spike(block2): de-risk social layer — key derivation, OfferTransport, NIP-99 round-trip

Throwaway research spike (spike/ branch, outside pub workspace, no prod deps
touched). Validates the open Block 2 decisions before committing a funded round:

1. Deterministic one-way secp256k1 (Nostr) key derived from the Ğ1 root seed
   via HKDF — user still backs up ONE thing. Reproducible + signs (BIP340).
2. OfferTransport abstraction over Nostr NIP-99 (kind 30402); Offer stays
   inventory/location-agnostic, geohash coarsened on the wire (tested).
3. publish->discover-by-geohash proven end-to-end against an in-process
   hermetic mini-relay (~34ms round-trip).

Findings + risks + recommendation in docs/design/spike-block2-findings.md.
Block 1 suite untouched. 14 tests green, analyzer clean.

spike/block2_spike/lib/src/bech32.dart

@@ -0,0 +1,54 @@
+/// Self-contained bech32 encode (BIP-173 checksum), enough for NIP-19
+/// `npub`/`nsec`. Written inline to avoid a dependency's version quirks; NIP-19
+/// is plain bech32 (checksum constant 1), not bech32m.
+const _charset = 'qpzry9x8gf2tvdw0s3jn54khce6mua7l';
+
+int _polymod(List<int> values) {
+  const gen = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3];
+  var chk = 1;
+  for (final v in values) {
+    final top = chk >> 25;
+    chk = ((chk & 0x1ffffff) << 5) ^ v;
+    for (var i = 0; i < 5; i++) {
+      if ((top >> i) & 1 == 1) chk ^= gen[i];
+    }
+  }
+  return chk;
+}
+
+List<int> _hrpExpand(String hrp) => [
+  ...hrp.codeUnits.map((c) => c >> 5),
+  0,
+  ...hrp.codeUnits.map((c) => c & 31),
+];
+
+List<int> _createChecksum(String hrp, List<int> data) {
+  final values = [..._hrpExpand(hrp), ...data, 0, 0, 0, 0, 0, 0];
+  final mod = _polymod(values) ^ 1;
+  return List.generate(6, (i) => (mod >> (5 * (5 - i))) & 31);
+}
+
+/// Converts 8-bit bytes to 5-bit groups (bech32 payload), padding with zeros.
+List<int> _convertBits8to5(List<int> data) {
+  var acc = 0;
+  var bits = 0;
+  final out = <int>[];
+  for (final value in data) {
+    acc = (acc << 8) | value;
+    bits += 8;
+    while (bits >= 5) {
+      bits -= 5;
+      out.add((acc >> bits) & 31);
+    }
+  }
+  if (bits > 0) out.add((acc << (5 - bits)) & 31);
+  return out;
+}
+
+/// bech32-encodes [data] (raw bytes) under [hrp] (e.g. `npub`).
+String bech32Encode(String hrp, List<int> data) {
+  final five = _convertBits8to5(data);
+  final checksum = _createChecksum(hrp, five);
+  final combined = [...five, ...checksum];
+  return '${hrp}1${combined.map((v) => _charset[v]).join()}';
+}

spike/block2_spike/lib/src/hkdf.dart

@@ -0,0 +1,32 @@
+import 'dart:typed_data';
+
+import 'package:crypto/crypto.dart';
+
+/// Minimal, self-contained HKDF-SHA256 (RFC 5869), synchronous.
+///
+/// Spike-local so the derivation stays pure Dart and easy to reason about. In
+/// production this would reuse `commons_core`'s `Hkdf` (the `cryptography`
+/// package already ships one — see backup_box.dart), keyed by a domain string.
+Uint8List hkdfSha256({
+  required List<int> ikm,
+  required String info,
+  List<int> salt = const [],
+  int length = 32,
+}) {
+  // Extract.
+  final actualSalt = salt.isEmpty ? Uint8List(32) : salt; // HashLen zeros
+  final prk = Hmac(sha256, actualSalt).convert(ikm).bytes;
+
+  // Expand.
+  final infoBytes = info.codeUnits;
+  final out = <int>[];
+  var previous = <int>[];
+  var counter = 1;
+  while (out.length < length) {
+    final input = <int>[...previous, ...infoBytes, counter];
+    previous = Hmac(sha256, prk).convert(input).bytes;
+    out.addAll(previous);
+    counter++;
+  }
+  return Uint8List.fromList(out.sublist(0, length));
+}

