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9.2: Using CLTV in Scripts

NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.

OP_CHECKLOCKTIMEVERIFY (or CLTV) is the natural complement to nLockTime. It moves the idea of locking transactions by an absolute time or blockheight into the realm of opcodes, allowing for the locking of individual UTXOs.

VERSION WARNING: CLTV became available with Bitcoin Core 0.11.2, but should be fairly widely deployed at this time.

Remember nLockTime

Before digging into CLTV, we should first recall how nLockTime works.

As detailed in §6.4: Sending a Transaction with a Locktime, locktime is enabled by setting two variables, nLockTime and the nSequence. The nSequence must be set to less than 0xffffffff (usually: 0xffffffff-1), then the nLockTime is interpreted as follows:

  • If the nLockTime is less than 500 million, it is interpreted as a blockheight.
  • If the nLockTime is 500 million or more, it is interpreted as a UNIX timestamp.

A transaction with nLockTime set cannot be spent (or even put on the block chain) until either the blockheight or time is reached. In the meantime, the transaction can be cancelled by respending any of the UTXOs that make up the transaction.

Understand the CLTV Opcode

OP_CHECKLOCKTIMEVERIFY works within the same paradigm of absolute blockheights or absolute UNIX times, but it runs as part of a Bitcoin Script. It reads one argument, which can be a blockheight or an absolute UNIX time. Through a somewhat convoluted methodology, it compares that argument to the current time. If it's too early, the script fails; if the time condition has been met, the script carries on.

Because CLTV is just part of a script (and presumably part of a P2SH transaction), a CLTV transaction is not kept out of the mempool like an nLockTime transaction is; as soon as it's verified, it goes onto the blockchain, and the funds are considered spent. The trick is that all the outputs that were locked with the CLTV aren't available for respending until the CLTV allows it.

Understand a CLTV Absolute Time

This is how OP_CHECKLOCKTIMEVERIFY would be used to check against May 24, 2017:

1495652013 OP_CHECKLOCKTIMEVERIFY

But we'll usually depict this in an abstraction like this:

<May24> OP_CHECKLOCKTIMEVERIFY

Or this:

<AbsoluteTime> OP_CHECKLOCKTIMEVERIFY

Understand a CLTV Absolute Block Height

This is how OPCHECKLOCKTIMEVERIFY would check against a blockheight that was reached on May 24, 2017:

467951 OP_CHECKLOCKTIMEVERIFY

But we'll usually abtract it like this:

<AbsoluteBlockHeight> OP_CHECKLOCKTIMEVERIFY

Understand How CLTV Really Works

The above explanation is sufficient to use and understand CLTV. However, BIP 65 lays out all the details.

A locking script will only allow a transaction to respend a UTXO locked with a CLTV if OP_CHECKLOCKTIMEVALUE verifies all of the following:

  • The nSequence field must be set to less than 0xffffffff, usually 0xffffffff-1 to avoid confilcts with relative timelocks.
  • CLTV must pop an operand off the stack and it must be 0 or greater.
  • Both the stack operand and nLockTime must either be above or below 500 million, to depict the same sort of absolute timelock.
  • The nLockTime value must be greater than or equal to the stack operand.

So the first thing to note here is that nLockTime is still used with CLTV. To be precise, it's required in the transaction that tries to respend a CLTV-timelocked UTXO. That means that it's not a part of the script's requirements. It's just the timer that's used to release the funds, as defined in the script.

This is managed through a clever understanding of how nLockTime works: a value for nLockTime must always be chosen that is less than or equal to the present time (or blockheight), so that the respending transaction can be put on the blockchain. However, due to CLTV's requirements, a value must also be chosen that is greater than or equal to CLTV's operand. The union of these two sets is NULL until the present time matches the CLTV operand. Afterward, any value can be chosen between CLTV's operand and the present time. Usually, you'd just set it to the present time (or block).

Write a CLTV Script

OP_CHECKLOCKTIMEVERIFY includes an OP_VERIFY, which means that it will immediately halt the script if its verification does not succeed. It has one other quirk: unlike most "verify" commands, it leaves what it's testing on the stack (just in case you want to make any other checks against the time). This means that an OP_CHECKLOCKTIMEVERIFY is usually followed by an OP_DROP to clear the stack.

