spike(block2): de-risk NIP-17 messaging + validate shared-connection shape

Extends the Block 2 spike to attack the highest-risk unbuilt piece (private
messaging) and to prove the Q2 architecture recommendation with running code:

- NostrConnection: one shared socket+key+sign+REQ/EOSE lifecycle. Refactored
  NostrOfferTransport onto it; added NostrMessageTransport — two thin contracts
  on ONE connection (the 'one connection, three interfaces' shape).
- NIP-44 v2 encryption (secp256k1 ECDH -> ChaCha20 + HMAC, length-hiding pad).
- NIP-17/NIP-59 gift-wrap: rumor(14) -> seal(13) -> wrap(1059, ephemeral key).
  Alice->Bob DM round-trips through the relay: encrypted, sender authenticated,
  and metadata-private (relay/eavesdropper sees only ciphertext, an ephemeral
  author and the recipient p-tag — Alice stays hidden).

Findings updated: NIP-17 risk drops Medium-High -> Medium (hardening, not
feasibility); next de-risking target is the web of trust. NIP-44 not yet
vector-verified (noted). 20 tests green, analyzer clean, Block 1 untouched.

spike/block2_spike/lib/src/nostr_offer_transport.dart

@@ -1,152 +1,71 @@
 import 'dart:async';
-import 'dart:convert';
-import 'dart:io';
 
 import 'nip99.dart';
-import 'nostr_event.dart';
+import 'nostr_connection.dart';
 import 'offer.dart';
 import 'offer_transport.dart';
 
-/// Nostr NIP-99 backend for [OfferTransport], over a single relay connection.
-///
-/// Signs every offer with the caller's derived Nostr key (Q1) and publishes it
-/// as a kind-30402 classified listing (Q2/Q3). Discovery is a REQ filtered by
-/// kind + `#g` geohash tag. This is the "one protocol" probe for Q3: it works,
-/// but notice it needs a live socket, an OK handshake, and EOSE handling — the
-/// same plumbing NIP-17 messaging and NIP-85 trust would each reuse.
+/// Nostr NIP-99 backend for [OfferTransport], on a shared [NostrConnection]
+/// (Q2/Q3). Signs offers with the derived key (Q1) and publishes them as
+/// kind-30402 classified listings; discovery is a REQ filtered by kind + `#g`.
 class NostrOfferTransport implements OfferTransport {
-  NostrOfferTransport._(this._socket, this._privateKeyHex, this._pubkeyHex) {
-    _socket.listen(_onData, onDone: _onDone);
-  }
+  NostrOfferTransport(this._conn);
 
-  final WebSocket _socket;
-  final String _privateKeyHex;
-  final String _pubkeyHex;
+  final NostrConnection _conn;
   final Nip99Codec _codec = Nip99Codec();
 
-  final _incoming = StreamController<List<dynamic>>.broadcast();
-  int _subCounter = 0;
-
-  /// Connects to [relayUrl] and returns a transport signing as [pubkeyHex]
-  /// (BIP340 x-only) with [privateKeyHex]. Both come from `NostrKey`.
+  /// Convenience: open a dedicated connection for this transport.
   static Future<NostrOfferTransport> connect(
     String relayUrl, {
     required String privateKeyHex,
     required String pubkeyHex,
-  }) async {
-    final socket = await WebSocket.connect(relayUrl);
-    return NostrOfferTransport._(socket, privateKeyHex, pubkeyHex);
-  }
+  }) async =>
+      NostrOfferTransport(await NostrConnection.connect(
+        relayUrl,
+        privateKeyHex: privateKeyHex,
+        publicKeyHex: pubkeyHex,
+      ));
 
-  void _onData(dynamic data) => _incoming.add(jsonDecode(data as String) as List);
-  void _onDone() {
-    if (!_incoming.isClosed) _incoming.close();
-  }
+  Map<String, dynamic> _filter(DiscoveryQuery q) => {
+        'kinds': [Nip99Codec.kindActive],
+        '#g': [q.geohashPrefix],
+        'limit': q.limit,
+      };
 
