Add optional KYC flow and service-backed account limits

frontend/packages/sxchnge_kyc/lib/src/services/photos.dart

@@ -0,0 +1,81 @@
+import 'dart:typed_data';
+import 'dart:ui' as ui;
+
+import 'package:file_selector/file_selector.dart';
+
+import 'package:image/image.dart' as metadata;
+
+import '../models/identity.dart';
+import 'contracts.dart';
+
+class FilePhotoService implements PhotoService {
+  const FilePhotoService();
+  static const maxBytes = 10 * 1024 * 1024;
+  @override
+  Future<Photo?> pick() async {
+    try {
+      final file = await openFile(
+        acceptedTypeGroups: const [
+          XTypeGroup(
+            label: 'JPEG / PNG',
+            extensions: ['jpg', 'jpeg', 'png'],
+            mimeTypes: ['image/jpeg', 'image/png'],
+            uniformTypeIdentifiers: ['public.jpeg', 'public.png'],
+          ),
+        ],
+      );
+      if (file == null) return null;
+      if (await file.length() > maxBytes)
+        throw const KycException(KycError.tooLarge);
+      final bytes = await file.readAsBytes();
+      return await validate(bytes);
+    } on KycException {
+      rethrow;
+    } catch (_) {
+      throw const KycException(KycError.permission);
+    }
+  }
+
+  static Future<Photo> validate(Uint8List bytes) async {
+    try {
+      if (bytes.length > maxBytes) throw const KycException(KycError.tooLarge);
+      final png =
+          bytes.length > 8 &&
+          bytes[0] == 137 &&
+          bytes[1] == 80 &&
+          bytes[2] == 78 &&
+          bytes[3] == 71;
+      final jpg =
+          bytes.length > 3 &&
+          bytes[0] == 255 &&
+          bytes[1] == 216 &&
+          bytes[2] == 255;
+      if (!png && !jpg) throw const KycException(KycError.format);
+      final metadata.Decoder decoder = png
+          ? metadata.PngDecoder()
+          : metadata.JpegDecoder();
+      final info = decoder.startDecode(bytes);
+      if (info == null) throw const KycException(KycError.unreadable);
+      if (info.width < 600 || info.height < 600)
+        throw const KycException(KycError.tooSmall);
+      if (info.width * info.height > 40000000)
+        throw const KycException(KycError.tooLarge);
+      final codec = await ui.instantiateImageCodec(
+        bytes,
+        targetWidth: 1000,
+        allowUpscaling: false,
+      );
+      try {
+        final frame = await codec.getNextFrame();
+        frame.image.dispose();
+      } finally {
+        codec.dispose();
+      }
+      return Photo(bytes);
+    } on KycException {
+      rethrow;
+    } catch (_) {
+      throw const KycException(KycError.unreadable);
+    }
+  }
+}