magpie_bridge

Magpie Bridge (MP) — UDP relay messaging protocol SDK for Dart.

A compact datagram protocol for messaging over UDP: a fixed 8-byte header with optional fields, two transport kinds (client↔server bridge and peer-to-peer direct), and an optional HMAC-SHA256 integrity tag.

Features

• Small wire format — fixed 8-byte prefix magic(4) | type(1) | headerLen(1) | payloadLen(2) plus optional target/source/sn/index/count/command fields, packed in a fixed order (big-endian).
• Two packet kinds — BridgePacket (B=1, exchanged with a relay server, carries target/source bids) and DirectPacket (B=0, peer-to-peer, no bids).
• Reliability layer — ReliablePacket appends a 32-byte HMAC-SHA256 tag; sign() / verify() are constant-time and re-signing is a single call.
• Strict validation — the factories check magic code, header-length formula, flag combinations, fragment ranges and the attachment tail on every received datagram; invalid ones return null.
• Pluggable MAC — no third-party runtime dependencies; the MAC implementation is injected through MacGenerator.
• No codegen, no async — plain Dart, synchronous, read-only packet objects.

Install

dependencies:
  magpie_bridge: ^0.2.0

import 'package:magpie_bridge/magpie_bridge.dart';

Quick start

Build and parse a bridge packet

import 'dart:convert';
import 'dart:typed_data';

final bridge = BridgePacketFactory();

// 1st handshake: ask the server for a bridge id (0 = let the server assign)
final syn = bridge.syn(utf8.encode('{"time":123.45}'), source: 12345);
final wire = syn.pack();   // header + payload, no attachment

// Parse a received datagram. Returns null when the datagram is invalid
// (bad magic code, wrong header length, out-of-range flags, ...).
final packet = bridge.parse(wire);
if (packet == null) {
  // drop it
} else {
  print(packet.source);      // 12345
  print(packet.command);     // Command.SYN
}

Build a data packet and its acknowledgement

// ordinary data: command stays 0 (C=0) to keep the header small
final pkt = bridge.data(
  Uint8List.fromList([1, 2, 3, 4, 5]),
  target: 100,
  source: 200,
  sn: 42,
  index: 0,
  count: 1,
);
final wire = pkt.pack();

// the receiver replies COPY (A=1) echoing sn/index/count
final copy = bridge.copy(pkt);

Direct (peer-to-peer) packets

final direct = DirectPacketFactory();

final data = direct.data(
  Uint8List.fromList([9, 9, 9]),
  sn: 7,
  index: 0,
  count: 1,
);
final packet = direct.parse(data.pack());
// packet is a DirectPacket (B=0, no bids)

Signing (HMAC-SHA256)

Signing is opt-in. Because the SDK deliberately has no third-party runtime dependency, you must inject a MAC implementation once before calling sign():

// 1. Provide an HMAC-SHA256 implementation (any library or your own)
//    implementing the MacGenerator interface, then inject it once.
//    A reference implementation built on package:crypto is provided
//    in example/hmac.dart (see registerHmacSha256()).
HmacSha256.generator = MyHmacSha256();   // your implementation

// 2. Sign a packet: returns a ReliablePacket (header + payload + 32-byte tag)
final ping = bridge.ping(Uint8List.fromList('{}'.codeUnits), source: 9);
final secret = Uint8List.fromList(List.filled(32, 7));
final signed = ping.sign(secret);
final wire = signed.pack();   // wire = header + payload + tag(32)

// 3. Verify on receipt (constant-time comparison)
final received = bridge.parse(wire) as ReliablePacket;
received.verify(secret);   // true
received.verify(other);    // false

// 4. Rotate the secret: re-signing replaces the attachment in one call
final rotated = signed.sign(newSecret);

The MacGenerator interface you inject is:

abstract interface class MacGenerator {
  Uint8List generate(Uint8List key, Uint8List data);
  bool verify(Uint8List key, Uint8List data, Uint8List tag);
}

Note: without an injected generator, sign() / verify() throw a Null check operator used on a null value error. This is intentional — the SDK stays dependency-free and lets the application choose its MAC implementation (e.g. package:crypto or a platform crypto API).

Protocol overview

field	bytes	description
magic	4	0x4D500001 ("MP\0\1")
type	1	flags A B C D + E width (bits 7..0)
header length	1	8 + 8·B + 4·D + 2·E + 4·C
payload length	2	big-endian
target / source	4×2	only when B=1
sn	4	only when D=1
index / count	E×2	only when E>0 (E = byte width)
command	4	only when C=1
payload	pLen
attachment	32	only for signed (Reliable) packets

Reserved commands: SYN? SYN! ACK! FAIL DONE ACPT DENY PING PONG FIN? FIN! NOOP DATA COPY — plus 0 for plain DATA (C=0). See docs/protocol.md in the repository for the full design.

Library layout

The package re-exports three facades from the entry point:

facade	contents
protocol.dart	constants, header calculations, Magpie / Packet interfaces, Command values
magpie.dart	MessagePacket, BasePacket, BridgePacket, DirectPacket, ReliablePacket
factory.dart	MessagePacketFactory, BridgePacketFactory, DirectPacketFactory

You can import the whole SDK with one line:

import 'package:magpie_bridge/magpie_bridge.dart';

Examples and tests

dart run example/quick_check.dart   # round-trip sanity checks
dart test                          # unit tests

License

MIT © 2026 Albert Moky