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/test/password_text_test.dart

@@ -0,0 +1,68 @@
+import 'dart:typed_data';
+
+import 'package:halkyon_keys/halkyon_keys.dart';
+import 'package:sodium/sodium_sumo.dart';
+import 'package:test/test.dart';
+
+Future<void> main() async {
+  group("a password as RFC 8265's OpaqueString takes it", () {
+    test('accents composed or decomposed, as systems type them, are one '
+        'password', () {
+      // é as one character, and as e with a combining accent (macOS).
+      expect(
+        normalizePassword('café au lait'),
+        normalizePassword('café au lait'),
+      );
+      expect(normalizePassword('café'), 'café');
+    });
+
+    test('every other space is a plain space', () {
+      for (final space in [
+        ' ', // no-break space
+        ' ', // en space
+        ' ', // thin space
+        ' ', // narrow no-break space
+        '　', // ideographic space
+      ]) {
+        expect(normalizePassword('two${space}words'), 'two words');
+      }
+    });
+
+    test('nothing else changes: case, width and what is typed stay', () {
+      expect(normalizePassword('Correct Horse'), 'Correct Horse');
+      expect(normalizePassword('ＦＵＬＬ'), 'ＦＵＬＬ');
+      expect(normalizePassword('  padded  '), '  padded  ');
+      expect(normalizePassword('пароль 密码 🔑'), 'пароль 密码 🔑');
+    });
+
+    test('an empty password, or one with control characters, is refused', () {
+      expect(() => normalizePassword(''), throwsA(isA<PasswordRefused>()));
+      for (final control in ['\u0000', '\t', '\n', '\u007f', '\u0085']) {
+        expect(
+          () => normalizePassword('pass${control}word'),
+          throwsA(isA<PasswordRefused>()),
+        );
+      }
+    });
+  });
+
+  test('the keys are made from the normalised password: typed either way, '
+      'the same keys', () async {
+    final sodium = await SodiumSumoInit.init();
+    const cheap = PasswordKdf(opsLimit: 1, memLimit: 8192);
+    final salt = Uint8List.fromList(List.generate(16, (i) => i));
+    PasswordKeys keysOf(String password) => derivePasswordKeys(
+      sodium,
+      password: password,
+      salt: salt,
+      kdf: cheap,
+    );
+    final composed = keysOf('café noir');
+    final decomposed = keysOf('café noir');
+    expect(composed.srpPrivateValue, decomposed.srpPrivateValue);
+    expect(
+      composed.keyEncryptionKey.extractBytes(),
+      decomposed.keyEncryptionKey.extractBytes(),
+    );
+  });
+}