1 (public geohash channels) shipped.** Phase 2 (encrypted DMs)
+is designed but not implemented.
+
+## What Bitchat does (verified against `permissionlesstech/bitchat`
+iOS + `bitchat-android`)
+
+Bitchat's Nostr side has two chat features. Amethyst is a pure-Nostr client
+(no BLE mesh / Noise identity), so only the Nostr halves are in scope.
+
+### Public geohash channels ("location channels") — SHIPPED
+- Message = **kind 20000** (ephemeral), content = plain UTF-8 text.
+  Tags: `["g", geohash]` (required), `["n", nickname]` (optional),
+  `["t","teleport"]` (optional). Optional NIP-13 `["nonce", …]` PoW, default 8
+  bits, used to relax per-sender relay rate limits.
+- Presence = **kind 20001**, only the `g` tag, empty content.
+- Subscribe: `kinds:[20000,20001]`, `#g:[geohash]` (exact cell).
+- Precision levels (geohash chars): building 8, block 7, neighborhood 6,
+  city 5, province 4, region 2.
+- Identity = a per-geohash throwaway key `HMAC-SHA256(deviceSeed, geohash)`,
+  deterministic per (device, cell), unlinkable to the user's npub.
+- **Relay routing is geographic and load-bearing:** a cell's traffic goes to the
+  5 relays nearest the cell center, chosen from the public MIT-licensed
+  `permissionlesstech/georelays` CSV both clients load. If Amethyst used any
+  other relay set its messages would not rendezvous with Bitchat clients.
+
+### Private DMs — NOT YET IMPLEMENTED (Phase 2)
+- Standard NIP-17/59: rumor kind 14, seal kind 13, gift wrap kind 1059, wrap
+  under a throwaway key, NIP-44 v2.
+- **The kind-14 rumor content is NOT plain text.** It is
+  `"bitchat1:" + base64url(<binary bitchat packet>)` — a `BitchatPacket`
+  (TLV + a `NoisePayloadType` byte) carrying the private message, delivery ACKs,
+  and read receipts. Full DM interop therefore requires porting that binary
+  framing (`NostrEmbeddedBitChat.swift` / `NostrEmbeddedBitChat.kt`).
+- Two DM flavors: geohash DMs (gift-wrapped to a participant's per-geohash
+  pubkey) and stable-identity DMs (to an npub learned via Bitchat's mesh
+  `[FAVORITED]:<npub>` handshake — mesh-specific, mostly N/A for a Nostr client).
+
+## What shipped (Phase 1)
+
+- **quartz** `experimental/bitchat/`: `GeohashChatEvent` (20000),
+  `GeohashPresenceEvent` (20001), `GeohashKeyDerivation` (per-geohash key),
+  registered in `EventFactory`. PoW reuses the existing `nip13Pow` `PoWTag`.
+- **commons** `service/georelay/`: `GeoRelayDirectory` (closest-N by haversine,
+  host tie-break, `:443` dedup), `GeoRelayCsvLoader` (runtime CSV fetch + fallback).
+- **amethyst**: `GeohashChatScreen` + `GeohashChatViewModel` (live subscription +
+  send), `GeohashChatDeviceSeed` (global encrypted seed store),
+  `Account.signWithAndSendPrivately`, `Route.GeohashChat`, and a chat action on
+  the geohash feed screen.
+- **cli**: `amy geochat listen|send|keys` — the interop harness. Verified with a
+  live send→relay→listen round-trip (kind 20000, PoW, `g`/`n` tags intact).
+
+## Follow-ups
+
+1. **Encrypted DMs (Phase 2).** Port the `bitchat1:` binary packet
+   (`BitchatPacket` TLV + `NoisePayloadType`) into quartz, wrap/unwrap it in the
+   existing NIP-17 stack (`GiftWrapEvent`/`SealedRumorEvent`/`ChatMessageEvent`),
+   handle geohash DMs (to a per-geohash pubkey) and delivery/read receipts.
+   Add `amy geochat dm` for interop testing.
+2. **Desktop UI.** The shared pieces (quartz events, `GeoRelayDirectory`) are
+   already cross-platform; add a desktop `GeohashChatScreen` equivalent.
+3. **LocalCache integration (optional).** The current Android screen manages its
+   own subscription/state rather than routing through `LocalCache`/the chatroom
+   list. Integrating would give unread badges and a unified chat list, at the
+   cost of a `Channel`/feed-filter/datasource fork.
+4. **Location-driven channel picker.** Use `LocationState` to offer the
+   region/province/city/neighborhood/block/building cells for the user's current
+   position, plus a manual/teleport entry.
+5. **Presence heartbeats + i18n.** Periodically emit kind 20001 while a channel
+   is open; extract the hardcoded screen strings into `strings.xml`.