fix(train): no row's ID tells the instance a date: the days' IDs are keyed by the account (ADR 0030, D57)

A day's, night's and heart chunk's ID was a UUID of version 5 of the
person and the date, which the instance can hash for every date itself:
each ID told it which days the band was worn. halkyon_keys' keyedRowId
hashes the name under the account's X25519 secret key (BLAKE2b, a UUID of
version 8), the same on every device of the account and unreadable
without its keys; ContentCipher.rowId and TrainKeys.rowId hand it on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_flutter/lib/src/app/train_dependencies.dart

@@ -384,11 +384,7 @@ Future<TrainPerson> _openPersonData(
     keys,
     person: person.authUserId,
   );
-  final days = DayRepository(
-    SyncDayGateway(db, sync),
-    keys,
-    person: person.authUserId,
-  );
+  final days = DayRepository(SyncDayGateway(db, sync), keys);
   final plans = PlanRepository(SyncPlanGateway(db, sync), keys);
   final routes = RouteRepository(SyncRouteGateway(db, sync), keys);
   final maps = MapSourceRepository(

apps/train/halkyon_train_flutter/lib/src/core/privacy/train_keys.dart

@@ -56,6 +56,15 @@ class TrainKeys(
     }
   }
 
+  /// Whether the account's keys are open here: [refresh] found them.
+  bool get unlocked => _unlocked;
+
+  /// The ID of the row named [name], keyed by the account (D57): the same
+  /// on each of its devices, nothing of [name] read from it by the
+  /// instance. Throws [KeysMissing] where [refresh] found no keys.
+  String rowId(String name) =>
+      _unlocked ? cipher.rowId(name) : throw KeysMissing();
+
   /// Whether what is sealed with the key [keyId] opens here.
   bool canOpen(String keyId) =>
       _unlocked && _madeAt(keyId) != null && cipher.has(keyId);

apps/train/halkyon_train_flutter/lib/src/feature/day/data/day_repository.dart

@@ -2,8 +2,6 @@ import 'dart:async';
 
 import 'package:flutter/foundation.dart';
 import 'package:halkyon_app/halkyon_app.dart';
-import 'package:halkyon_train_client/halkyon_train_client.dart'
-    show Namespace, Uuid;
 import 'package:halkyon_train_flutter/src/core/privacy/train_keys.dart';
 import 'package:halkyon_train_flutter/src/feature/day/data/day_gateway.dart';
 import 'package:halkyon_train_flutter/src/feature/day/data/day_payload.dart';
@@ -17,17 +15,12 @@ const _chunkHours = 3;
 /// The person's days and nights from the Loop (ADR 0030, D37), sealed with
 /// their key as their workouts are (D7): one `day` row a date, one `night`
 /// row a date, a day's heart rate in rows of three hours sealed for their
-/// day; every ID made from the person and the date (and the chunk's place),
-/// so a day read again, here or on another phone, writes the same rows, the
-/// last writer winning. A row that doesn't open here is left out: what is
+/// day; every ID made from the date (and the chunk's place) under a key of
+/// the account's (D57), so a day read again, here or on another phone,
+/// writes the same rows, the last writer winning, and the instance reads no
+/// date from an ID. A row that doesn't open here is left out: what is
 /// sealed doesn't say its date.
-class DayRepository(
-  final DayGateway _gateway,
-  final TrainKeys _keys, {
-
-  /// Whose days they are: their auth user ID.
-  required final String person,
-}) {
+class DayRepository(final DayGateway _gateway, final TrainKeys _keys) {
   late final _envelope = PayloadEnvelope(_keys.cipher);
   StreamSubscription<void>? _remote;
 
@@ -39,15 +32,12 @@ class DayRepository(
   /// Fires when the days changed, here or on another device.
   Stream<void> get changes => _changes.stream;
 
-  String _dayId(DateTime date) => _id('day/$person/${dayKey(date)}');
+  String _dayId(DateTime date) => _keys.rowId('train/day/${dayKey(date)}');
 
-  String _nightId(DateTime date) => _id('night/$person/${dayKey(date)}');
+  String _nightId(DateTime date) => _keys.rowId('train/night/${dayKey(date)}');
 
   String _chunkId(DateTime date, int index) =>
-      _id('day/$person/${dayKey(date)}/heart/$index');
-
-  String _id(String path) =>
-      const Uuid().v5(Namespace.url.value, 'https://halkyon.app/train/$path');
+      _keys.rowId('train/day/${dayKey(date)}/heart/$index');
 
   /// Every date with a day or a night that opens here, newest first.
   Future<List<StoredDay>> all() async {
@@ -80,12 +70,18 @@ class DayRepository(
   /// The day of [date], where one opens here.
   Future<DayContent?> day(DateTime date) async {
     await _keys.refresh();
+    if (!_keys.unlocked) {
+      return null;
+    }
     return await _day(date);
   }
 
