perf(mobile): isolate native signing and measure Dart comparison

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/README.md

@@ -83,8 +83,8 @@ runner. The internal release toolchain is configured separately in
 
 Use `shared/crypto/event_signer.dart` for signed events. iOS calls the existing
 CSecp256k1 library through synchronous FFI; other platforms retain the Dart
-signer. Burst-producing media authentication retains its bounded worker path
-because Debug Simulator bursts still exceed a frame budget. Event hashing remains in `nostr.Event.unsigned`. The native context is
+signer. This change preserves existing call scheduling; worker offloading is a
+separate change. Event hashing remains in `nostr.Event.unsigned`. The native context is
 randomized once and shared read-only; each call creates, self-verifies with,
 and destroys its own keypair. It never caches account secrets.
 
@@ -94,6 +94,12 @@ cross-implementation tests additionally accept `BUZZ_SIGNER_LIBRARY` pointing
 to a locally compiled adapter library. `tool/benchmark_event_signer.dart`
 measures complete host calls; `tool/native_signing_probe.dart` is an explicit
 Simulator entry point for complete event, media-auth, and NIP-98 measurements.
+`tool/compare_event_signers.dart` compares the existing Dart signer (with a
+cached public key) and native signing in the same iOS process, on the UI isolate,
+with identical inputs and alternating order. Seven warm samples on a Debug
+iPhone 17 Pro Simulator measured median 64-event bursts of 209.7 ms (Dart) and
+50.2 ms (native), a 76% reduction. These are synchronous event-construction
+timings, not UIKit main-thread CPU measurements or rendered-frame timings.
 Neither benchmark represents physical-device release performance.
 
 ## Run

mobile/lib/shared/crypto/event_signer.dart

@@ -7,11 +7,13 @@ import 'package:nostr/nostr.dart' as nostr;
 /// Signs canonical Nostr events with libsecp256k1 on iOS.
 ///
 /// Native keypairs exist only for this call and are wiped before returning.
-/// Other platforms retain the existing Dart implementation.
+/// Other platforms retain the existing Dart implementation, including the
+/// optional cached [pubkey]. iOS derives its public key from the native keypair.
 nostr.Event signEvent({
   required int kind,
   required String content,
   required String secretKey,
+  String? pubkey,
   int? createdAt,
   List<List<String>> tags = const [],
   bool verify = false,
@@ -21,6 +23,7 @@ nostr.Event signEvent({
       kind: kind,
       content: content,
       secretKey: secretKey,
+      pubkey: pubkey,
       createdAt: createdAt,
       tags: tags,
       verify: verify,

mobile/tool/compare_event_signers.dart

@@ -0,0 +1,87 @@
+import 'dart:io';
+import 'package:buzz/shared/crypto/event_signer.dart';
+import 'package:flutter/material.dart';
+import 'package:nostr/nostr.dart' as nostr;
+
+// Run on iOS with the same event construction on the UI isolate for both paths.
+void main() {
+  WidgetsFlutterBinding.ensureInitialized();
+  try {
+    if (!Platform.isIOS) throw StateError('Requires iOS');
+    final results = <String>[];
+    final key = '0' * 63 + '3';
+    final publicKey = nostr.Keys(key).public;
+    nostr.Event create(bool native) {
+      final tags = [
+        ['t', 'get'],
+        ['expiration', '1800000060'],
+      ];
+      return native
+          ? signEvent(
+              kind: 24242,
+              content: 'Get buzz-media',
+              tags: tags,
+              createdAt: 1800000000,
+              secretKey: key,
+              pubkey: publicKey,
+            )
+          : nostr.Event.from(
+              kind: 24242,
+              content: 'Get buzz-media',
+              tags: tags,
+              createdAt: 1800000000,
+              secretKey: key,
+              pubkey: publicKey,
+            );
+    }
+
+    for (final native in [false, true]) {
+      final watch = Stopwatch()..start();
+      final event = create(native);
+      watch.stop();
+      results.add(
+        'first measured ${native ? "native" : "Dart"}: ${watch.elapsedMicroseconds} us',
+      );
+      if (!event.isValid()) throw StateError('Invalid signature');
+    }
+    if (create(false).id != create(true).id) {
+      throw StateError('Different events');
+    }
+    for (var warm = 0; warm < 10; warm++) {
+      create(false);
+      create(true);
+    }
+    for (final count in [1, 8, 32, 64]) {
+      final samples = {false: <int>[], true: <int>[]};
+      for (var round = 0; round < 7; round++) {
+        for (final native in round.isEven ? [false, true] : [true, false]) {
+          final watch = Stopwatch()..start();
+          for (var i = 0; i < count; i++) {
+            create(native);
+          }
+          watch.stop();
+          samples[native]!.add(watch.elapsedMicroseconds);
+        }
+      }
+      for (final native in [false, true]) {
+        final values = samples[native]!..sort();
+        results.add(
+          'burst $count ${native ? "native" : "Dart"}: '
+          'median ${values[3]} min ${values.first} max ${values.last} us',
+        );
+      }
+    }
+    for (final result in results) {
+      debugPrint('SIGNING_PROBE $result');
+    }
+    debugPrint('SIGNING_PROBE COMPLETE');
+    runApp(
+      MaterialApp(
+        home: Scaffold(body: SafeArea(child: Text(results.join('\n')))),
+      ),
+    );
+  } catch (error) {
+    debugPrint('SIGNING_PROBE FAILED: $error');
+    rethrow;
+  }
+}

mobile/tool/native_signing_probe.dart

@@ -61,7 +61,7 @@ Future<void> runProbe() async {
   final watch = Stopwatch()..start();
   final proofs = <Map<String, String>>[];
   for (var i = 0; i < 64; i++) {
-    proofs.add(await media.headersForAsync('https://relay.example/media/$i'));
+    proofs.add(media.headersFor('https://relay.example/media/$i'));
   }
   results.add('64 complete media headers: ${watch.elapsedMicroseconds} us');
   for (final proof in proofs) {
@@ -69,9 +69,7 @@ Future<void> runProbe() async {
     final event = nostr.Event.fromJson(utf8.decode(base64Decode(encoded)));
     if (!event.isValid()) throw StateError('Invalid media proof');
   }
-  if ((await media.headersForAsync(
-    'https://foreign.example/media/x',
-  )).isNotEmpty) {
+  if ((media.headersFor('https://foreign.example/media/x')).isNotEmpty) {
     throw StateError('Foreign host received credentials');
   }
   results.add('media signatures and foreign-host isolation: PASS');

scripts/test-mobile-linked-signing.sh

@@ -11,7 +11,7 @@ xcodebuild -workspace mobile/ios/Runner.xcworkspace -scheme Runner \
   -configuration Debug -sdk iphonesimulator -destination "id=$device" \
   -derivedDataPath "${BUZZ_SIGNING_DERIVED_DATA:-$fixture/build}" -jobs 2 \
   BUNDLE_IDENTIFIER="$bundle" CODE_SIGNING_ALLOWED=NO ONLY_ACTIVE_ARCH=YES \
-  FLUTTER_TARGET=tool/native_signing_probe.dart build > "$fixture/build.log" 2>&1 || {
+  FLUTTER_TARGET="${BUZZ_SIGNING_PROBE_TARGET:-tool/native_signing_probe.dart}" build > "$fixture/build.log" 2>&1 || {
     cat "$fixture/build.log"
     exit 1
   }