spike/block2_spike/lib/src/mini_relay.dart

@@ -0,0 +1,124 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'nostr_event.dart';
+
+/// A tiny, in-process Nostr relay (NIP-01 subset): EVENT / REQ / EOSE / CLOSE,
+/// filtering by `kinds`, `authors`, `ids` and `#g` (exact tag). Enough to prove
+/// the publish→discover round-trip WITHOUT touching the network, so the test is
+/// hermetic and CI-safe. Not a real relay — no persistence, no NIP-11, no auth.
+///
+/// Addressable events (kind 30000–39999, NIP-99's 30402) replace by
+/// (kind, pubkey, `d`) like a real relay, so re-publishing an offer updates it.
+class MiniRelay {
+  MiniRelay._(this._server);
+
+  final HttpServer _server;
+  final List<NostrEvent> _events = [];
+  final Set<_Sub> _subs = {};
+
+  int get port => _server.port;
+  String get url => 'ws://127.0.0.1:$port';
+
+  /// Number of events currently stored (for assertions/metrics).
+  int get storedCount => _events.length;
+
+  static Future<MiniRelay> start() async {
+    final server = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
+    final relay = MiniRelay._(server);
+    relay._accept();
+    return relay;
+  }
+
+  void _accept() {
+    _server.listen((req) async {
+      if (!WebSocketTransformer.isUpgradeRequest(req)) {
+        req.response.statusCode = HttpStatus.badRequest;
+        await req.response.close();
+        return;
+      }
+      final ws = await WebSocketTransformer.upgrade(req);
+      final socketSubs = <_Sub>{};
+      ws.listen(
+        (data) => _onMessage(ws, socketSubs, data as String),
+        onDone: () => _subs.removeAll(socketSubs),
+        onError: (_) => _subs.removeAll(socketSubs),
+      );
+    });
+  }
+
+  void _onMessage(WebSocket ws, Set<_Sub> socketSubs, String data) {
+    final msg = jsonDecode(data) as List;
+    switch (msg[0]) {
+      case 'EVENT':
+        _handleEvent(ws, NostrEvent.fromJson(msg[1] as Map<String, dynamic>));
+      case 'REQ':
+        final subId = msg[1] as String;
+        final filters = msg.sublist(2).cast<Map<String, dynamic>>();
+        final sub = _Sub(ws, subId, filters);
+        _subs.add(sub);
+        socketSubs.add(sub);
+        for (final e in _events) {
+          if (sub.matches(e)) _send(ws, ['EVENT', subId, e.toJson()]);
+        }
+        _send(ws, ['EOSE', subId]);
+      case 'CLOSE':
+        final subId = msg[1] as String;
+        socketSubs.removeWhere((s) => s.id == subId);
+        _subs.removeWhere((s) => s.ws == ws && s.id == subId);
+    }
+  }
+
+  void _handleEvent(WebSocket ws, NostrEvent event) {
+    if (!event.verify()) {
+      _send(ws, ['OK', event.id, false, 'invalid: bad signature']);
+      return;
+    }
+    if (event.kind >= 30000 && event.kind < 40000) {
+      final d = event.tag('d') ?? '';
+      _events.removeWhere(
+        (e) => e.kind == event.kind && e.pubkey == event.pubkey && (e.tag('d') ?? '') == d,
+      );
+    }
+    _events.add(event);
+    _send(ws, ['OK', event.id, true, '']);
+    for (final sub in _subs) {
+      if (sub.matches(event)) _send(sub.ws, ['EVENT', sub.id, event.toJson()]);
+    }
+  }
+
+  void _send(WebSocket ws, Object message) => ws.add(jsonEncode(message));
+
+  Future<void> stop() => _server.close(force: true);
+}
+
+class _Sub {
+  _Sub(this.ws, this.id, this.filters);
+
+  final WebSocket ws;
+  final String id;
+  final List<Map<String, dynamic>> filters;
+
+  bool matches(NostrEvent e) => filters.any((f) => _matchesFilter(e, f));
+
+  bool _matchesFilter(NostrEvent e, Map<String, dynamic> f) {
+    if (f['kinds'] != null && !(f['kinds'] as List).contains(e.kind)) {
+      return false;
+    }
+    if (f['authors'] != null && !(f['authors'] as List).contains(e.pubkey)) {
+      return false;
+    }
+    if (f['ids'] != null && !(f['ids'] as List).contains(e.id)) return false;
+    for (final entry in f.entries) {
+      if (!entry.key.startsWith('#')) continue;
+      final tagName = entry.key.substring(1);
+      final wanted = (entry.value as List).cast<String>();
+      final present = e.tags
+          .where((t) => t.isNotEmpty && t[0] == tagName && t.length > 1)
+          .map((t) => t[1]);
+      if (!wanted.any(present.contains)) return false;
+    }
+    return true;
+  }
+}

