Add NIP-19 (npub/nsec), NIP-05 verification, and arbitrary-pubkey profile fetch

- nip19.dart: bech32 encodeNpub/encodeNsec for raw keys (no TLV entities),
  verified against the canonical NIP-19 test vectors.
- nip05.dart: verifyNip05 with an injectable http.Client plus a pure
  nip05MatchesPubkey helper for the .well-known/nostr.json check.
- fetchProfileFor(fetcher, pubkey): fetches and parses any user's kind-0 into
  a typed NostrProfile (newest-wins), generalizing the own-key FetchProfile.
- Export all three from swarm.dart; unit tests for each (bech32 vectors,
  MockClient HTTP cases, fake-relay profile parsing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/src/nip05.dart

@@ -0,0 +1,45 @@
+import 'dart:convert';
+
+import 'package:http/http.dart' as http;
+
+/// Verifies a NIP-05 identifier (`name@domain`) by fetching the domain's
+/// `.well-known/nostr.json?name=<name>` and checking it maps `name` to
+/// [pubkeyHex]. Returns false on any malformed input, network error, or
+/// mismatch. [client] is injectable for testing.
+Future<bool> verifyNip05(
+  String nip05,
+  String pubkeyHex, {
+  http.Client? client,
+}) async {
+  final parts = nip05.split('@');
+  if (parts.length != 2 || parts[0].isEmpty || parts[1].isEmpty) return false;
+  final name = parts[0];
+  final domain = parts[1];
+
+  final c = client ?? http.Client();
+  try {
+    final url = Uri.https(domain, '/.well-known/nostr.json', {'name': name});
+    final resp = await c.get(url);
+    if (resp.statusCode != 200) return false;
+    return nip05MatchesPubkey(resp.body, name, pubkeyHex);
+  } catch (_) {
+    return false;
+  } finally {
+    if (client == null) c.close();
+  }
+}
+
+/// Pure check: does the `.well-known/nostr.json` document [body] map [name] to
+/// [pubkeyHex] under its `names` object? Pubkey comparison is case-insensitive.
+bool nip05MatchesPubkey(String body, String name, String pubkeyHex) {
+  try {
+    final json = jsonDecode(body);
+    if (json is! Map) return false;
+    final names = json['names'];
+    if (names is! Map) return false;
+    final mapped = names[name];
+    return mapped is String && mapped.toLowerCase() == pubkeyHex.toLowerCase();
+  } catch (_) {
+    return false;
+  }
+}

lib/src/nip19.dart

@@ -0,0 +1,82 @@
+// NIP-19 bech32 entity encoding for raw keys (`npub` / `nsec`). Pure and
+// dependency-free. Only the bare key entities are supported (no TLV types like
+// `nprofile`/`nevent`), which is all the app needs for copy/export.
+
+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) {
+  final out = <int>[];
+  for (final c in hrp.codeUnits) {
+    out.add(c >> 5);
+  }
+  out.add(0);
+  for (final c in hrp.codeUnits) {
+    out.add(c & 31);
+  }
+  return out;
+}
+
+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);
+}
+
+String _bech32Encode(String hrp, List<int> data) {
+  final combined = [...data, ..._createChecksum(hrp, data)];
+  final sb = StringBuffer(hrp)..write('1');
+  for (final d in combined) {
+    sb.write(_charset[d]);
+  }
+  return sb.toString();
+}
+
+/// Regroups 8-bit bytes into 5-bit groups (with zero padding) for bech32.
+List<int> _to5Bit(List<int> bytes) {
+  var acc = 0;
+  var bits = 0;
+  final out = <int>[];
+  for (final b in bytes) {
+    acc = (acc << 8) | b;
+    bits += 8;
+    while (bits >= 5) {
+      bits -= 5;
+      out.add((acc >> bits) & 31);
+    }
+  }
+  if (bits > 0) out.add((acc << (5 - bits)) & 31);
+  return out;
+}
+
+List<int> _hexToBytes(String hex) {
+  if (hex.length != 64) {
+    throw ArgumentError(
+      'expected a 32-byte key (64 hex chars), got ${hex.length}',
+    );
+  }
+  return [
+    for (var i = 0; i < 64; i += 2) int.parse(hex.substring(i, i + 2), radix: 16),
+  ];
+}
+
+String _encodeKey(String hrp, String hex) =>
+    _bech32Encode(hrp, _to5Bit(_hexToBytes(hex)));
+
+/// Encodes a 32-byte hex public key as a NIP-19 `npub1…` string.
+String encodeNpub(String pubkeyHex) => _encodeKey('npub', pubkeyHex);
+
+/// Encodes a 32-byte hex private key as a NIP-19 `nsec1…` string.
+String encodeNsec(String privkeyHex) => _encodeKey('nsec', privkeyHex);

lib/src/nostr_account_actor.dart

@@ -57,6 +57,44 @@ class NostrProfile {
       };
 }
 
+/// Fetches and parses the kind-0 profile for an arbitrary [pubkeyHex] via
+/// [fetcher], returning the newest one (or null if none is found or it can't be
+/// parsed). Unlike [NostrAccountActor]'s [FetchProfile], this works for any
+/// user, not just the loaded account.
+Future<NostrProfile?> fetchProfileFor(
+  NostrFetchActor fetcher,
+  String pubkeyHex,
+) async {
+  final filter = jsonEncode({
+    'kinds': [0],
+    'authors': [pubkeyHex],
+    'limit': 5,
+  });
+  final result = await fetcher.handle(FetchEvents(filter));
+  final events = (result as EventsFetched).events;
+
+  Map<String, dynamic>? newest;
+  for (final raw in events) {
+    try {
+      final ev = jsonDecode(raw) as Map<String, dynamic>;
+      final ts = ev['created_at'];
+      if (ts is! int) continue;
+      if (newest == null || ts > (newest['created_at'] as int)) newest = ev;
+    } catch (_) {
+      // skip malformed events
+    }
+  }
+  if (newest == null) return null;
+
+  try {
+    return NostrProfile.fromJson(
+      jsonDecode(newest['content'] as String) as Map<String, dynamic>,
+    );
+  } catch (_) {
+    return null;
+  }
+}
+
 // =============== COMMANDS ===============
 
 sealed class AccountMessage {

lib/swarm.dart

@@ -8,3 +8,5 @@ export 'src/file_upload_actor.dart';
 export 'src/file_download_actor.dart';
 export 'src/nostr_account_actor.dart';
 export 'src/nip46_signer_actor.dart';
+export 'src/nip19.dart';
+export 'src/nip05.dart';