BA-05 · LESSON 10

Timelocks

KEY DEFINITIONS

Terms used in this lesson

CLTV: CheckLockTimeVerify: a Bitcoin Script operation that enforces an absolute time or block-height condition.
CSV: CheckSequenceVerify: a Bitcoin Script operation that enforces a relative delay after an earlier output confirms.
BIP68: The consensus rule that gives version-2 transaction sequence fields relative block-height or relative-time meaning.
BIP65: The Bitcoin Improvement Proposal that introduced CHECKLOCKTIMEVERIFY for absolute height or time requirements.
CHECKLOCKTIMEVERIFY: The full name of CLTV, which checks an absolute height or time requirement.
BIP112: The Bitcoin Improvement Proposal that introduced CHECKSEQUENCEVERIFY for relative-delay requirements.
CHECKSEQUENCEVERIFY: The full name of CSV, which checks a relative height or time requirement.
MTP: Median Time Past: the median timestamp of the previous 11 blocks, used by Bitcoin for time-based lock evaluation.
Proof-of-work target: The maximum header-hash value accepted for a block; lowering it makes valid hashes harder to find.

nLockTime, sequence, CLTV, and CSV constrain when transactions or spending paths become valid.

45 min Difficulty 2/5 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 · timelock unitsAbsolute and relative timelocks use different transaction fieldsSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
LOCK TYPEAbsolute height
CLTV checks the transaction's nLockTime against a height or Unix-time threshold.
02
SCRIPT840100 CLTV DROP
The locking script states the minimum age or absolute boundary before later authorization can succeed.
03
TX FIELDnLockTime = 840100
The transaction must enable and encode the same unit domain required by the script.
04
CHAIN CONTEXTCandidate height >= 840100
Nodes evaluate the boundary against the candidate block and the confirmed history of the input.
05
RESULTThen check signature
A satisfied timelock does not authorize the spend by itself. Every remaining Script condition must also pass.

Rule shown: nLockTime sets an absolute height or time boundary; nSequence can encode a relative age under BIP68. Script opcodes enforce the matching field and unit.

STAGE 1 OF 5LOCK TYPE: Absolute height

CLTV checks the transaction's nLockTime against a height or Unix-time threshold.

Governing relationship: nLockTime sets an absolute height or time boundary; nSequence can encode a relative age under BIP68. Script opcodes enforce the matching field and unit. Complete path: 1. LOCK TYPE, Absolute height. CLTV checks the transaction's nLockTime against a height or Unix-time threshold. 2. SCRIPT, 840100 CLTV DROP. The locking script states the minimum age or absolute boundary before later authorization can succeed. 3. TX FIELD, nLockTime = 840100. The transaction must enable and encode the same unit domain required by the script. 4. CHAIN CONTEXT, Candidate height >= 840100. Nodes evaluate the boundary against the candidate block and the confirmed history of the input. 5. RESULT, Then check signature. A satisfied timelock does not authorize the spend by itself. Every remaining Script condition must also pass.

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

BEFORE

Output X confirms at height 800,000 and has a CSV recovery branch requiring 144 blocks.

MECHANISM

Absolute locks target a specific block height or timestamp. Relative locks target an age after the prior output confirms.

AFTER

Relative timelocks move with an output's confirmation; absolute timelocks name a chain height or time threshold.

Text equivalent: Starting from Output X confirms at height 800,000 and has a CSV recovery branch requiring 144 blocks. The mechanism begins when Absolute locks target a specific block height or timestamp. Relative locks target an age after the prior output confirms. The resulting lesson is Relative timelocks move with an output's confirmation; absolute timelocks name a chain height or time threshold.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Choose absolute or relative time

Absolute locks target a specific block height or timestamp. Relative locks target an age after the prior output confirms.

Complete path: 1. Choose absolute or relative time: Absolute locks target a specific block height or timestamp. Relative locks target an age after the prior output confirms. 2. Encode the transaction fields: nLockTime and sequence interact for transaction finality. BIP68 gives version-2 transaction sequences relative height or 512-second time semantics. 3. Commit Script requirements: CLTV compares the transaction's nLockTime with a script threshold. CSV compares an input's sequence-derived delay with a script threshold. 4. Check chain context: Validity depends on current chain height or median-time-past and, for relative locks, the confirmation point of the spent output. 5. Satisfy every other condition: A timelock does not authorize a spend alone. Required signatures, hashes, and output availability must also pass.

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

ACCEPTED PATH

BIP65, BIP68, and BIP112 checks use a signed 32-bit transaction version plus exact uint32 lock, sequence, and chain-context fields, height-versus-time flags, MTP, 512-second units, masks, disable flags, and nonfinal sequence.

CHANGED CONDITION

A height/time mismatch, final or disabled sequence, insufficient encoded delay, or immature candidate context rejects while the spent output remains unchanged.

Important distinction: A timelocked transaction is automatically broadcast and executed when its timer expires. Expiration only removes one validity barrier. Someone must still construct, authorize, broadcast, relay, and mine a valid transaction.

Text equivalent: The accepted path is enforced by BIP65, BIP68, and BIP112 checks use a signed 32-bit transaction version plus exact uint32 lock, sequence, and chain-context fields, height-versus-time flags, MTP, 512-second units, masks, disable flags, and nonfinal sequence. If the required condition changes, A height/time mismatch, final or disabled sequence, insufficient encoded delay, or immature candidate context rejects while the spent output remains unchanged.

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Learn the idea

A timelock restricts when a transaction or input may become valid. BIP65 introduced CHECKLOCKTIMEVERIFY, or CLTV, for absolute requirements. BIP112 introduced CHECKSEQUENCEVERIFY, or CSV, for relative requirements. nLockTime is a transaction-level absolute height or time used with nonfinal sequence values. Relative locktime uses input sequence under BIP68. Median Time Past, or MTP, is the median timestamp of the previous 11 blocks and supplies the chain time used by time-based locks.

GUIDED EXPLANATION1/5 ideas inspected

Choose absolute or relative time

Absolute locks target a specific block height or timestamp. Relative locks target an age after the prior output confirms.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“A timelocked transaction is automatically broadcast and executed when its timer expires.”

Expiration only removes one validity barrier. Someone must still construct, authorize, broadcast, relay, and mine a valid transaction.

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