spike/block2_spike/lib/src/nip99.dart

@@ -0,0 +1,118 @@
+import 'nostr_event.dart';
+import 'offer.dart';
+
+/// Maps [Offer] ↔ Nostr NIP-99 "Classified Listing" events (kind 30402), the
+/// first concrete `OfferTransport` backend (sharing-model.md §2).
+///
+/// This codec is where the privacy contract is *enforced on the wire*: the only
+/// location that ever leaves the device is a coarse geohash, truncated here to
+/// [maxGeohashChars]. There is no field that could carry the inventory or an
+/// exact address, because [Offer] itself has none — the seam does the work.
+class Nip99Codec {
+  /// NIP-99 addressable classified listing.
+  static const kindActive = 30402;
+
+  /// A geohash of 5 chars ≈ ±2.4 km cell — the coarse "near you" of the mocks.
+  /// Anything finer is refused so a slip in the caller can't leak a precise fix.
+  static const maxGeohashChars = 5;
+
+  /// Builds an UNSIGNED event from [offer]. Caller signs with the derived key.
+  NostrEvent encode(Offer offer, {required int createdAt}) {
+    final coarse = _coarsen(offer.approxGeohash);
+
+    final tags = <List<String>>[
+      ['d', offer.id],
+      ['title', offer.summary],
+      ['status', offer.status == OfferStatus.active ? 'active' : 'sold'],
+      // Reciprocity mode is a Tanemaki concept NIP-99 lacks; carry it explicitly.
+      ['offer_type', offer.type.name],
+    ];
+    // NIP-52 geohash ladder: one `g` tag per prefix length so an area query
+    // matches by *exact* tag (how Nostr relays filter) yet still finds coarser
+    // searches. Coarse only — NO "location"/address/exact-fix tag ever.
+    for (var i = 1; i <= coarse.length; i++) {
+      tags.add(['g', coarse.substring(0, i)]);
+    }
+    if (offer.category != null) tags.add(['t', offer.category!]);
+    if (offer.radiusKm != null) {
+      tags.add(['radius_km', '${offer.radiusKm}']);
+    }
+    if (offer.type == OfferType.sale && offer.priceAmount != null) {
+      tags.add([
+        'price',
+        '${offer.priceAmount}',
+        offer.priceCurrency ?? 'EUR',
+      ]);
+    }
+    if (offer.type == OfferType.exchange && offer.exchangeTerms != null) {
+      tags.add(['exchange_terms', offer.exchangeTerms!]);
+    }
+    if (offer.imageUrl != null) tags.add(['image', offer.imageUrl!]);
+    if (offer.expiresAt != null) {
+      tags.add([
+        'expiration',
+        '${offer.expiresAt!.millisecondsSinceEpoch ~/ 1000}',
+      ]);
+    }
+
+    return NostrEvent(
+      pubkey: offer.authorPubkeyHex,
+      createdAt: createdAt,
+      kind: kindActive,
+      tags: tags,
+      content: offer.summary,
+    );
+  }
+
+  /// Reconstructs an [Offer] from a received event.
+  Offer decode(NostrEvent event) {
+    final priceTag = _fullTag(event, 'price');
+    return Offer(
+      id: event.tag('d') ?? event.id,
+      authorPubkeyHex: event.pubkey,
+      summary: event.tag('title') ?? event.content,
+      type: _parseType(event.tag('offer_type')),
+      status: event.tag('status') == 'sold'
+          ? OfferStatus.closed
+          : OfferStatus.active,
+      category: event.tag('t'),
+      approxGeohash: _longestGeohash(event),
+      radiusKm: int.tryParse(event.tag('radius_km') ?? ''),
+      priceAmount: priceTag != null && priceTag.length > 1
+          ? num.tryParse(priceTag[1])
+          : null,
+      priceCurrency: priceTag != null && priceTag.length > 2
+          ? priceTag[2]
+          : null,
+      exchangeTerms: event.tag('exchange_terms'),
+      imageUrl: event.tag('image'),
+    );
+  }
+
+  String _coarsen(String geohash) => geohash.length > maxGeohashChars
+      ? geohash.substring(0, maxGeohashChars)
+      : geohash;
+
+  /// The finest `g` tag present (top of the prefix ladder).
+  String _longestGeohash(NostrEvent event) {
+    var best = '';
+    for (final t in event.tags) {
+      if (t.isNotEmpty && t[0] == 'g' && t.length > 1 && t[1].length > best.length) {
+        best = t[1];
+      }
+    }
+    return best;
+  }
+
+  List<String>? _fullTag(NostrEvent event, String name) {
+    for (final t in event.tags) {
+      if (t.isNotEmpty && t[0] == name) return t;
+    }
+    return null;
+  }
+
+  OfferType _parseType(String? name) => OfferType.values.firstWhere(
+    (t) => t.name == name,
+    orElse: () => OfferType.gift,
+  );
+}

