<pre>
  BIP: 65
  Layer: Consensus (soft fork)
  Title: OP_CHECKLOCKTIMEVERIFY
  Authors: Peter Todd <pete@petertodd.org>
  Status: Deployed
  Type: Specification
  Assigned: 2014-10-01
  License: PD
</pre>

==Abstract==

This BIP describes a new opcode (OP_CHECKLOCKTIMEVERIFY) for the Bitcoin
scripting system that allows a transaction output to be made unspendable until
some point in the future.


==Summary==

CHECKLOCKTIMEVERIFY redefines the existing NOP2 opcode. When executed, if
any of the following conditions are true, the script interpreter will terminate
with an error:

* the stack is empty; or
* the top item on the stack is less than 0; or
* the lock-time type (height vs. timestamp) of the top stack item and the nLockTime field are not the same; or
* the top stack item is greater than the transaction's nLockTime field; or
* the nSequence field of the txin is 0xffffffff;

Otherwise, script execution will continue as if a NOP had been executed.

The nLockTime field in a transaction prevents the transaction from being mined
until either a certain block height, or block time, has been reached. By
comparing the argument to CHECKLOCKTIMEVERIFY against the nLockTime field, we
indirectly verify that the desired block height or block time has been reached;
until that block height or block time has been reached the transaction output
remains unspendable.


==Motivation==

The nLockTime field in transactions can be used to prove that it is
''possible'' to spend a transaction output in the future, by constructing a
valid transaction spending that output with the nLockTime field set.

However, the nLockTime field can't prove that it is ''impossible'' to spend a
transaction output until some time in the future, as there is no way to know if
a valid signature for a different transaction spending that output has been
created.


===Escrow===

If Alice and Bob jointly operate a business they may want to
ensure that all funds are kept in 2-of-2 multisig transaction outputs that
require the co-operation of both parties to spend. However, they recognise that
in exceptional circumstances such as either party getting "hit by a bus" they
need a backup plan to retrieve the funds. So they appoint their lawyer, Lenny,
to act as a third-party.

With a standard 2-of-3 CHECKMULTISIG at any time Lenny could conspire with
either Alice or Bob to steal the funds illegitimately. Equally Lenny may prefer
not to have immediate access to the funds to discourage bad actors from
attempting to get the secret keys from him by force.

However, with CHECKLOCKTIMEVERIFY the funds can be stored in scriptPubKeys of
the form:

    IF
        <now + 3 months> CHECKLOCKTIMEVERIFY DROP
        <Lenny's pubkey> CHECKSIGVERIFY
        1
    ELSE
        2
    ENDIF
    <Alice's pubkey> <Bob's pubkey> 2 CHECKMULTISIG

At any time the funds can be spent with the following scriptSig:

    0 <Alice's signature> <Bob's signature> 0

After 3 months have passed Lenny and one of either Alice or Bob can spend the
funds with the following scriptSig:

    0 <Alice/Bob's signature> <Lenny's signature> 1


===Non-interactive time-locked refunds===

There exist a number of protocols where a transaction output is created that
requires the co-operation of both parties to spend the output. To ensure the
failure of one party does not result in the funds becoming lost, refund
transactions are setup in advance using nLockTime. These refund transactions
need to be created interactively, and additionally, are currently vulnerable to
transaction malleability. CHECKLOCKTIMEVERIFY can be used in these protocols,
replacing the interactive setup with a non-interactive setup, and additionally,
making transaction malleability a non-issue.


====Two-factor wallets====

Services like GreenAddress store bitcoins with 2-of-2 multisig scriptPubKey's
such that one keypair is controlled by the user, and the other keypair is
controlled by the service. To spend funds the user uses locally installed
wallet software that generates one of the required signatures, and then uses a
2nd-factor authentication method to authorize the service to create the second
SIGHASH_NONE signature that is locked until some time in the future and sends
the user that signature for storage. If the user needs to spend their funds and
the service is not available, they wait until the nLockTime expires.

