refactor: login form

mobile/lib/services/oauth.service.dart

@@ -1,5 +1,11 @@
+import 'dart:convert';
+import 'dart:math';
+
+import 'package:crypto/crypto.dart';
 import 'package:flutter_web_auth_2/flutter_web_auth_2.dart';
+import 'package:immich_mobile/models/auth/oauth_login_data.model.dart';
 import 'package:immich_mobile/services/api.service.dart';
+import 'package:immich_mobile/utils/url_helper.dart';
 import 'package:logging/logging.dart';
 import 'package:openapi/api.dart';
 
@@ -11,6 +17,50 @@ class OAuthService {
   final log = Logger('OAuthService');
   OAuthService(this._apiService);
 
+  String _generateRandomString(int length) {
+    const chars = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz1234567890';
+    final random = Random.secure();
+    return String.fromCharCodes(Iterable.generate(length, (_) => chars.codeUnitAt(random.nextInt(chars.length))));
+  }
+
+  List<int> _randomBytes(int length) {
+    final random = Random.secure();
+    return List<int>.generate(length, (i) => random.nextInt(256));
+  }
+
+  /// Per specification, the code verifier must be 43-128 characters long
+  /// and consist of characters [A-Z, a-z, 0-9, "-", ".", "_", "~"]
+  /// https://datatracker.ietf.org/doc/html/rfc7636#section-4.1
+  String _randomCodeVerifier() {
+    return base64Url.encode(_randomBytes(42));
+  }
+
+  String _generatePKCECodeChallenge(String codeVerifier) {
+    final bytes = utf8.encode(codeVerifier);
+    final digest = sha256.convert(bytes);
+    return base64Url.encode(digest.bytes).replaceAll('=', '');
+  }
+
+  /// Initiates OAuth login flow.
+  /// Returns the OAuth server URL to redirect to, along with PKCE parameters.
+  Future<OAuthLoginData?> getOAuthLoginData(String serverUrl) async {
+    final state = _generateRandomString(32);
+    final codeVerifier = _randomCodeVerifier();
+    final codeChallenge = _generatePKCECodeChallenge(codeVerifier);
+
+    final oAuthServerUrl = await getOAuthServerUrl(sanitizeUrl(serverUrl), state, codeChallenge);
+
+    if (oAuthServerUrl == null) {
+      return null;
+    }
+
+    return OAuthLoginData(serverUrl: oAuthServerUrl, state: state, codeVerifier: codeVerifier);
+  }
+
+  Future<LoginResponseDto?> completeOAuthLogin(OAuthLoginData oAuthData) {
+    return oAuthLogin(oAuthData.serverUrl, oAuthData.state, oAuthData.codeVerifier);
+  }
+
   Future<String?> getOAuthServerUrl(String serverUrl, String state, String codeChallenge) async {
     // Resolve API server endpoint from user provided serverUrl
     await _apiService.resolveAndSetEndpoint(serverUrl);