spike/block2_spike/lib/src/nostr_event.dart

@@ -0,0 +1,77 @@
+import 'dart:convert';
+
+import 'package:bip340/bip340.dart' as bip340;
+import 'package:crypto/crypto.dart';
+
+/// A minimal Nostr event (NIP-01): enough to build, id, sign and serialise
+/// NIP-99 classified listings. Throwaway — production would use a vetted client.
+class NostrEvent {
+  NostrEvent({
+    required this.pubkey,
+    required this.createdAt,
+    required this.kind,
+    required this.tags,
+    required this.content,
+    this.id = '',
+    this.sig = '',
+  });
+
+  final String pubkey; // x-only hex
+  final int createdAt; // unix seconds
+  final int kind;
+  final List<List<String>> tags;
+  final String content;
+  String id;
+  String sig;
+
+  /// NIP-01 id: sha256 of the compact JSON array
+  /// `[0, pubkey, created_at, kind, tags, content]`.
+  String computeId() {
+    final serialized = jsonEncode([0, pubkey, createdAt, kind, tags, content]);
+    final digest = sha256.convert(utf8.encode(serialized));
+    return digest.toString(); // lower-case hex
+  }
+
+  /// Fills [id] and [sig] (BIP340 Schnorr over the id) with [privateKeyHex].
+  void signWith(String privateKeyHex, {String? auxRandHex}) {
+    id = computeId();
+    // Deterministic aux in the spike (derived from id) so round-trip tests are
+    // reproducible; production uses fresh randomness per BIP340.
+    final aux = auxRandHex ?? id;
+    sig = bip340.sign(privateKeyHex, id, aux);
+  }
+
+  /// Verifies the signature against [pubkey]. Cross-checks that a real relay /
+  /// peer would accept what we produced.
+  bool verify() => id == computeId() && bip340.verify(pubkey, id, sig);
+
+  Map<String, dynamic> toJson() => {
+    'id': id,
+    'pubkey': pubkey,
+    'created_at': createdAt,
+    'kind': kind,
+    'tags': tags,
+    'content': content,
+    'sig': sig,
+  };
+
+  static NostrEvent fromJson(Map<String, dynamic> j) => NostrEvent(
+    id: j['id'] as String,
+    pubkey: j['pubkey'] as String,
+    createdAt: j['created_at'] as int,
+    kind: j['kind'] as int,
+    tags: (j['tags'] as List)
+        .map((t) => (t as List).map((e) => e as String).toList())
+        .toList(),
+    content: j['content'] as String,
+    sig: j['sig'] as String,
+  );
+
+  /// First value of the first tag named [name], or null.
+  String? tag(String name) {
+    for (final t in tags) {
+      if (t.isNotEmpty && t[0] == name && t.length > 1) return t[1];
+    }
+    return null;
+  }
+}

