Add NIP-17 gift-wrapped direct messages

Add outbound NIP-17 private messaging: build the kind-14 rumor -> kind-13
seal -> kind-1059 gift wrap chain on the existing NIP-44 primitives, wrapped
in NostrDmActor (WrapDirectMessage/GiftWrapped). Publishing is left to
NostrPublishActor.

Extract nostrEventId() from signNostrEvent so the unsigned rumor can compute
its id without a signature. Export the new actor and nip17 helpers. Add a
round-trip test (wrap -> unwrap) and a negative decryption test.

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

lib/src/nip17.dart

@@ -0,0 +1,136 @@
+import 'dart:convert';
+import 'dart:math';
+
+import 'package:bip340/bip340.dart' as bip340;
+
+import 'nip44.dart';
+import 'nostr_event.dart';
+
+/// NIP-17 private direct messages over NIP-59 gift wraps.
+///
+/// A message is built in three layers:
+///   1. an unsigned **rumor** (the real event, here kind 14) carrying the
+///      recipient `p` tag;
+///   2. a **seal** (kind 13) signed by the sender, whose content is the rumor
+///      NIP-44-encrypted to the recipient;
+///   3. a **gift wrap** (kind 1059) signed by a throwaway ephemeral key, whose
+///      content is the seal NIP-44-encrypted to the recipient.
+///
+/// Only the gift wrap is published. The recipient's pubkey appears just in the
+/// gift wrap's `p` tag; the sender's identity is hidden until the recipient
+/// decrypts the seal. Seal and wrap timestamps are randomized into the past
+/// (up to two days) so they leak nothing about send time.
+
+const int kSealKind = 13;
+const int kGiftWrapKind = 1059;
+const int kDirectMessageKind = 14;
+
+const int _twoDaysSeconds = 2 * 24 * 60 * 60;
+
+final Random _rng = Random.secure();
+
+int _nowSeconds() => DateTime.now().millisecondsSinceEpoch ~/ 1000;
+
+/// A timestamp randomly displaced up to two days into the past (NIP-59).
+int _randomPastTimestamp() => _nowSeconds() - _rng.nextInt(_twoDaysSeconds);
+
+String _randomPrivateKey() {
+  final bytes = List.generate(32, (_) => _rng.nextInt(256));
+  return bytes.map((b) => b.toRadixString(16).padLeft(2, '0')).join();
+}
+
+/// Builds a NIP-59 gift wrap (kind 1059) carrying a NIP-17 direct message.
+///
+/// [rumorKind] defaults to kind 14 (the NIP-17 chat message). [rumorTags] are
+/// merged with the recipient `p` tag. Returns the signed gift-wrap event as a
+/// JSON string ready to publish.
+String giftWrap({
+  required String senderPrivKey,
+  required String recipientPubKey,
+  required String content,
+  int rumorKind = kDirectMessageKind,
+  List<List<String>> rumorTags = const [],
+}) {
+  final senderPubKey = bip340.getPublicKey(senderPrivKey);
+
+  // 1. Unsigned rumor — has an id but no signature.
+  final rumorCreatedAt = _nowSeconds();
+  final rumorTagsWithRecipient = <List<String>>[
+    ['p', recipientPubKey],
+    ...rumorTags,
+  ];
+  final rumorId = nostrEventId(
+    publicKey: senderPubKey,
+    createdAt: rumorCreatedAt,
+    kind: rumorKind,
+    tags: rumorTagsWithRecipient,
+    content: content,
+  );
+  final rumor = <String, dynamic>{
+    'id': rumorId,
+    'pubkey': senderPubKey,
+    'created_at': rumorCreatedAt,
+    'kind': rumorKind,
+    'tags': rumorTagsWithRecipient,
+    'content': content,
+  };
+
+  // 2. Seal (kind 13) signed by the sender; content = rumor encrypted to
+  //    the recipient. No tags, randomized timestamp.
+  final sealContent =
+      nip44Encrypt(jsonEncode(rumor), senderPrivKey, recipientPubKey);
+  final seal = signNostrEvent(
+    privateKey: senderPrivKey,
+    publicKey: senderPubKey,
+    kind: kSealKind,
+    tags: const [],
+    content: sealContent,
+    createdAt: _randomPastTimestamp(),
+  );
+
+  // 3. Gift wrap (kind 1059) signed by an ephemeral key; content = seal
+  //    encrypted to the recipient, `p` tag addresses the recipient.
+  final ephemeralPrivKey = _randomPrivateKey();
+  final ephemeralPubKey = bip340.getPublicKey(ephemeralPrivKey);
+  final wrapContent = nip44Encrypt(seal, ephemeralPrivKey, recipientPubKey);
+  return signNostrEvent(
+    privateKey: ephemeralPrivKey,
+    publicKey: ephemeralPubKey,
+    kind: kGiftWrapKind,
+    tags: [
+      ['p', recipientPubKey],
+    ],
+    content: wrapContent,
+    createdAt: _randomPastTimestamp(),
+  );
+}
+
+/// Unwraps a gift wrap (kind 1059) received by the holder of [recipientPrivKey].
+/// Returns the inner rumor as a decoded map (`id`, `pubkey`, `kind`, `tags`,
+/// `content`, …). Throws if decryption or the kind chain is invalid.
+///
+/// Provided so receivers (a later version) and tests can reverse [giftWrap].
+Map<String, dynamic> unwrapGift(
+  String giftWrapEvent,
+  String recipientPrivKey,
+) {
+  final wrap = jsonDecode(giftWrapEvent) as Map<String, dynamic>;
+  if (wrap['kind'] != kGiftWrapKind) {
+    throw ArgumentError('Not a gift wrap (kind ${wrap['kind']})');
+  }
+  final sealJson = nip44Decrypt(
+    wrap['content'] as String,
+    recipientPrivKey,
+    wrap['pubkey'] as String,
+  );
+  final seal = jsonDecode(sealJson) as Map<String, dynamic>;
+  if (seal['kind'] != kSealKind) {
+    throw ArgumentError('Not a seal (kind ${seal['kind']})');
+  }
+  final rumorJson = nip44Decrypt(
+    seal['content'] as String,
+    recipientPrivKey,
+    seal['pubkey'] as String,
+  );
+  return jsonDecode(rumorJson) as Map<String, dynamic>;
+}

