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_prefix you 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 (implementation-defined)

There is no watchtower BOLT. As of September 2026 the lightning/bolts repo root contains BOLTs 00-05, 07-12 plus the bolt-simple-taproot.md extension document — there is no 13-*.md and no watchtower spec, and lightning/blips contains no watchtower bLIP at all (September 2026). An unmerged personal draft titled "Watchtower protocol specification (BOLT DRAFT REV.1)" lives at sr-gi/bolt13 (last pushed August 2023) and is what The Eye of Satoshi implements; Core Lightning's beginners guide links to it. It is a different protocol from LND's, using register_top_up / subscription_details / add_update_appointment messages.

Consequence: there is no cross-implementation tower interop. An LND client can only use an LND-protocol tower. The wire protocol below is LND's wtwire (watchtower/wtwire, message types 600-607 as of lnd v0.21.3-beta, September 2026) — the de-facto deployed one.

Registration

Client → Watchtower (create_session, type 602):

create_session {
  blob_type,       // altruist/reward + commit/anchor/taproot flags
  max_updates,     // uint16, updates honored for this session
  reward_base,     // uint32, flat tower cut if blob_type has reward
  reward_rate,     // uint32, millionths of revoked commitment balance
  sweep_fee_rate,  // sat/kw the justice tx must be built at
}

Watchtower replies create_session_reply (603) with the session parameters. The session is identified by the client's brontide key, not by an explicit session ID field.

Update push

When client signs new commitment, sends state_update (604):

state_update {
  seq_num,         // uint16, 1-indexed, <= max_updates
  last_applied,    // uint16, echoes tower's last_applied
  is_complete,     // 1 = tower closes the connection after replying
  hint,            // 16-byte prefix of the revoked commitment txid
  encrypted_blob,  // justice kit, encrypted under the FULL txid
}

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_local of cheating commitment using revocation key.
Spends the to_remote output when the blob carries one.
Pays output to a client-specified address.

HTLC outputs are not swept by an LND tower: as of lnd v0.21.3-beta (September 2026) watchtower/lookout/justice_descriptor.go still carries // TODO(conner): sweep htlc outputs, and the blob Type flags cover commitment outputs only. In-flight HTLC value is outside watchtower coverage.

The watchtower fee model:

Altruist — tower takes nothing, client pays only on-chain fees. This is what LND ships and the only mode in practice.
Reward — reward_base + reward_rate negotiated in create_session add a tower output to the justice tx. Wire support exists; lnd's own docs/watchtower.md still says reward towers "will be enabled in a subsequent release" as of v0.21.3-beta (September 2026).
Subscription — the sr-gi/bolt13 draft and The Eye of Satoshi use a prepaid slot count topped up over Lightning, independent of LND's model.

Implementations

LND watchtower

LND has built-in client (wtclient) and tower (watchtower), enabled with watchtower.active=1; it listens on :9911 by default:

# Tower (your friend's node)
lncli tower info

# Client side: add tower
lncli wtclient add <pubkey>@<addr>:<port>

The tower is private altruist only — it takes no cut, so there are no tower fees to configure (lnd v0.21.3-beta, September 2026). Access control is by whoever knows the tower URI.

Eclair

Eclair has no watchtower, client or server: as of September 2026 there is no watchtower code in ACINQ/eclair and no eclair.conf setting for one. Eclair relies on the node itself being online.

Core Lightning

CLN has no built-in tower. It exposes the commitment_revocation hook, which watchtower plugins (e.g. rust-teos' watchtower-plugin) use to capture the penalty tx for each revoked state.

Standalone services

The Eye of Satoshi (talaia-labs/rust-teos) — open-source tower implementing the sr-gi/bolt13 draft, with a CLN plugin. Last tagged release v0.2.0 (February 2023); repo still receives occasional commits (latest October 2025 as of September 2026).
Verify which protocol a vendor speaks before assuming your client can talk to it — see the interop note above.

LN-Symmetry / eltoo (would-be replacement)

LN-Symmetry — the design long discussed as "eltoo" — replaces the punish-on-cheating model with "latest-state-wins" semantics:

No revocation; an old state published on chain is overridden by rebinding the latest update transaction onto it.
Watchtowers are no longer per-state justice-blob stores. The draft's on-chain requirements make publishing the re-bound complete update transaction the response to an old state (XX-eltoo-onchain.md), so a tower acting for an offline client republishes the latest state instead of punishing — a design consequence, not a tower requirement the draft states.

Status as of September 2026: still a proposal, and the active work has moved off SIGHASH_ANYPREVOUT. BIP 118 (SIGHASH_ANYPREVOUT for Taproot Scripts, Decker/Towns) is still Status: Draft and unactivated. The current vehicle is BIP 448, "Taproot-native (Re)bindable Transactions" (Sanders/Poinsot/Roose, assigned 2026-03-11, Status: Draft), which bundles BIP 446 OP_TEMPLATEHASH, BIP 348 OP_CHECKSIGFROMSTACK and BIP 349 OP_INTERNALKEY — all Draft, none activated. The BIP 448 abstract names LN-Symmetry ("Eltoo") as a motivating use case.

Work is collected in the bip448 GitHub organization (created June 2026). Both Lightning pieces are open, unmerged drafts opened 2026-06-26 by instagibbs: draft BOLTs in bip448/bolts PR #1 (XX-eltoo-peer-protocol.md, XX-eltoo-transactions.md, XX-eltoo-onchain.md) and a Core Lightning implementation in bip448/lightning PR #1. Optech newsletter #421 (2026-09-04) covers the state of this work.

What the BIP 448 draft BOLTs change for towers:

Settlement is enforced by OP_TEMPLATEHASH <expected_hash> OP_EQUAL with no signature. The draft calls this "a key advantage over BIP118: watchtowers can enforce settlement by simply spending with the correct settlement transaction, without needing to recreate any signatures."
Update and settlement transactions are re-bound at broadcast time by rewriting the prevout; the template hash excludes prevout data (sha_prevouts, sha_scriptpubkeys, sha_amounts), so one signature stays valid against any earlier state output.
Fee bumping uses a keyless P2A anchor under TRUC (BIP 431), which the draft says lets third parties (e.g. watchtowers) bump fees without requiring key material; replacement is by package RBF on the anchor spend.

None of this is deployed: no BIP 448 opcode is active on mainnet as of September 2026, so every production tower today is still an ln-penalty tower.

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	Free on altruist towers; fees on reward/subscription
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.
Assuming a tower is interchangeable across implementations — it is not; pick a tower that speaks your node's protocol.
Treating watchtower coverage as covering HTLCs — LND towers sweep commitment outputs only (September 2026).

lightning-watchtowers