spike/block2_spike/lib/src/nostr_key.dart

@@ -0,0 +1,77 @@
+import 'dart:typed_data';
+
+import 'package:bip340/bip340.dart' as bip340;
+
+import 'bech32.dart';
+import 'hkdf.dart';
+
+/// A Nostr identity (secp256k1 / BIP340 x-only) deterministically derived from
+/// the Duniter/Ğ1-style root seed. This is Spike Question 1: prove we can keep
+/// "one identity" (user backs up ONE seed) while still having a viable Nostr
+/// transport key.
+///
+/// Derivation is one-way by construction: the root seed feeds HKDF, and HKDF's
+/// PRF (HMAC-SHA256) is not invertible, so neither the private nor the public
+/// Nostr key leaks the root seed. See spike-block2-findings.md §1.
+class NostrKey {
+  NostrKey._({required this.privateKeyHex, required this.publicKeyHex});
+
+  /// 32-byte secp256k1 scalar, lower-case hex (64 chars).
+  final String privateKeyHex;
+
+  /// 32-byte BIP340 x-only public key, lower-case hex (64 chars). This is the
+  /// Nostr identity that appears on events and in `npub`.
+  final String publicKeyHex;
+
+  /// NIP-19 `npub…` encoding of [publicKeyHex] — what a user would share.
+  String get npub => bech32Encode('npub', _hexToBytes(publicKeyHex));
+
+  /// NIP-19 `nsec…` encoding of [privateKeyHex] — secret, never shared.
+  String get nsec => bech32Encode('nsec', _hexToBytes(privateKeyHex));
+
+  /// Domain-separated derivation label. Versioned so a future scheme can coexist
+  /// with old backups (same discipline as BackupBox's HKDF info string).
+  static const derivationInfo = 'org.comunes.tane/nostr/secp256k1/v1';
+
+  /// secp256k1 group order n.
+  static final BigInt _n = BigInt.parse(
+    'FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141',
+    radix: 16,
+  );
+
+  /// Deterministically derives the Nostr key from a root [seed] (the 32 bytes
+  /// [IdentityService.generateRootSeed] produces).
+  ///
+  /// Reduces to a valid scalar in `[1, n-1]` by rejection: on the (astronomically
+  /// rare) miss it bumps a counter in the HKDF info, so the result stays a pure
+  /// function of the seed.
+  factory NostrKey.deriveFromSeed(List<int> seed) {
+    var counter = 0;
+    while (true) {
+      final okm = hkdfSha256(ikm: seed, info: '$derivationInfo/$counter');
+      final d = _bytesToBigInt(okm);
+      if (d != BigInt.zero && d < _n) {
+        final privHex = d.toRadixString(16).padLeft(64, '0');
+        final pubHex = bip340.getPublicKey(privHex);
+        return NostrKey._(privateKeyHex: privHex, publicKeyHex: pubHex);
+      }
+      counter++;
+    }
+  }
+}
+
+BigInt _bytesToBigInt(List<int> bytes) {
+  var result = BigInt.zero;
+  for (final b in bytes) {
+    result = (result << 8) | BigInt.from(b);
+  }
+  return result;
+}
+
+Uint8List _hexToBytes(String hex) {
+  final out = Uint8List(hex.length ~/ 2);
+  for (var i = 0; i < out.length; i++) {
+    out[i] = int.parse(hex.substring(i * 2, i * 2 + 2), radix: 16);
+  }
+  return out;
+}

