Private Channels 私有通道

Public/Announced Channels:

After channel opens:
  1. Send channel_announcement message
  2. Send channel_update message
  3. All nodes update routing graph

Everyone knows:
  - Channel exists
  - Both node IDs
  - Channel capacity
  - Fee settings
  - Timelock requirements

Use case: Routing nodes, public infrastructure

---

Private/Unannounced Channels:

After channel opens:
  1. No channel_announcement sent
  2. No public channel_update sent
  3. Channel doesn't exist in network graph

Only channel parties know:
  - Channel exists
  - Capacity and balance
  - Detailed settings

External observers can only see:
  - On-chain funding TX (can't confirm it's LN channel)

Use case: End-user wallets, privacy needs

Creating Private Channels:

channel_flags in open_channel message:

  channel_flags:
    bit 0: announce_channel
           0 = private (don't announce)
           1 = public (announce)

LND Command:
lncli openchannel --private \
  --node_key <peer_pubkey> \
  --local_amt 1000000

CLN Command:
lightning-cli fundchannel \
  <peer_id> 1000000 \
  announce=false

Negotiation Flow:
  1. Initiator sets channel_flags=0 in open_channel
  2. Receiver agrees in accept_channel
  3. Channel opens without broadcasting
  4. Both parties maintain channel info locally

Receiving Payments via Private Channels:

Problem: Sender doesn't know private channel exists. How to route?

Solution: Route Hints

Invoice includes private channel info:
lnbc100n1...
  - payment_hash
  - amount
  - description
  - r (route hints):
      [{
        "pubkey": "<LSP_node_id>",
        "short_channel_id": "123x456x0",
        "fee_base_msat": 1000,
        "fee_proportional_millionths": 100,
        "cltv_expiry_delta": 40
      }]

Sender Path Calculation:
  1. Sender receives invoice
  2. Reads route hints
  3. Routes to node in hints
  4. Uses hint channel to reach receiver

  Sender -> [Public Network] -> LSP -> [Private Channel] -> Receiver
                                 ^
                           route hint provided

Privacy Leakage:
  - Sender knows private channel exists (from invoice)
  - Sender knows capacity estimate (from fee limits)
  - LSP knows payment source and destination

SCID Alias (Short Channel ID Alias):

Problem:
  - Traditional SCID format: block_height:tx_index:output_index
  - Example: 700000:1234:0
  - This leaks on-chain funding TX location!

SCID Alias Solution:
  - Use randomly generated alias SCID
  - Still 8-byte format, but doesn't map to real block
  - Only channel parties know the mapping

Feature bit: option_scid_alias (bit 46/47)

Example:
  Real SCID: 700000x1234x0 (not public)
  Alias SCID: 0x00abcdef12345678 (used in route hints)

Benefits:
  - Invoice doesn't leak funding TX location
  - Supports Zero-Conf channels (no real SCID yet)
  - Better privacy protection

Using alias in route hints:
{
  "short_channel_id": "alias_scid",
  ...
}

Enhanced Privacy: Route Blinding + Private Channels

Route Hints Problem:
  - Exposes private channel info to sender
  - Sender knows receiver's connected node

Route Blinding Solution:
  - Receiver creates blinded path
  - Sender only knows entry node
  - Doesn't know subsequent path or final destination

Combined Usage:

  Sender -> Entry Node -> [Blinded Path] -> LSP -> [Private] -> You
                               ^            ^
                         Route Blinding   Private
                               |
                    Sender only knows this node

See: /tech/lightning/route-blinding