The following simple locking script could be used to transform a P2PKH output to a timelocked-P2PKH transaction:

<NextYear> OP_CHECKLOCKTIMEVERIFY OP_DROP OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG

Encode a CLTV Script

Of course, as with any complex Bitcoin Scripts, this CLTV script would actually be encoded in a P2SH script, as explained in §8.1: Understanding the Foundation of P2SH and §8.2: Building the Structure of P2SH.

Assuming that <NextYear> were the integer "1546288031" (little-endian hex: 0x9f7b2a5c) and <pubKeyHash> were "371c20fb2e9899338ce5e99908e64fd30b789313", this redeemScript would be built as:

OP_PUSHDATA (4 bytes) 0x9f7b2a5c OP_CHECKLOCKTIMEVERIFY OP_DROP OP_DUP OP_HASH160 OP_PUSHDATA (20 bytes) 0x371c20fb2e9899338ce5e99908e64fd30b789313 OP_EQUALVERIFY OP_CHECKSIG

Which translates into hex as:

04 9f7b2a5c b1 75 76 a9 14 371c20fb2e9899338ce5e99908e64fd30b789313 88 ac

The decodescript RPC can verify that we got it right:

$ bitcoin-cli -named decodescript hexstring=049f7b2a5cb17576a914371c20fb2e9899338ce5e99908e64fd30b78931388ac
{
  "asm": "1546288031 OP_CHECKLOCKTIMEVERIFY OP_DROP OP_DUP OP_HASH160 371c20fb2e9899338ce5e99908e64fd30b789313 OP_EQUALVERIFY OP_CHECKSIG",
  "type": "nonstandard",
  "p2sh": "2MxANZMPo1b2jGaeKTv9rwcBEiXcXYCc3x9"
}

We're not going to continuously show how all Bitcoin Scripts are encoded into P2SH transactions, but will instead offer these shorthands: when we describe a script, it will be a redeemScript, which would normally be serialized and hashed in a locking script and serialized in the unlocking script; when we show an unlocking procedure, it will be the second round of validation, following the confirmation of the locking script hash.

Spend a CLTV UTXO

In order to spend a UTXO that is locked with a CLTV, you must set nLockTime on your new transaction. Usually, you just want to set it to the present time or the present block, as appropriate. As long the CLTV time or blockheight is in the past, and as long as you supply any other data required by the unlocking script, you'll be able to process the UTXO.

In the case of the above example, the following unlocking script would suffice, provided that nLockTime was set to somewhere in advance of the <NextYear> date, and provided it was indeed at least <NextYear>:

<signature> <pubKey>

Run a CLTV Script

To run the Script, you would first concatenate the unlocking and locking scripts:

Script: <signature> <pubKey> <NextYear> OP_CHECKLOCKTIMEVERIFY OP_DROP OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Stack: [ ]

The three constants would be pushed onto the stack:

Script: OP_CHECKLOCKTIMEVERIFY OP_DROP OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Stack: [ <signature> <pubKey> <NextYear> ]

Then, OP_CHECKLOCKTIMEVERIFY runs. It finds something on the stack and verifies that nSequence isn't 0xffffffff. Finally, it compares <NextYear> with nLockTime. If they are both the same sort of representation and if nLockTime ≥ <NextYear>, then it successfully processes (else, it ends the script):

Script: OP_DROP OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Running: <NextYear> OP_CHECKLOCKTIMEVERIFY
Stack: [ <signature> <pubKey> <NextYear> ]

Then, OP_DROP gets rid of that <NextYear> left around:

Script: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Running: <NextYear> OP_DROP
Stack: [ <signature> <pubKey> ]

Finally, the remainder of the script runs, which is a normal check of a signature and public key.

Summary: Using CLTV in Scripts

OP-CHECKLOCKTIMEVERIFY is a simple opcode that looks at a single argument, interprets it as a blockheight or UNIX timestamp, and only allows its UTXO to be unlocked if that blockheight or UNIX timestamp is in the past. Setting nLockTime on the spending transaction is what allows Bitcoin to make this calculation.

What is the Power of CLTV? You've already seem that simple locktimes were one of the bases of Smart Contracts. CLTV takes the next step. Now you can both guarantee that a UTXO can't be spent before a certain time and guarantee that it won't be spent either. In its simplest form, this could be used to create a trust that someone could only access when they reached 18 or a retirement fund that they could only access when they turned 50. However it's true power comes when combined with conditionals, where the CLTV only activates in certain situations.

What's Next?

Continue "Empowering Timelock" with §9.3: Using CSV in Scripts.