feat(keys): passwords taken as RFC 8265's OpaqueString

Other spaces become a plain space and the text is composed (NFC) before
Argon2id, so a password typed on another system, accents composed or
not, unlocks the same keys; empty passwords and control characters are
refused. The normalisation (unorm_dart) is pinned exactly, as it must
never change once an account exists. ADR 0017 records both decisions.

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_keys/lib/halkyon_keys.dart

@@ -13,6 +13,8 @@ export 'src/data_keys.dart' show DataKey, openDataKey, sealDataKey;
 export 'src/key_refused.dart';
 export 'src/password_keys.dart'
     show PasswordKdf, PasswordKeys, derivePasswordKeys, newPasswordSalt;
+export 'src/password_text.dart'
+    show PasswordRefusal, PasswordRefused, normalizePassword;
 export 'src/recovery_phrase.dart'
     show
         entropyOfRecoveryPhrase,

packages/halkyon_keys/lib/src/password_keys.dart

@@ -1,6 +1,7 @@
 import 'dart:typed_data';
 
 import 'package:halkyon_keys/src/key_refused.dart';
+import 'package:halkyon_keys/src/password_text.dart';
 import 'package:sodium/sodium_sumo.dart';
 
 /// The password's keys (ADR 0017): Argon2id over the password and the
@@ -95,7 +96,8 @@ final class PasswordKeys._(
   }
 }
 
-/// The keys [password] unlocks for the account with [salt], at the costs
+/// The keys [password] (normalised, [normalizePassword]) unlocks for the
+/// account with [salt], at the costs
 /// [kdf] names (the account's own; [PasswordKdf.current] for a new one).
 PasswordKeys derivePasswordKeys(
   SodiumSumo sodium, {
@@ -112,7 +114,8 @@ PasswordKeys derivePasswordKeys(
   }
   final masterKey = sodium.crypto.pwhash.callStr(
     outLen: _keyBytes,
-    password: password,
+    // As RFC 8265's OpaqueString takes it, so it unlocks alike everywhere.
+    password: normalizePassword(password),
     salt: salt,
     opsLimit: kdf.opsLimit,
     memLimit: kdf.memLimit,

packages/halkyon_keys/lib/src/password_text.dart

@@ -0,0 +1,55 @@
+import 'package:unorm_dart/unorm_dart.dart' as unorm;
+
+/// A password as RFC 8265's OpaqueString profile takes it (decided 20 Oct
+/// 2026), before anything is derived from it: every space other than the
+/// plain one becomes the plain one, and the text is composed (Unicode NFC),
+/// so a password typed on another system (macOS can write an accent as a
+/// letter and a combining mark) unlocks the same keys. Nothing else changes:
+/// case, width and every other character stay as typed. Refused
+/// ([PasswordRefused]) when empty or holding a control character.
+///
+/// Once an account exists its password's normalisation must never change:
+/// the library is pinned exactly, and NFC is stable for every assigned
+/// character by Unicode's own policy.
+String normalizePassword(String password) {
+  final spaced = String.fromCharCodes([
+    for (final rune in password.runes)
+      if (_otherSpaces.contains(rune)) 0x20 else rune,
+  ]);
+  final normalized = unorm.nfc(spaced);
+  if (normalized.isEmpty) {
+    throw const PasswordRefused(PasswordRefusal.empty);
+  }
+  if (normalized.runes.any(_isControl)) {
+    throw const PasswordRefused(PasswordRefusal.controlCharacter);
+  }
+  return normalized;
+}
+
+/// Unicode's spaces (category Zs) other than U+0020.
+const _otherSpaces = {
+  0x00a0, 0x1680, 0x2000, 0x2001, 0x2002, 0x2003, 0x2004, 0x2005, 0x2006, //
+  0x2007, 0x2008, 0x2009, 0x200a, 0x202f, 0x205f, 0x3000,
+};
+
+/// Unicode's control characters (category Cc).
+bool _isControl(int rune) => rune <= 0x1f || (rune >= 0x7f && rune <= 0x9f);
+
+/// Why a password can't be taken.
+final class const PasswordRefused(
+  /// Why.
+  final PasswordRefusal reason,
+) implements Exception {
+  @override
+  String toString() => 'PasswordRefused(${reason.name})';
+}
+
+/// Why a password was refused.
+enum PasswordRefusal() {
+  /// Nothing was typed.
+  empty,
+
+  /// It holds a control character (a tab, a line break, …), which no
+  /// keyboard types the same way everywhere.
+  controlCharacter,
+}