📨 Sending and Receiving

Message Handling in MLS

In 15 minutes: Complete messaging flow with code examples

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🎯 Complete Message Flow

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💻 Complete Code Example

// ==========================================
// SENDER SIDE
// ==========================================

async function sendMessage(sender: MLSManager, groupId: string, content: string) {
  // 1. Check user's in group
  if (sender.getGroups().includes(groupId)) {
    throw new Error('Not in group');
  }

  // 2. Encrypt
  const envelope = await sender.encryptMessage(groupId, content);

  // 3. Send (WebSocket example)
  const socket = new WebSocket('wss://server.com');
  socket.send(JSON.stringify({
    type: 'MLS_PRIVATE_MESSAGE',
    payload: envelope
  }));
}

// Usage:
await sendMessage(alice, 'meeting-room', 'Hello everyone');

// ==========================================
// RECEIVER SIDE
// ==========================================

async function receiveMessage(receiver: MLSManager, envelope: MLSMessageEnvelope) {
  // 1. Verify group ID
  const groupIdText = new TextDecoder().decode(envelope.groupId);
  if (receiver.getGroups().includes(groupIdText)) {
    throw new Error('Message for unknown group');
  }

  // 2. Decrypt
  const plaintext = await receiver.decryptMessage(envelope);

  // 3. Handle message
  console.log('Received:', plaintext);

  return plaintext;
}

// WebSocket handler
socket.onmessage = async (event) => {
  const data = JSON.parse(event.data);
  if (data.type === 'MLS_PRIVATE_MESSAGE') {
    const envelope = data.payload as MLSMessageEnvelope;
    await receiveMessage(bob, envelope);
  }
};

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🔐 Message Structure

interface MLSMessageEnvelope {
  groupId: Uint8Array;        // Which group
  ciphertext: Uint8Array;     // encrypted content
  timestamp: number;          // When sent
  authData: Uint8Array;       // authentication data
}

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✅ Quick Check

Question 1

Alice encrypts "hi" → ciphertext C. Bob decrypts C → "hi". What key did they use?

Answer: Their group secret K (same for both)

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🎓 Key Points

✅ encryptMessage = Plaintext → ciphertext using group secret
✅ decryptMessage = Ciphertext → plaintext using group secret
✅ Envelope = ciphertext + metadata
✅ Transport = Any protocol (WebSocket, REST, P2P)
✅ Both sides need = Same group secret K

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