The problem is there exist numerous occasions the user will not have a valid
signature for some or all of their transaction outputs. With
CHECKLOCKTIMEVERIFY rather than creating refund signatures on demand
scriptPubKeys of the following form are used instead:

    IF
        <service pubkey> CHECKSIGVERIFY
    ELSE
        <expiry time> CHECKLOCKTIMEVERIFY DROP
    ENDIF
    <user pubkey> CHECKSIG

Now the user is always able to spend their funds without the co-operation of
the service by waiting for the expiry time to be reached.


====Payment Channels====

Jeremy Spilman style payment channels first setup a deposit controlled by
2-of-2 multisig, tx1, and then adjust a second transaction, tx2, that spends
the output of tx1 to payor and payee. Prior to publishing tx1 a refund
transaction is created, tx3, to ensure that should the payee vanish the payor
can get their deposit back. The process by which the refund transaction is
created is currently vulnerable to transaction malleability attacks, and
additionally, requires the payor to store the refund. Using the same
scriptPubKey form as in the Two-factor wallets example solves both these issues.


===Trustless Payments for Publishing Data===

The PayPub protocol makes it possible to pay for information in a trustless way
by first proving that an encrypted file contains the desired data, and secondly
crafting scriptPubKeys used for payment such that spending them reveals the
encryption keys to the data. However the existing implementation has a
significant flaw: the publisher can delay the release of the keys indefinitely.

This problem can be solved interactively with the refund transaction technique;
with CHECKLOCKTIMEVERIFY the problem can be non-interactively solved using
scriptPubKeys of the following form:

    IF
        HASH160 <Hash160(encryption key)> EQUALVERIFY
        <publisher pubkey> CHECKSIG
    ELSE
        <expiry time> CHECKLOCKTIMEVERIFY DROP
        <buyer pubkey> CHECKSIG
    ENDIF

The buyer of the data is now making a secure offer with an expiry time. If the
publisher fails to accept the offer before the expiry time is reached the buyer
can cancel the offer by spending the output.


===Proving sacrifice to miners' fees===

Proving the sacrifice of some limited resource is a common technique in a
variety of cryptographic protocols. Proving sacrifices of coins to mining fees
has been proposed as a ''universal public good'' to which the sacrifice could
be directed, rather than simply destroying the coins. However doing so is
non-trivial, and even the best existing technique - announce-commit sacrifices
- could encourage mining centralization. CHECKLOCKTIMEVERIFY can be used to
create outputs that are provably spendable by anyone (thus to mining fees
assuming miners behave optimally and rationally) but only at a time
sufficiently far into the future that large miners can't profitably sell the
sacrifices at a discount.


===Freezing Funds===

In addition to using cold storage, hardware wallets, and P2SH multisig outputs
to control funds, now funds can be frozen in UTXOs directly on the blockchain.
With the following scriptPubKey, nobody will be able to spend the encumbered
output until the provided expiry time. This ability to freeze funds reliably may
be useful in scenarios where reducing duress or confiscation risk is desired.

    <expiry time> CHECKLOCKTIMEVERIFY DROP DUP HASH160 <pubKeyHash> EQUALVERIFY CHECKSIG


===Replacing the nLockTime field entirely===

As an aside, note how if the SignatureHash() algorithm could optionally cover
part of the scriptSig the signature could require that the scriptSig contain
CHECKLOCKTIMEVERIFY opcodes, and additionally, require that they be executed.
(the CODESEPARATOR opcode came very close to making this possible in v0.1 of
Bitcoin) This per-signature capability could replace the per-transaction
nLockTime field entirely as a valid signature would now be the proof that a
transaction output ''can'' be spent.


