Watchtower

Lightning channels stay honest because either side can take the entire channel balance if the other broadcasts a stale state, but only if somebody notices in time. A watchtower is that somebody: it holds a 16 byte hint and an encrypted blob it cannot read until the matching transaction appears on chain. If your node lives on a laptop that sleeps, you want one.

How it works

A watchtower never holds your keys and never learns which channel it is protecting until something goes wrong.

Each time your channel balance changes, your node builds the punishment transaction for the state it is about to revoke, encrypts it, and hands the tower two things: a hint, which is the first 16 bytes of the revoked commitment transaction's txid, and the ciphertext. The tower stores fixed size blobs and scans every block for a transaction whose txid starts with one of its hints. Nothing else. When a match turns up, the tower derives the decryption key from that same txid, unpacks the sweep instructions, signs nothing new of its own, and broadcasts the finished punishment transaction. The recovered coins land at an address your wallet picked in advance.

Two limits are worth knowing before you lean on one. In lnd, a tower session accepts up to 1024 backups before the client opens another, and the justice transaction is assembled at a default fee rate of 10 sat/byte, which you can raise with the wtclient.sweep-fee-rate setting. The second limit is coverage: lnd towers back up the to_local and to_remote outputs of a revoked commitment, not the outputs of payments still in flight, so hiring a tower shrinks your exposure rather than erasing it.

Where you see it

Watchtowers appear in node software configuration, not in the apps most people use to send sats.

Running lnd, you add a tower with its public key and host, and you can operate one for friends on port 9911, which is the default. Every deployed tower is an altruist tower: it hands back everything it recovers minus the mining fee instead of taking a cut, because the paid version of the protocol was specified but never finished. Custodial Lightning apps expose none of this, and neither do most phone wallets, since their channels are managed by a provider whose own machines never sleep.

The honest version for somebody who mainly wants to buy and hold: you probably do not need one. You need one if you operate a routing node, or if you park a meaningful balance in channels on a computer that gets shut in a drawer for weeks. If Lightning is a phone app holding coffee money, the loss you should plan around is the phone going in a river, not a cheating peer.

Watchtower vs routing node

A watchtower defends channels, while a routing node forwards other people's payments for a fee, and the two jobs share nothing but a machine. A routing node stays online because being offline costs it income, which is exactly why serious routing operators hire towers rather than act as one. You can run both on one box. They answer opposite questions: how do I keep earning, and how do I avoid being robbed while I am not looking.