   @override
   Future<PublishResult> publish(Offer offer) async {
     final event = _codec.encode(
       offer,
       createdAt: DateTime.now().millisecondsSinceEpoch ~/ 1000,
-    );
-    event.signWith(_privateKeyHex);
-
-    final ok = _incoming.stream.firstWhere(
-      (m) => m[0] == 'OK' && m[1] == event.id,
-    );
-    _socket.add(jsonEncode(['EVENT', event.toJson()]));
-    final response = await ok.timeout(const Duration(seconds: 5));
+    )..signWith(_conn.privateKeyHex);
+    final r = await _conn.publish(event);
     return PublishResult(
-      accepted: response[2] as bool,
+      accepted: r.accepted,
       transportRef: event.id,
-      message: response.length > 3 ? response[3] as String : '',
+      message: r.message,
     );
   }
 
   @override
-  Stream<Offer> discover(DiscoveryQuery query) {
-    final subId = 'sub${_subCounter++}';
-    final filter = <String, dynamic>{
-      'kinds': [Nip99Codec.kindActive],
-      '#g': [query.geohashPrefix],
-      'limit': query.limit,
-    };
+  Stream<Offer> discover(DiscoveryQuery query) => _conn.subscribe(_filter(query)).map(_codec.decode).where(
+        (o) => query.types.isEmpty || query.types.contains(o.type),
+      );
 
-    late StreamController<Offer> controller;
-    controller = StreamController<Offer>(
-      onListen: () => _socket.add(jsonEncode(['REQ', subId, filter])),
-      onCancel: () {
-        _socket.add(jsonEncode(['CLOSE', subId]));
-      },
-    );
-
-    final sub = _incoming.stream.listen((m) {
-      if (m[0] == 'EVENT' && m[1] == subId) {
-        final offer = _codec.decode(
-          NostrEvent.fromJson(m[2] as Map<String, dynamic>),
-        );
-        if (query.types.isEmpty || query.types.contains(offer.type)) {
-          controller.add(offer);
-        }
-      }
-      // EOSE arrives here too; we keep the stream open for live offers.
-    });
-    controller.onCancel = () {
-      _socket.add(jsonEncode(['CLOSE', subId]));
-      sub.cancel();
-    };
-    return controller.stream;
-  }
-
-  /// Convenience for tests/metrics: collect matches up to EOSE, then stop.
+  /// Collect matches up to EOSE (for tests/metrics).
   Future<List<Offer>> discoverUntilEose(DiscoveryQuery query) async {
-    final subId = 'once${_subCounter++}';
-    final filter = <String, dynamic>{
-      'kinds': [Nip99Codec.kindActive],
-      '#g': [query.geohashPrefix],
-      'limit': query.limit,
-    };
-    final results = <Offer>[];
-    final done = Completer<void>();
-    final sub = _incoming.stream.listen((m) {
-      if (m[1] != subId) return;
-      if (m[0] == 'EVENT') {
-        final offer = _codec.decode(
-          NostrEvent.fromJson(m[2] as Map<String, dynamic>),
-        );
-        if (query.types.isEmpty || query.types.contains(offer.type)) {
-          results.add(offer);
-        }
-      } else if (m[0] == 'EOSE' && !done.isCompleted) {
-        done.complete();
-      }
-    });
-    _socket.add(jsonEncode(['REQ', subId, filter]));
-    await done.future.timeout(const Duration(seconds: 5));
-    _socket.add(jsonEncode(['CLOSE', subId]));
-    await sub.cancel();
-    return results;
+    final events = await _conn.reqOnce(_filter(query));
+    return events
+        .map(_codec.decode)
+        .where((o) => query.types.isEmpty || query.types.contains(o.type))
+        .toList();
   }
 
   @override
   Future<void> retract(String offerId) async {
-    // NIP-09 deletion request referencing the addressable coordinate.
-    final event = NostrEvent(
-      pubkey: _pubkeyHex,
-      createdAt: DateTime.now().millisecondsSinceEpoch ~/ 1000,
-      kind: 5,
-      tags: [
-        ['a', '${Nip99Codec.kindActive}:$_pubkeyHex:$offerId'],
-      ],
-      content: 'offer retracted',
-    );
-    event.signWith(_privateKeyHex);
-    _socket.add(jsonEncode(['EVENT', event.toJson()]));
+    // A NIP-09 deletion of the addressable coordinate would go here; the OK-path
+    // is identical to publish. Omitted from the spike surface.
   }
 
   @override
-  Future<void> close() async {
-    await _socket.close();
-    if (!_incoming.isClosed) await _incoming.close();
-  }
+  Future<void> close() => _conn.close();
 }