Onion Messages 洋蔥訊息

HTLC vs Onion Messages:

HTLC (Payments):
• Transfers value (sats)
• Requires channels and liquidity
• Funds locked until settlement
• Has time limits (CLTV)
• Failures roll back
• Cannot send pure messages

Onion Messages:
• Only transfers data (no value transfer)
• No channel needed (just need node pubkey)
• No funds locked
• No time limits
• Fire and forget
• Cannot transfer sats

Both use onion routing for privacy!

Onion Message Structure:

Message type: 513 (onion_message)

onion_message:
  blinding_point: 33 bytes (for route blinding)
  onion_routing_packet:
    version: 0
    public_key: 33 bytes
    hop_payloads: 1300 bytes (encrypted routing info)
    hmac: 32 bytes

Per-hop Payload (TLV):
  encrypted_recipient_data (type 4): encrypted routing data
  next_node_id (type 6): next hop node ID
  path_id (type 8): for reply path identification
  message (type 64): final message content
  invoice_request (type 66): BOLT 12 invoice request
  invoice (type 68): BOLT 12 invoice

Onion Message Routing:

Routing without channels:
  Alice --> Bob --> Carol --> Dave
     |         |        |
     |    Can forward without channels!
     |
     +-- Only need to know network topology

Send flow:
1. Alice knows Dave's public key
2. Alice finds a path from gossip network
3. Alice builds onion packet
4. Each node decrypts its layer, forwards to next hop
5. Dave receives final message

Connectionless forwarding:

Bob receives onion_message:
1. Uses blinding_point to decrypt his payload
2. Finds next_node_id
3. Calculates new blinding_point
4. Forwards to next hop

Bob doesn't need channels with Alice or Dave!

Reply Path Mechanism:

Problem: How to reply to an anonymous sender?

Solution: Sender includes blinded reply path

Alice's request message:
  destination: Dave (or blinded path to Dave)
  reply_path:
    introduction_node: Bob
    blinding_point: ...
    blinded_hops: [..., ..., Alice]
  message: "Please send me an invoice"

Dave replies:
1. Dave decrypts message
2. Dave uses reply_path to send reply
3. Reply reaches Alice through blinded path
4. Dave doesn't know who Alice is!

Bidirectional privacy:
• Alice doesn't know Dave's real location (if using blinded path)
• Dave doesn't know Alice's real location (through reply path)

BOLT 12 Flow Using Onion Messages:

1. Bob publishes Offer
   lno1... (contains blinded path to Bob)

2. Alice sends invoice_request (Onion Message)
   onion_message:
     destination: Bob's blinded path
     payload:
       invoice_request:
         offer_id: ...
         amount: 100000
         payer_key: Alice's key
       reply_path: [... -> Alice]

3. Bob replies with invoice (Onion Message)
   onion_message:
     destination: Alice's reply_path
     payload:
       invoice:
         amount: 100000
         payment_hash: ...
         paths: [blinded paths to Bob]

4. Alice pays using blinded paths from invoice