verifyNostrEvent helper

lib/src/nip17.dart

@@ -9,23 +9,8 @@ import 'nostr_event.dart';
 /// Recomputes a signed event's id from its fields and checks the BIP-340
 /// signature against its pubkey. Throws if either is inconsistent.
 void _verifySignedEvent(Map<String, dynamic> event) {
-  final id = nostrEventId(
+  if (!verifyNostrEvent(event)) {
-    publicKey: event['pubkey'] as String,
+    throw ArgumentError('Event id or signature is invalid');
-    createdAt: event['created_at'] as int,
-    kind: event['kind'] as int,
-    tags:
-        (event['tags'] as List).map((t) => (t as List).cast<String>()).toList(),
-    content: event['content'] as String,
-  );
-  if (id != event['id']) {
-    throw ArgumentError('Event id does not match its contents');
-  }
-  if (!bip340.verify(
-    event['pubkey'] as String,
-    id,
-    event['sig'] as String,
-  )) {
-    throw ArgumentError('Event signature is invalid');
   }
 }
 

lib/src/nostr_event.dart

@@ -4,6 +4,30 @@ import 'dart:math';
 import 'package:bip340/bip340.dart' as bip340;
 import 'package:crypto/crypto.dart';
 
+/// Verifies a decoded Nostr event: recomputes its id from the canonical
+/// serialization and checks the BIP-340 signature against its pubkey. Returns
+/// false (rather than throwing) for a malformed map, a mismatched id, or a bad
+/// signature, so callers can drop untrusted events fetched from a relay.
+bool verifyNostrEvent(Map<String, dynamic> event) {
+  try {
+    final publicKey = event['pubkey'] as String;
+    final sig = event['sig'] as String;
+    final id = nostrEventId(
+      publicKey: publicKey,
+      createdAt: event['created_at'] as int,
+      kind: event['kind'] as int,
+      tags: (event['tags'] as List)
+          .map((t) => (t as List).cast<String>())
+          .toList(),
+      content: event['content'] as String,
+    );
+    if (id != event['id']) return false;
+    return bip340.verify(publicKey, id, sig);
+  } catch (_) {
+    return false;
+  }
+}
+
 /// 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

lib/swarm.dart

@@ -3,6 +3,7 @@ export 'src/relay_connection.dart';
 export 'src/nostr_publish_actor.dart';
 export 'src/nostr_fetch_actor.dart';
 export 'src/nostr_signer_actor.dart';
+export 'src/nostr_event.dart' show verifyNostrEvent;
 export 'src/geo_actor.dart';
 export 'src/file_upload_actor.dart';
 export 'src/file_download_actor.dart';

test/nostr_event_test.dart

@@ -0,0 +1,53 @@
+import 'dart:convert';
+
+import 'package:swarm/swarm.dart';
+import 'package:test/test.dart';
+
+void main() {
+  // Produces a genuinely-signed event so verifyNostrEvent has a real signature
+  // to check (signed with the throwaway key below).
+  const privKey =
+      '0000000000000000000000000000000000000000000000000000000000000001';
+
+  Future<Map<String, dynamic>> signedEvent() async {
+    final signer = NostrSignerActor(privKey);
+    final result =
+        await signer.handle(
+              const SignEvent(kind: 1985, content: '', tags: [
+                ['L', 'app.xfay.moderation'],
+                ['l', 'hide', 'app.xfay.moderation'],
+                ['e', 'imageeventid'],
+              ]),
+            )
+            as EventSigned;
+    return jsonDecode(result.event) as Map<String, dynamic>;
+  }
+
+  test('accepts a genuinely signed event', () async {
+    expect(verifyNostrEvent(await signedEvent()), isTrue);
+  });
+
+  test('rejects a tampered content', () async {
+    final event = await signedEvent();
+    event['content'] = 'tampered';
+    expect(verifyNostrEvent(event), isFalse);
+  });
+
+  test('rejects a tampered id', () async {
+    final event = await signedEvent();
+    event['id'] = '${event['id']}'.replaceRange(0, 1, '0');
+    expect(verifyNostrEvent(event), isFalse);
+  });
+
+  test('rejects a forged author (pubkey swapped)', () async {
+    final event = await signedEvent();
+    event['pubkey'] =
+        '0000000000000000000000000000000000000000000000000000000000000002';
+    expect(verifyNostrEvent(event), isFalse);
+  });
+
+  test('returns false for a malformed map instead of throwing', () {
+    expect(verifyNostrEvent(const {}), isFalse);
+    expect(verifyNostrEvent(const {'kind': 1}), isFalse);
+  });
+}