lib/src/nostr_dm_actor.dart

@@ -0,0 +1,61 @@
+import 'package:actors/actors.dart';
+
+import 'nip17.dart';
+
+sealed class DmMessage {
+  const DmMessage();
+}
+
+/// Wrap [content] (with optional [rumorTags]) as a NIP-17 direct message
+/// gift-wrapped to [recipientPubKey]. The result is a signed kind-1059 event
+/// ready to publish via `NostrPublishActor`.
+final class WrapDirectMessage extends DmMessage {
+  final String recipientPubKey;
+  final String content;
+  final int rumorKind;
+  final List<List<String>> rumorTags;
+  const WrapDirectMessage({
+    required this.recipientPubKey,
+    required this.content,
+    this.rumorKind = kDirectMessageKind,
+    this.rumorTags = const [],
+  });
+}
+
+sealed class DmResult {
+  const DmResult();
+}
+
+final class GiftWrapped extends DmResult {
+  final String event;
+  const GiftWrapped(this.event);
+}
+
+/// Builds NIP-17 gift-wrapped direct messages with the holder's private key.
+/// Construction only — publishing is left to `NostrPublishActor`.
+class NostrDmActor with Handler<DmMessage, DmResult> {
+  final String _privateKey;
+
+  NostrDmActor(String privateKey) : _privateKey = privateKey;
+
+  @override
+  Future<DmResult> handle(DmMessage message) => switch (message) {
+        WrapDirectMessage(
+          :final recipientPubKey,
+          :final content,
+          :final rumorKind,
+          :final rumorTags,
+        ) =>
+          Future.value(
+            GiftWrapped(
+              giftWrap(
+                senderPrivKey: _privateKey,
+                recipientPubKey: recipientPubKey,
+                content: content,
+                rumorKind: rumorKind,
+                rumorTags: rumorTags,
+              ),
+            ),
+          ),
+      };
+}

lib/src/nostr_event.dart

@@ -4,6 +4,22 @@ import 'dart:math';
 import 'package:bip340/bip340.dart' as bip340;
 import 'package:crypto/crypto.dart';
 
+/// Computes the Nostr event id: the lowercase hex SHA-256 of the canonical
+/// serialization `[0, pubkey, created_at, kind, tags, content]`. Shared by
+/// [signNostrEvent] and by unsigned events such as NIP-59 rumors, which carry
+/// an id but no signature.
+String nostrEventId({
+  required String publicKey,
+  required int createdAt,
+  required int kind,
+  required List<List<String>> tags,
+  required String content,
+}) {
+  final ser = jsonEncode([0, publicKey, createdAt, kind, tags, content]);
+  final idBytes = sha256.convert(utf8.encode(ser)).bytes;
+  return idBytes.map((b) => b.toRadixString(16).padLeft(2, '0')).join();
+}
+
 String signNostrEvent({
   required String privateKey,
   required String publicKey,
@@ -13,9 +29,13 @@ String signNostrEvent({
   int? createdAt,
 }) {
   final ts = createdAt ?? DateTime.now().millisecondsSinceEpoch ~/ 1000;
-  final ser = jsonEncode([0, publicKey, ts, kind, tags, content]);
-  final idBytes = sha256.convert(utf8.encode(ser)).bytes;
-  final id = idBytes.map((b) => b.toRadixString(16).padLeft(2, '0')).join();
+  final id = nostrEventId(
+    publicKey: publicKey,
+    createdAt: ts,
+    kind: kind,
+    tags: tags,
+    content: content,
+  );
 
   final rng = Random.secure();
   final aux = List.generate(32, (_) => rng.nextInt(256))

lib/swarm.dart

@@ -10,3 +10,5 @@ export 'src/nostr_account_actor.dart';
 export 'src/nip46_signer_actor.dart';
 export 'src/nip19.dart';
 export 'src/nip05.dart';
+export 'src/nip17.dart';
+export 'src/nostr_dm_actor.dart';