Watchtower
Also · Tower
A watchtower watches the blockchain on behalf of a Lightning user and punishes a counterparty who publishes an old channel state while the user is offline.
Lightning’s penalty mechanism only works if someone is watching. A cheating peer publishes an old commitment, and the victim has a timelock window to respond with the revocation secret and sweep the funds. A phone that is switched off for a week misses that window.
A watchtower is a server that keeps encrypted “justice” transactions for each revoked state and scans every new block for the matching channel closure. If it sees one, it broadcasts the penalty. It learns nothing about the channel until a breach happens. Towers can be self-hosted or run by a third party. They trade a little privacy and a service dependency for the right to go offline.
In French · Watchtower