BA-11 · LESSON 04

Watchtowers and backups

Offline protection and channel-state recovery address different failure modes and have distinct trust assumptions.

65 min Difficulty 3/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · state machineOne valid action transforms a known stateSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
BEFOREKnown state
Identify the facts and objects that exist before the action.
02
ACTIONWatchtowers and backups
The Watchtowers and backups lesson isolates the event that proposes a change.
03
RULEDeterministic check
The enforcing rule accepts or rejects the proposed transition.
04
AFTERSuccessor state
Accepted work creates a new state without silently changing the prior facts.
05
FAILUREState unchanged
A failed condition must not produce a partially accepted successor.

Rule shown: A valid transition names the prior state, action, enforcing rule, and successor state.

STAGE 1 OF 5BEFORE: Known state

Identify the facts and objects that exist before the action.

Governing relationship: A valid transition names the prior state, action, enforcing rule, and successor state. Complete path: 1. BEFORE, Known state. Identify the facts and objects that exist before the action. 2. ACTION, Watchtowers and backups. The Watchtowers and backups lesson isolates the event that proposes a change. 3. RULE, Deterministic check. The enforcing rule accepts or rejects the proposed transition. 4. AFTER, Successor state. Accepted work creates a new state without silently changing the prior facts. 5. FAILURE, State unchanged. A failed condition must not produce a partially accepted successor.

1. State transitionWhat exists, what changes, and what results

BEFORE

Compare four separate cases: a matching tower remedy that confirms first, a delayed owner spend that confirms first, a locator mismatch, and static-backup-only recovery.

MECHANISM

After each state update, the client creates information sufficient to recognize and respond to that state's revoked commitment.

AFTER

Monitoring protects against counterparty breach; backup supports data-loss recovery. They solve different problems.

Text equivalent: Starting from Compare four separate cases: a matching tower remedy that confirms first, a delayed owner spend that confirms first, a locator mismatch, and static-backup-only recovery. The mechanism begins when After each state update, the client creates information sufficient to recognize and respond to that state's revoked commitment. The resulting lesson is Monitoring protects against counterparty breach; backup supports data-loss recovery. They solve different problems.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Prepare breach remedies

After each state update, the client creates information sufficient to recognize and respond to that state's revoked commitment.

Complete path: 1. Prepare breach remedies: After each state update, the client creates information sufficient to recognize and respond to that state's revoked commitment. 2. Upload privacy-limited blobs: A locator is a short lookup value derived from a candidate commitment transaction identifier. A tower uses it to select a small set of encrypted appointments; only a full matching breach transaction can decrypt or validate the remedy, so collisions do not authorize punishment. 3. Monitor while client is offline: The tower watches new blocks or mempool data for a commitment transaction matching one of its locators. 4. Broadcast justice response: A match enables the tower to reconstruct or decrypt a pre-signed response. Operationally it should broadcast before maturity, but the hard conflict boundary is whether justice confirms before the competing delayed spend, not whether the delay clock has merely elapsed. 5. Use backups for a different failure: A static backup helps locate peers and initiate data-loss recovery or safe closure. Restoring stale live state as current could itself risk publishing revoked data.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The observed breach transaction must match the locator, authenticate and authorize the appointment, fund a valid remedy, and win the actual confirmation conflict.

CHANGED CONDITION

A locator mismatch, bad tag or signature, missing remedy, static backup alone, or delayed owner confirmation first prevents punishment; maturity alone never chooses the winner.

Important distinction: A static channel backup restores a Lightning channel exactly like a seed restores an on-chain wallet. Channel state changes off chain and includes revocation history. Static backups usually support recovery procedures, not resuming from an arbitrary latest balance.

Text equivalent: The accepted path is enforced by The observed breach transaction must match the locator, authenticate and authorize the appointment, fund a valid remedy, and win the actual confirmation conflict. If the required condition changes, A locator mismatch, bad tag or signature, missing remedy, static backup alone, or delayed owner confirmation first prevents punishment; maturity alone never chooses the winner.

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Learn the idea

A watchtower monitors Bitcoin for revoked Lightning commitments when a user may be offline. The client gives it encrypted breach-remedy data keyed so a matching transaction can unlock the needed response. Static channel backups are different: they help a wallet rediscover channels and request recovery, but they do not safely restore arbitrary latest commitment state.

GUIDED EXPLANATION1/5 ideas inspected

Prepare breach remedies

After each state update, the client creates information sufficient to recognize and respond to that state's revoked commitment.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“A static channel backup restores a Lightning channel exactly like a seed restores an on-chain wallet.”

Channel state changes off chain and includes revocation history. Static backups usually support recovery procedures, not resuming from an arbitrary latest balance.

FINISH LEARNING FIRST

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