diff --git a/lib/service/dfu_protocol.dart b/lib/service/dfu_protocol.dart
new file mode 100644
index 0000000..6a83147
--- /dev/null
+++ b/lib/service/dfu_protocol.dart
@@ -0,0 +1,137 @@
+import 'dart:typed_data';
+
+import 'package:abawo_bt_app/model/shifter_types.dart';
+
+const int _startPayloadLength = 11;
+
+class DfuStartPayload {
+  const DfuStartPayload({
+    required this.totalLength,
+    required this.imageCrc32,
+    required this.sessionId,
+    required this.flags,
+  });
+
+  final int totalLength;
+  final int imageCrc32;
+  final int sessionId;
+  final int flags;
+}
+
+class DfuDataFrame {
+  const DfuDataFrame({
+    required this.sequence,
+    required this.offset,
+    required this.payloadLength,
+    required this.bytes,
+  });
+
+  final int sequence;
+  final int offset;
+  final int payloadLength;
+  final Uint8List bytes;
+}
+
+class DfuProtocol {
+  const DfuProtocol._();
+
+  static Uint8List encodeStartPayload(DfuStartPayload payload) {
+    final data = ByteData(_startPayloadLength);
+    data.setUint8(0, universalShifterDfuOpcodeStart);
+    data.setUint32(1, payload.totalLength, Endian.little);
+    data.setUint32(5, payload.imageCrc32, Endian.little);
+    data.setUint8(9, payload.sessionId);
+    data.setUint8(10, payload.flags);
+    return data.buffer.asUint8List();
+  }
+
+  static Uint8List encodeFinishPayload() {
+    return Uint8List.fromList([universalShifterDfuOpcodeFinish]);
+  }
+
+  static Uint8List encodeAbortPayload() {
+    return Uint8List.fromList([universalShifterDfuOpcodeAbort]);
+  }
+
+  static List<DfuDataFrame> buildDataFrames(
+    List<int> imageBytes, {
+    int startSequence = 0,
+  }) {
+    final frames = <DfuDataFrame>[];
+    var seq = _asU8(startSequence);
+    var offset = 0;
+    while (offset < imageBytes.length) {
+      final remaining = imageBytes.length - offset;
+      final chunkLength = remaining < universalShifterDfuFramePayloadSizeBytes
+          ? remaining
+          : universalShifterDfuFramePayloadSizeBytes;
+
+      final frame = Uint8List(universalShifterDfuFrameSizeBytes);
+      frame[0] = seq;
+      frame.setRange(1, 1 + chunkLength, imageBytes, offset);
+
+      frames.add(
+        DfuDataFrame(
+          sequence: seq,
+          offset: offset,
+          payloadLength: chunkLength,
+          bytes: frame,
+        ),
+      );
+
+      offset += chunkLength;
+      seq = nextSequence(seq);
+    }
+
+    return frames;
+  }
+
+  static int nextSequence(int sequence) {
+    return _asU8(sequence + 1);
+  }
+
+  static int rewindSequenceFromAck(int acknowledgedSequence) {
+    return nextSequence(acknowledgedSequence);
+  }
+
+  static int sequenceDistance(int from, int to) {
+    return _asU8(to - from);
+  }
+
+  static int parseAckPayload(List<int> payload) {
+    if (payload.length != 1) {
+      throw const FormatException('ACK payload must be exactly 1 byte.');
+    }
+    return _asU8(payload.first);
+  }
+
+  static const int crc32Initial = 0xFFFFFFFF;
+  static const int _crc32PolynomialReflected = 0xEDB88320;
+
+  static int crc32Update(int crc, List<int> bytes) {
+    var next = crc & 0xFFFFFFFF;
+    for (final byte in bytes) {
+      next ^= byte;
+      for (var bit = 0; bit < 8; bit++) {
+        if ((next & 0x1) != 0) {
+          next = (next >> 1) ^ _crc32PolynomialReflected;