spike/block2_spike/lib/src/nostr_offer_transport.dart

@@ -0,0 +1,152 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'nip99.dart';
+import 'nostr_event.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.
+class NostrOfferTransport implements OfferTransport {
+  NostrOfferTransport._(this._socket, this._privateKeyHex, this._pubkeyHex) {
+    _socket.listen(_onData, onDone: _onDone);
+  }
+
+  final WebSocket _socket;
+  final String _privateKeyHex;
+  final String _pubkeyHex;
+  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`.
+  static Future<NostrOfferTransport> connect(
+    String relayUrl, {
+    required String privateKeyHex,
+    required String pubkeyHex,
+  }) async {
+    final socket = await WebSocket.connect(relayUrl);
+    return NostrOfferTransport._(socket, privateKeyHex, pubkeyHex);
+  }
+
+  void _onData(dynamic data) => _incoming.add(jsonDecode(data as String) as List);
+  void _onDone() {
+    if (!_incoming.isClosed) _incoming.close();
+  }
+
+  @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));
+    return PublishResult(
+      accepted: response[2] as bool,
+      transportRef: event.id,
+      message: response.length > 3 ? response[3] as String : '',
+    );
+  }
+
+  @override
+  Stream<Offer> discover(DiscoveryQuery query) {
+    final subId = 'sub${_subCounter++}';
+    final filter = <String, dynamic>{
+      'kinds': [Nip99Codec.kindActive],
+      '#g': [query.geohashPrefix],
+      'limit': query.limit,
+    };
+
+    late StreamController<Offer> controller;
+    controller = StreamController<Offer>(
+      onListen: () => _socket.add(jsonEncode(['REQ', subId, filter])),
+      onCancel: () {
+        _socket.add(jsonEncode(['CLOSE', subId]));
+      },
+    );
+
+    final sub = _incoming.stream.listen((m) {
+      if (m[0] == 'EVENT' && m[1] == subId) {
+        final offer = _codec.decode(
+          NostrEvent.fromJson(m[2] as Map<String, dynamic>),
+        );
+        if (query.types.isEmpty || query.types.contains(offer.type)) {
+          controller.add(offer);
+        }
+      }
+      // EOSE arrives here too; we keep the stream open for live offers.
+    });
+    controller.onCancel = () {
+      _socket.add(jsonEncode(['CLOSE', subId]));
+      sub.cancel();
+    };
+    return controller.stream;
+  }
+
+  /// Convenience for tests/metrics: collect matches up to EOSE, then stop.
+  Future<List<Offer>> discoverUntilEose(DiscoveryQuery query) async {
+    final subId = 'once${_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;
+  }
+
+  @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()]));
+  }
+
+  @override
+  Future<void> close() async {
+    await _socket.close();
+    if (!_incoming.isClosed) await _incoming.close();
+  }
+}