   /// The night that ended [date], where one opens here.
   Future<NightContent?> night(DateTime date) async {
     await _keys.refresh();
+    if (!_keys.unlocked) {
+      return null;
+    }
     final id = _nightId(date);
     return switch (await _gateway.night(id)) {
       final envelope? => switch (_open(envelope, table: 'night', id: id)) {
@@ -99,6 +95,9 @@ class DayRepository(
   /// The 24/7 heart rate of [date] that opens here, in time order.
   Future<List<HeartRun>> heart(DateTime date) async {
     await _keys.refresh();
+    if (!_keys.unlocked) {
+      return [];
+    }
     return await _heart(date);
   }
 

apps/train/halkyon_train_flutter/test/app/train_app_test.dart

@@ -185,7 +185,6 @@ void main() {
                       FakeContentCipher(),
                       () async => null,
                     ),
-                    person: person.authUserId,
                   ),
               plans:
                   plans ??
@@ -538,7 +537,7 @@ void main() {
         keys,
         person: 'alice',
       );
-      days = DayRepository(MemoryDayGateway(), keys, person: 'alice');
+      days = DayRepository(MemoryDayGateway(), keys);
       platform = FakeLoopPlatform()
         ..nearby.add(band)
         ..inReach.add('A1');
@@ -1731,7 +1730,7 @@ void main() {
           FakeLoopPlatform(),
           LoopStore(File('${folder.path}/loop.json')),
           ProfileRepository(MemoryProfileGateway(), keys, person: 'alice'),
-          days: DayRepository(MemoryDayGateway(), keys, person: 'alice'),
+          days: DayRepository(MemoryDayGateway(), keys),
           workouts: workouts,
           backgroundLines: File('${folder.path}/loop_days.txt'),
         );
@@ -1845,7 +1844,7 @@ void main() {
         FakeContentCipher(),
         () async => FakeAccountCrypto().keysNumbered(1),
       );
-      days = DayRepository(MemoryDayGateway(), keys, person: 'alice');
+      days = DayRepository(MemoryDayGateway(), keys);
     });
 
     tearDown(() => keys.dispose());

apps/train/halkyon_train_flutter/test/feature/day/day_repository_test.dart

@@ -3,6 +3,8 @@ import 'dart:typed_data';
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_train_client/halkyon_train_client.dart'
+    show Namespace, Uuid;
 import 'package:halkyon_train_flutter/src/core/privacy/train_keys.dart';
 import 'package:halkyon_train_flutter/src/feature/day/data/day_repository.dart';
 import 'package:halkyon_train_flutter/src/feature/day/data/loop_lines.dart';
@@ -69,7 +71,7 @@ void main() {
     final keys = TrainKeys(keyRows, SodiumContentCipher(), () async => account);
     opened.add(keys);
     await keys.refresh();
-    return DayRepository(rows, keys, person: 'alice');
+    return DayRepository(rows, keys);
   }
 
   setUpAll(() async {
@@ -227,6 +229,48 @@ void main() {
     }
   });
 
+  test("no row's ID tells its date: the instance, hashing every date as an "
+      'ID of version 5 would be, finds none of them (D57); another phone of '
+      'the account names the rows alike', () async {
+    final phone = await device(alice);
+    await phone.take(
+      linesOf([
+        summary(tuesday, 9876),
+        heart(tuesday, [1, 4]),
+        sleep,
+      ]),
+    );
+    final ids = {
+      ...rows.dayRows.keys,
+      ...rows.nightRows.keys,
+      ...rows.seriesRows.keys,
+    };
+    expect(ids, hasLength(4));
+
+    String v5(String path) => const Uuid().v5(
+      Namespace.url.value,
+      'https://halkyon.app/train/$path',
+    );
+    final guesses = {
+      for (final person in ['alice', ''])
+        for (final kind in ['day', 'night'])
+          for (final date in [monday, tuesday]) ...[
+            v5('$kind/$person/${dayKey(date)}'),
+            for (var i = 0; i < 8; i++)
+              v5('$kind/$person/${dayKey(date)}/heart/$i'),
+          ],
+    };
+    expect(ids.intersection(guesses), isEmpty);
+    for (final id in ids) {
+      expect(id[14], '8', reason: 'a UUID of version 8, keyed: $id');
+    }
+
+    final other = await device(alice);
+    await other.take(linesOf([summary(tuesday, 9877)]));
+    expect(rows.dayRows.keys, hasLength(1));
+    expect((await other.day(tuesday))!.steps, 9877);
+  });
+
   test(
     "a device without the account's keys sees no day and keeps none",
     () async {

apps/train/halkyon_train_flutter/test/feature/day/days_cubit_test.dart

@@ -38,7 +38,7 @@ void main() {
       () async => FakeAccountCrypto().keysNumbered(1),
     );
     rows = MemoryDayGateway();
-    days = DayRepository(rows, keys, person: 'alice');
+    days = DayRepository(rows, keys);
   });
 
   tearDown(() async {

apps/train/halkyon_train_flutter/test/feature/day/sync_day_gateway_test.dart

@@ -63,7 +63,7 @@ void main() {
       () async => alice,
     );
     await keys.refresh();
-    days = DayRepository(SyncDayGateway(db, sync), keys, person: 'alice');
+    days = DayRepository(SyncDayGateway(db, sync), keys);
   });
 
   tearDown(() async {

apps/train/halkyon_train_flutter/test/feature/loop/polar_loop_test.dart

@@ -101,7 +101,7 @@ void main() {
       person: 'person',
     );
     await profile.save(body);
-    days = DayRepository(MemoryDayGateway(), keys, person: 'person');
+    days = DayRepository(MemoryDayGateway(), keys);
     workouts = WorkoutRepository(MemoryWorkoutGateway(), keys);
     now = DateTime.utc(2026, 10, 28, 9);
     day = today;
@@ -344,7 +344,7 @@ void main() {
       );
       addTearDown(keyless.dispose);
       await keyless.refresh();
-      days = DayRepository(MemoryDayGateway(), keyless, person: 'person');
+      days = DayRepository(MemoryDayGateway(), keyless);
       platform.days.add([summary('2026-10-27', 9876)]);
       final loop = await setUpBand();
       await settled(loop);