+        } else {
+          next >>= 1;
+        }
+      }
+    }
+    return next & 0xFFFFFFFF;
+  }
+
+  static int crc32Finalize(int crc) {
+    return (crc ^ 0xFFFFFFFF) & 0xFFFFFFFF;
+  }
+
+  static int crc32(List<int> bytes) {
+    return crc32Finalize(crc32Update(crc32Initial, bytes));
+  }
+
+  static int _asU8(int value) {
+    return value & 0xFF;
+  }
+}
diff --git a/test/service/dfu_protocol_test.dart b/test/service/dfu_protocol_test.dart
new file mode 100644
index 0000000..1b5b74f
--- /dev/null
+++ b/test/service/dfu_protocol_test.dart
@@ -0,0 +1,86 @@
+import 'package:abawo_bt_app/model/shifter_types.dart';
+import 'package:abawo_bt_app/service/dfu_protocol.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('DfuProtocol CRC32', () {
+    test('matches known vector', () {
+      final crc = DfuProtocol.crc32('123456789'.codeUnits);
+      expect(crc, 0xCBF43926);
+    });
+  });
+
+  group('DfuProtocol control payload encoding', () {
+    test('encodes START payload with exact 11-byte LE layout', () {
+      final payload = DfuProtocol.encodeStartPayload(
+        const DfuStartPayload(
+          totalLength: 0x1234,
+          imageCrc32: 0x89ABCDEF,
+          sessionId: 0x22,
+          flags: universalShifterDfuFlagEncrypted,
+        ),
+      );
+
+      expect(payload.length, 11);
+      expect(
+        payload,
+        [
+          universalShifterDfuOpcodeStart,
+          0x34,
+          0x12,
+          0x00,
+          0x00,
+          0xEF,
+          0xCD,
+          0xAB,
+          0x89,
+          0x22,
+          universalShifterDfuFlagEncrypted,
+        ],
+      );
+    });
+
+    test('encodes FINISH and ABORT payloads as one byte', () {
+      expect(
+          DfuProtocol.encodeFinishPayload(), [universalShifterDfuOpcodeFinish]);
+      expect(
+          DfuProtocol.encodeAbortPayload(), [universalShifterDfuOpcodeAbort]);
+    });
+  });
+
+  group('DfuProtocol data frame building', () {
+    test('builds 64-byte frames and handles final partial payload', () {
+      final image = List<int>.generate(80, (index) => index);
+      final frames = DfuProtocol.buildDataFrames(image);
+
+      expect(frames.length, 2);
+
+      expect(frames[0].sequence, 0);
+      expect(frames[0].offset, 0);
+      expect(frames[0].payloadLength, universalShifterDfuFramePayloadSizeBytes);
+      expect(frames[0].bytes.length, universalShifterDfuFrameSizeBytes);
+      expect(frames[0].bytes.sublist(1, 64), image.sublist(0, 63));
+
+      expect(frames[1].sequence, 1);
+      expect(frames[1].offset, 63);
+      expect(frames[1].payloadLength, 17);
+      expect(frames[1].bytes.length, universalShifterDfuFrameSizeBytes);
+      expect(frames[1].bytes.sublist(1, 18), image.sublist(63, 80));
+    });
+  });
+
+  group('DfuProtocol sequence and ACK helpers', () {
+    test('wraps sequence values and computes ack+1 rewind', () {
+      expect(DfuProtocol.nextSequence(0x00), 0x01);
+      expect(DfuProtocol.nextSequence(0xFF), 0x00);
+
+      expect(DfuProtocol.rewindSequenceFromAck(0x05), 0x06);
+      expect(DfuProtocol.rewindSequenceFromAck(0xFF), 0x00);
+    });
+
+    test('computes wrapping sequence distance', () {
+      expect(DfuProtocol.sequenceDistance(250, 2), 8);
+      expect(DfuProtocol.sequenceDistance(1, 1), 0);
+    });
+  });
+}