fix(app): what a password unlocked is wiped once done with

The reading of the full reading's fixes noted that the main isolate's
copies of SRP's x and the key-encryption key, Dart's own bytes, stayed
for as long as the sign-in sheet's cubit lived, and after every change's
proof.

PasswordUnlock.forget zeroes them: the sign-in cubit wipes what the
password unlocked once signed in, once another password replaced it, and
as it closes; a change's or deletion's proof wipes what it unlocked once
its answer is made. The fake crypto's proof copies its bytes, as a real
one is new bytes. halkyon_keys' README says what is wiped and what isn't.

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

packages/halkyon_app/lib/src/password/bloc/password_sign_in_cubit.dart

@@ -124,7 +124,9 @@ class PasswordSignInCubit(
   /// until the code.
   Future<void> credentialsEntered(String address, String password) =>
       _run(() async {
+        final previous = _unlocked;
         _unlocked = await _passwords.unlock(address, password);
+        previous?.forget();
         emit(
           state.copyWith(
             step: PasswordSignInStep.secondFactor,
@@ -344,11 +346,22 @@ class PasswordSignInCubit(
 
   Future<void> _putInPlace() async {
     await _passwords.enter(_pending!);
+    // Done with what the password unlocked (the reading of the fixes).
+    _unlocked?.forget();
+    _unlocked = null;
     if (!isClosed) {
       emit(state.copyWith(step: PasswordSignInStep.done, clearProblem: true));
     }
   }
 
+  /// Wipes what the password unlocked, whatever step it closes on.
+  @override
+  Future<void> close() {
+    _unlocked?.forget();
+    _unlocked = null;
+    return super.close();
+  }
+
   /// Runs [step], busy meanwhile; a refusal is shown on the step it came
   /// from.
   Future<void> _run(Future<void> Function() step) async {

packages/halkyon_app/lib/src/password/data/account_crypto.dart

@@ -18,6 +18,13 @@ final class const PasswordUnlock({
   /// The key-encryption key, 32 bytes.
   required final Uint8List keyEncryptionKey,
 }) {
+  /// Zeroes [srpSecret] and [keyEncryptionKey] once nothing needs them (the
+  /// reading of the fixes): Dart's own bytes, which no collector wipes.
+  void forget() {
+    srpSecret.fillRange(0, srpSecret.length, 0);
+    keyEncryptionKey.fillRange(0, keyEncryptionKey.length, 0);
+  }
+
   /// Whether these were derived for [salt] and [costs].
   bool fits(Uint8List salt, protocol.PasswordCosts costs) =>
       _same(this.salt, salt) &&

packages/halkyon_app/lib/src/password/data/password_repository.dart

@@ -29,7 +29,10 @@ final class PasswordUnlocked._(
   final PasswordUnlock _unlock,
   var protocol.PasswordChallenge? _challenge,
   var DateTime _begunAt,
-);
+) {
+  /// Wipes what the password unlocked, once the sign-in is done with it.
+  void forget() => _unlock.forget();
+}
 
 /// A session granted but not yet this device's: a sign-in or sign-up,
 /// held back while an authenticator is set up with it (`needsAuthenticator`),
@@ -382,7 +385,8 @@ class PasswordRepository(
   }) async {
     final challenge = await _gateway.beginChange(as: as);
     final salt = Uint8List.sublistView(challenge.salt);
-    final unlock = switch (unlocked?._unlock) {
+    final kept = unlocked?._unlock;
+    final unlock = switch (kept) {
       final kept? when kept.fits(salt, challenge.costs) => kept,
       _ when password != null => await _crypto.unlock(
         password,
@@ -392,18 +396,25 @@ class PasswordRepository(
       // Its password changed since: begin again.
       _ => throw const PasswordFailed(PasswordProblem.challengeExpired),
     };
-    return (
-      challenge: challenge,
-      answer: await _crypto.answer(
-        unlock,
-        challenge,
-        instanceUrl: _gateway.instanceUrl,
-        purpose: purpose,
-        digest: digest,
-      ),
-      totpCode: totpCode?.replaceAll(' ', ''),
-      recoveryCode: recoveryCode,
-    );
+    // One unlocked here is wiped once its answer is made; one kept by a
+    // sign-in under way is its to wipe.
+    final ownUnlock = identical(unlock, kept) ? null : unlock;
+    try {
+      return (
+        challenge: challenge,
+        answer: await _crypto.answer(
+          unlock,
+          challenge,
+          instanceUrl: _gateway.instanceUrl,
+          purpose: purpose,
+          digest: digest,
+        ),
+        totpCode: totpCode?.replaceAll(' ', ''),
+        recoveryCode: recoveryCode,
+      );
+    } finally {
+      ownUnlock?.forget();
+    }
   }
 
   /// What keeps the signed-in person's account.
@@ -498,14 +509,20 @@ class PasswordRepository(
       salt: Uint8List.sublistView(challenge.salt),
       costs: challenge.costs,
     );
-    await _gateway.deleteAccount(
-      challenge,
-      await _crypto.answer(
+    final SrpAnswer answer;
+    try {
+      answer = await _crypto.answer(
         unlock,
         challenge,
         instanceUrl: _gateway.instanceUrl,
         purpose: srp.PasswordProofPurpose.deleteAccount,
-      ),
+      );
+    } finally {
+      unlock.forget();
+    }
+    await _gateway.deleteAccount(
+      challenge,
+      answer,
       totpCode: totpCode?.replaceAll(' ', ''),
       recoveryCode: recoveryCode,
       request: request,

packages/halkyon_app/lib/src/testing/fake_account_crypto.dart

@@ -26,6 +26,10 @@ const otherFakePhrase =
 class FakeAccountCrypto() implements AccountCrypto {
   var _accounts = 0;
 
+  /// What each password unlocked, as [unlock] made it, in order: tests see
+  /// it wiped once done with.
+  final unlocks = <PasswordUnlock>[];
+
   /// The keys of a new account, numbered so each differs.
   UnlockedAccountKeys keysNumbered(int number) => UnlockedAccountKeys(
     encryptionPublicKey: Uint8List.fromList(List.filled(32, number)),
@@ -143,12 +147,14 @@ class FakeAccountCrypto() implements AccountCrypto {
     required protocol.PasswordCosts costs,
   }) async {
     _check(password);
-    return PasswordUnlock(
+    final unlock = PasswordUnlock(
       salt: salt,
       costs: costs,
       srpSecret: Uint8List.fromList(utf8.encode(password)),
       keyEncryptionKey: Uint8List.fromList(utf8.encode(password)),
     );
+    unlocks.add(unlock);
+    return unlock;
   }
 
   @override
@@ -160,7 +166,8 @@ class FakeAccountCrypto() implements AccountCrypto {
     List<int> digest = const [],
   }) async => SrpAnswer(
     clientValue: ByteData.sublistView(bindingOf(purpose, digest)),
-    clientProof: ByteData.sublistView(unlock.srpSecret),
+    // A copy, as a real proof is new bytes: the unlock is wiped after.
+    clientProof: ByteData.sublistView(Uint8List.fromList(unlock.srpSecret)),
     verifies: (serverProof) =>
         utf8.decode(Uint8List.sublistView(serverProof)) == 'M2',
   );