==Detailed Specification==

Refer to the reference implementation, reproduced below, for the precise
semantics and detailed rationale for those semantics.


    case OP_NOP2:
    {
        // CHECKLOCKTIMEVERIFY
        //
        // (nLockTime -- nLockTime )

        if (!(flags & SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY))
            break; // not enabled; treat as a NOP

        if (stack.size() < 1)
            return false;

        // Note that elsewhere numeric opcodes are limited to
        // operands in the range -2**31+1 to 2**31-1, however it is
        // legal for opcodes to produce results exceeding that
        // range. This limitation is implemented by CScriptNum's
        // default 4-byte limit.
        //
        // If we kept to that limit we'd have a year 2038 problem,
        // even though the nLockTime field in transactions
        // themselves is uint32 which only becomes meaningless
        // after the year 2106.
        //
        // Thus as a special case we tell CScriptNum to accept up
        // to 5-byte bignums, which are good until 2**32-1, the
        // same limit as the nLockTime field itself.
        const CScriptNum nLockTime(stacktop(-1), 5);

        // In the rare event that the argument may be < 0 due to
        // some arithmetic being done first, you can always use
        // 0 MAX CHECKLOCKTIMEVERIFY.
        if (nLockTime < 0)
            return false;

        // There are two types of nLockTime: lock-by-blockheight
        // and lock-by-blocktime, distinguished by whether
        // nLockTime < LOCKTIME_THRESHOLD.
        //
        // We want to compare apples to apples, so fail the script
        // unless the type of nLockTime being tested is the same as
        // the nLockTime in the transaction.
        if (!(
              (txTo.nLockTime <  LOCKTIME_THRESHOLD && nLockTime <  LOCKTIME_THRESHOLD) ||
              (txTo.nLockTime >= LOCKTIME_THRESHOLD && nLockTime >= LOCKTIME_THRESHOLD)
             ))
            return false;

        // Now that we know we're comparing apples-to-apples, the
        // comparison is a simple numeric one.
        if (nLockTime > (int64_t)txTo.nLockTime)
            return false;

        // Finally the nLockTime feature can be disabled and thus
        // CHECKLOCKTIMEVERIFY bypassed if every txin has been
        // finalized by setting nSequence to maxint. The
        // transaction would be allowed into the blockchain, making
        // the opcode ineffective.
        //
        // Testing if this vin is not final is sufficient to
        // prevent this condition. Alternatively we could test all
        // inputs, but testing just this input minimizes the data
        // required to prove correct CHECKLOCKTIMEVERIFY execution.
        if (txTo.vin[nIn].IsFinal())
            return false;

        break;

    }

https://github.com/petertodd/bitcoin/commit/ab0f54f38e08ee1e50ff72f801680ee84d0f1bf4


==Deployment==

We reuse the double-threshold IsSuperMajority() switchover mechanism used in
BIP66 with the same thresholds, but for nVersion = 4. The new rules are
in effect for every block (at height H) with nVersion = 4 and at least
750 out of 1000 blocks preceding it (with heights H-1000..H-1) also
have nVersion >= 4. Furthermore, when 950 out of the 1000 blocks
preceding a block do have nVersion >= 4, nVersion < 4 blocks become
invalid, and all further blocks enforce the new rules.

It should be noted that BIP9 involves permanently setting a high-order bit to
1 which results in nVersion >= all prior IsSuperMajority() soft-forks and thus
no bits in nVersion are permanently lost.


===SPV Clients===

While SPV clients are (currently) unable to validate blocks in general,
trusting miners to do validation for them, they are able to validate block
headers and thus can validate a subset of the deployment rules. SPV clients
should reject nVersion < 4 blocks if 950 out of 1000 preceding blocks have
nVersion >= 4 to prevent false confirmations from the remaining 5% of
non-upgraded miners when the 95% threshold has been reached.


==Credits==

Thanks goes to Gregory Maxwell for suggesting that the argument be compared
against the per-transaction nLockTime, rather than the current block height and
time.


==References==

PayPub

* https://github.com/unsystem/paypub

Jeremy Spilman Payment Channels

* https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2013-April/002433.html


==Implementations==

Python / python-bitcoinlib

* https://github.com/petertodd/checklocktimeverify-demos

JavaScript / Node.js / bitcore

* https://github.com/mruddy/bip65-demos


==Copyright==

This document is placed in the public domain.

