claude-dev-suite Watchtowers
A Lightning channel partner can cheat by broadcasting an old commitment
tx. To detect and revert, the honest party must broadcast a "justice
tx" within to_self_delay blocks. Watchtowers do this monitoring on
your behalf when you're offline.
Threat model
Watchtower is untrusted for fund custody. It only learns:
- Channel txid (from
txid_prefixyou provide). - Encrypted blob (the justice tx).
It does NOT learn:
- Channel balance.
- Counterparty identity.
- Payment activity.
If the watchtower colludes with the cheating party, it gains nothing (it doesn't have funds; it only fails to broadcast → channel partner might steal anyway).
Protocol (BOLT 13 draft)
Registration
Client → Watchtower:
session_request {
features,
...
}
Watchtower replies with session ID + parameters.
Update push
When client signs new commitment, sends to watchtower:
update {
session_id,
commitment_txid_hint (first 16 bytes of txid as identifier),
encrypted_blob (sweep tx + revocation key, encrypted with hint as key),
}
Watchtower stores (hint → blob) mapping. Cannot decrypt without
seeing the actual commitment tx on chain.
Monitoring
Watchtower watches the chain:
- For each block, scan all txs.
- For each tx in block, compute hint = first-16-bytes-of-txid.
- If hint matches a stored update, decrypt blob using full txid.
- Decrypt yields a justice tx; broadcast it.
Justice tx
Pre-built sweep tx:
- Spends
to_localof cheating commitment using revocation key. - Spends each HTLC output similarly.
- Pays output to a client-specified address.
The watchtower fee model:
- Free for first N updates (Eclair / cheap).
- Per-update payment via LN (LN-watchtower setups).
Implementations
LND watchtower
LND has built-in client (wtclient) and tower (watchtower):
# Tower (your friend's node)
lncli tower info
# Client side: add tower
lncli wtclient add <pubkey>@<addr>:<port>
Tower fees configurable; small per-update.
Eclair watchtower
Eclair has integrated watchtower; configurable via eclair.conf.
Standalone services
- Boltz Watchtower — paid service.
- Voltage Watchtower — included with hosted plans.
- The Eye of Satoshi — open-source watchtower implementation.
Eltoo (would-be replacement)
Eltoo (BIP118 dependency) replaces the punish-on-cheating model with "latest-state-wins" semantics:
- No revocation; old commitments simply unable to be settled (override).
- Watchtowers no longer needed in the same way; replaced by
"online check after
to_self_delay".
Status: depends on SIGHASH_ANYPREVOUT (BIP118), not yet activated.
Self-hosted vs outsourced
| Aspect | Self-hosted | Outsourced |
|---|---|---|
| Privacy | Best (no one else knows your channels) | Encrypted blobs hide details |
| Reliability | Single point of failure | Multiple towers possible |
| Cost | Server cost | Per-update fee |
| Trust | None | Privacy-leak risk if multiple hashers cluster you |
Best practice: subscribe to multiple watchtowers for redundancy.
Common bugs
- Sending the justice tx blob unencrypted → watchtower learns channel state.
- Not retrying tower delivery on failure → missed updates.
- Watchtower with insufficient mempool watch → misses cheating broadcast.
- Delayed justice tx broadcast (post
to_self_delay) → too late. - Zero-conf channels: watchtower doesn't recognize the channel until funding tx confirms; gap in coverage.