BA-11 · LESSON 02

Lightning commitments

KEY DEFINITIONS

Terms used in this lesson

HTLC: Hashed Timelock Contract: a conditional payment that requires a secret before a deadline or permits a later refund.
Preimage: The original secret data whose hash equals a previously committed value.

Revocable commitments, HTLCs, timelocks, and justice or replacement mechanisms manage unilateral closes.

45 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
ACTIONLightning commitments
The Lightning commitments 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, Lightning commitments. The Lightning commitments 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

State 7 pays Alice 65,000 sats and Bob 35,000 sats. State 8 pays 45,000 sats to Alice and 55,000 sats to Bob. Alice broadcasts revoked state 7 at height 200; her local output…

MECHANISM

Each party holds a different commitment transaction with delayed local outputs and immediately claimable remote outputs, arranged to support punishment.

AFTER

Revocation makes old-state publication risky, but only a confirmed conflicting spend determines who receives the output.

Text equivalent: Starting from State 7 pays Alice 65,000 sats and Bob 35,000 sats. State 8 pays 45,000 sats to Alice and 55,000 sats to Bob. Alice broadcasts revoked state 7 at height 200; her local output has a six-block relative delay. The mechanism begins when Each party holds a different commitment transaction with delayed local outputs and immediately claimable remote outputs, arranged to support punishment. The resulting lesson is Revocation makes old-state publication risky, but only a confirmed conflicting spend determines who receives the output.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Build asymmetric commitments

Each party holds a different commitment transaction with delayed local outputs and immediately claimable remote outputs, arranged to support punishment.

Complete path: 1. Build asymmetric commitments: Each party holds a different commitment transaction with delayed local outputs and immediately claimable remote outputs, arranged to support punishment. 2. Add HTLC outputs: Pending routed payments become outputs with hash, timeout, and signature paths rather than immediately changing settled balances. 3. Exchange a successor state: Parties first secure valid signatures for the new state, then disclose revocation material for the old state under the protocol's ordering. 4. Detect an obsolete close: If one broadcasts a revoked commitment, the counterparty can use the stored revocation secret immediately. The broadcaster's ordinary local path becomes eligible only after its relative delay. If both spends are possible after maturity, transaction confirmation ordering decides the conflict. 5. Resolve current close: A latest-state close pays uncontested outputs and resolves HTLCs through direction-specific signatures and second-stage transactions. Before expiry only the authorized preimage path may be ready; after expiry preimage and timeout spends can race until one confirms.

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

ACCEPTED PATH

The exact commitment number selects current or revoked handling; Script authorization, relative-delay or absolute-expiry eligibility, and actual confirmation order are checked separately.

CHANGED CONDITION

An impossible future state, bad direction-specific signature, absent preimage before expiry, immature timeout, or losing conflicting confirmation leaves the attempted path unauthorized or unconfirmed.

Important distinction: Lightning nodes delete old commitments, so they cannot be broadcast. A party may retain old signed transactions. Revocation secrets and delayed outputs make broadcasting them punishable rather than technically impossible.

Text equivalent: The accepted path is enforced by The exact commitment number selects current or revoked handling; Script authorization, relative-delay or absolute-expiry eligibility, and actual confirmation order are checked separately. If the required condition changes, An impossible future state, bad direction-specific signature, absent preimage before expiry, immature timeout, or losing conflicting confirmation leaves the attempted path unauthorized or unconfirmed.

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Learn the idea

Lightning commitment transactions represent each channel party's current unilateral settlement. Revocable commitment designs punish publication of an old state by revealing a revocation secret to the counterparty. A contest delay postpones the obsolete broadcaster's ordinary local-output path; it does not make the revocation path disappear at maturity. After the delay, conflicting spends race until one confirms. HTLC means Hashed Timelock Contract, a conditional output with separately authorized preimage and timeout paths. Expiry enables the timeout path; it does not necessarily disable a still-valid preimage path, so post-expiry conflicts require a confirmation winner.

GUIDED EXPLANATION1/5 ideas inspected

Build asymmetric commitments

Each party holds a different commitment transaction with delayed local outputs and immediately claimable remote outputs, arranged to support punishment.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Lightning nodes delete old commitments, so they cannot be broadcast.”

A party may retain old signed transactions. Revocation secrets and delayed outputs make broadcasting them punishable rather than technically impossible.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.