spike/block2_spike/lib/src/offer.dart

@@ -0,0 +1,90 @@
+/// The four (well, five) reciprocity modes from sharing-model.md §3, plus the
+/// generic `lend` the core reserves for the future "library of things".
+enum OfferType { gift, exchange, sale, wanted, lend }
+
+enum OfferStatus { active, reserved, closed }
+
+/// A publishable offer — the "shop window", deliberately DECOUPLED from Lot /
+/// inventory (sharing-model.md §2, core-domain-boundary.md §4.3).
+///
+/// It carries only a chosen, denormalised *summary* of what's offered — never a
+/// foreign key into the seed tables, never the full inventory, never an exact
+/// address. This is the privacy seam: the core can publish and sync offers
+/// WITHOUT understanding seeds. Everything here is what the person elected to
+/// reveal.
+class Offer {
+  const Offer({
+    required this.id,
+    required this.authorPubkeyHex,
+    required this.summary,
+    required this.type,
+    required this.approxGeohash,
+    this.status = OfferStatus.active,
+    this.category,
+    this.priceAmount,
+    this.priceCurrency,
+    this.exchangeTerms,
+    this.radiusKm,
+    this.imageUrl,
+    this.expiresAt,
+  });
+
+  /// Stable per-offer identifier (NIP-99 `d` tag → addressable event).
+  final String id;
+
+  /// The publishing identity (BIP340 x-only pubkey hex). Derived, per Q1.
+  final String authorPubkeyHex;
+
+  /// Human summary the author chose to publish (e.g. "Tomate rosa de Barbastro").
+  final String summary;
+
+  final OfferType type;
+  final OfferStatus status;
+
+  /// Free-text category, prefilled in-app from Species.family but opaque here.
+  final String? category;
+
+  /// LOW-PRECISION geohash only (sharing-model.md §2: "10 km near you", never
+  /// exact coordinates). The spike enforces a max length in the transport.
+  final String approxGeohash;
+  final int? radiusKm;
+
+  final num? priceAmount; // only for sale
+  final String? priceCurrency; // ISO 4217 or a community currency (e.g. "G1")
+  final String? exchangeTerms; // only for exchange
+
+  final String? imageUrl;
+  final DateTime? expiresAt;
+}
+
+/// Result of publishing to a transport.
+class PublishResult {
+  const PublishResult({
+    required this.accepted,
+    required this.transportRef,
+    this.message = '',
+  });
+
+  /// Whether the relay/instance accepted the event.
+  final bool accepted;
+
+  /// The transport-side id (Nostr event id / ActivityPub object id).
+  final String transportRef;
+  final String message;
+}
+
+/// A discovery query: coarse location + optional facets. Note there is NO way to
+/// ask for "everything from person X's inventory" — you can only browse the
+/// public shop window by area and kind.
+class DiscoveryQuery {
+  const DiscoveryQuery({
+    required this.geohashPrefix,
+    this.types = const {},
+    this.limit = 100,
+  });
+
+  /// Coarse geohash prefix to search near (e.g. "u09" ≈ tens of km).
+  final String geohashPrefix;
+  final Set<OfferType> types;
+  final int limit;
+}

spike/block2_spike/lib/src/offer_transport.dart

@@ -0,0 +1,29 @@
+import 'offer.dart';
+
+/// The abstraction the plan promises lives in `commons_core` (core-domain-
+/// boundary.md §2, PLAN §4): publish an [Offer], discover offers by area. The
+/// concrete Nostr NIP-99 backend sits behind this so the domain never talks to
+/// a relay directly, and a second backend (ActivityPub / FEP-0837) could slot
+/// in without the app noticing.
+///
+/// Spike Question 2 asks whether messaging (NIP-17) and trust (NIP-85) fit
+/// behind this same seam, or whether they couple more than the plan implies.
+/// The finding: they do NOT belong here — they share the *key* and the *relay
+/// connection*, but their verbs differ (send/receive a private DM; assert/read
+/// a certification). Forcing them behind `OfferTransport` would overload it.
+/// See spike-block2-findings.md §2 for the proposed 3-interface split over one
+/// shared `NostrConnection`.
+abstract interface class OfferTransport {
+  /// Publishes (or replaces) [offer]. Idempotent on the offer id.
+  Future<PublishResult> publish(Offer offer);
+
+  /// Streams offers matching [query]. Emits already-stored matches, then live
+  /// ones as they arrive, until the caller cancels the subscription.
+  Stream<Offer> discover(DiscoveryQuery query);
+
+  /// Retracts a previously published offer (NIP-99: publish a `closed` status /
+  /// NIP-09 deletion). Relays that already replicated it drop it over time.
+  Future<void> retract(String offerId);
+
+  Future<void> close();
+}