Bitcoin Universe · Model 18
The Lightning security machine
Why both sides of a Lightning channel must protect the latest agreement, and what happens when someone tries to close with an old one.
Monitoring and timing
Protecting the latest state
A watchtower is optional. It helps only if it is running, sees the old state and gets its transaction confirmed in time, and it does not remove every risk.
Compare states
| Commitment | Alice (sat) | Bob (sat) |
|---|
What is simulated. One channel under the LN-penalty construction used by today's Lightning implementations (BOLT #2, #3): revocable commitment transactions, a relative time lock on the broadcaster's own output, and a penalty that gives the honest side the whole channel if a revoked state is published and the revocation key is used in time. Other designs (for example LN-Symmetry, also called eltoo) replace penalties with different mechanisms and are not shown. Left out: commitment and penalty transaction fees, anchor outputs, payments in flight (HTLCs), fee spikes and congestion that can delay a penalty, and channel reserves. Bob reacts the block after he comes back online; the watchtower reacts after 6 blocks. Demonstration amounts only.