Subject: [bitcoin-dev] [BIP-draft] CHECKSEQUENCEVERIFY - An opcode for relative locktime

From: Btc Drak <[email-masked]>
Date: Thu, 13 Aug 2015 12:06:44 +0100

I have written the following draft BIP for a new opcode
CHECKSEQUENCEVERIFY by Mark Friedenbach, which introduces a form of
relative-locktime to Bitcoin's scripting language.

https://github.com/btcdrak/bips/blob/bip-checksequenceverify/bip-csv.mediawiki

<pre>
  BIP: XX
  Title: CHECKSEQUENCEVERIFY
  Authors: BtcDrak <[email-masked]>
           Mark Friedenbach <[email-masked]>
  Status: Draft
  Type: Standards Track
  Created: 2015-08-10
</pre>

==Abstract==

This BIP describes a new opcode (CHECKSEQUENCEVERIFY) for the Bitcoin
scripting system that in combination with BIP 68 allows execution
pathways of a script to be restricted based on the age of the output
being spent.

==Summary==

CHECKSEQUENCEVERIFY redefines the existing NOP3 opcode. When executed
it compares the top item on the stack to the inverse of the nSequence
field of the transaction input containing the scriptSig. If the
inverse of nSequence is less than the sequence threshold (1 << 31),
the transaction version is greater than or equal to 2, and the top
item on the stack is less than or equal to the inverted nSequence,
script evaluation continues as though a NOP was executed. Otherwise
the script fails immediately.

BIP 68's redefinition of nSequence prevents a non-final transaction
from being selected for inclusion in a block until the corresponding
input has reached the specified age, as measured in block heiht or
block time. By comparing the argument to CHECKSEQUENCEVERIFY against
the nSequence field, we indirectly verify a desired minimum age of the
the output being spent; until that relative age has been reached any
script execution pathway including the CHECKSEQUENCEVERIFY will fail
to validate, causing the transaction not to be selected for inclusion
in a block.

==Motivation==

BIP 68 repurposes the transaction nSequence field meaning by giving
sequence numbers new consensus-enforced semantics as a relative
lock-time. However, there is no way to build Bitcoin scripts to make
decisions based on this field.

By making the nSequence field accessible to script, it becomes
possible to construct code pathways that only become accessible some
minimum time after proof-of-publication. This enables a wide variety
of applications in phased protocols such as escrow, payment channels,
or bidirectional pegs.

==Specification==

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

    case OP_NOP3:
    {
        if (!(flags & SCRIPT_VERIFY_CHECKSEQUENCEVERIFY)) {
            // not enabled; treat as a NOP3
            if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) {
                return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
            }
            break;
        }

        if (stack.size() < 1)
            return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);

        // Note that unlike CHECKLOCKTIMEVERIFY we do not need to
        // accept 5-byte bignums since any value greater than or
        // equal to SEQUENCE_THRESHOLD (= 1 << 31) will be rejected
        // anyway. This limitation just happens to coincide with
        // CScriptNum's default 4-byte limit with an explicit sign
        // bit.
        //
        // This means there is a maximum relative lock time of 52
        // years, even though the nSequence field in transactions
        // themselves is uint32_t and could allow a relative lock
        // time of up to 120 years.
        const CScriptNum nInvSequence(stacktop(-1), fRequireMinimal);

        // In the rare event that the argument may be < 0 due to
        // some arithmetic being done first, you can always use
        // 0 MAX CHECKSEQUENCEVERIFY.
        if (nInvSequence < 0)
            return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME);

        // Actually compare the specified inverse sequence number
        // with the input.
        if (!CheckSequence(nInvSequence))
            return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME);

        break;
    }

    bool CheckSequence(const CScriptNum& nInvSequence) const
    {
        int64_t txToInvSequence;

        // Fail under all circumstances if the transaction's version
        // number is not set high enough to enable enforced sequence
        // number rules.
        if (txTo->nVersion < 2)
            return false;

        // Sequence number must be inverted to convert it into a
        // relative lock-time.
        txToInvSequence = (int64_t)~txTo->vin[nIn].nSequence;

        // Sequence numbers under SEQUENCE_THRESHOLD are not consensus
        // constrained.
        if (txToInvSequence >= SEQUENCE_THRESHOLD)
            return false;

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

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

        return true;
    }

https://github.com/maaku/bitcoin/commit/33be476a60fcc2afbe6be0ca7b93a84209173eb2

==Example: Escrow with Timeout==

An escrow that times out automatically 30 days after being funded can be
established in the following way. Alice, Bob and Escrow create a 2-of-3
address with the following redeemscript.

    IF
        2 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3
CHECKMULTISIGVERIFY
    ELSE
        <LOCKTIME_THRESHOLD + 30*24*60*60> CHECKSEQUENCEVERIFY DROP
        <Alice's pubkey> CHECKSIGVERIFY
    ENDIF

At any time funds can be spent using signatures from any two of Alice,
Bob or the Escrow.

After 30 days Alice can sign alone.

The clock does not start ticking until the payment to the escrow address
confirms.

==Reference Implementation==

A reference implementation is provided in the following git repository:

https://github.com/maaku/bitcoin/tree/checksequenceverify

==Deployment==

We reuse the double-threshold switchover mechanism from BIPs 34 and
66, 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 = 3 blocks become
invalid, and all further blocks enforce the new rules.

It is recommended that this soft-fork deployment trigger include other
related proposals for improving Bitcoin's lock-time capabilities, including:

[https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki BIP 65]:
OP_CHECKLOCKTIMEVERIFY,

[https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki BIP 68]:
Consensus-enforced transaction replacement signalled via sequence numbers,

and [https://github.com/bitcoin/bips/blob/master/bip-00XX.mediawiki BIP XX]:
Median-Past-Time-Lock.

==Credits==

Mark Friedenbach invented the application of sequence numbers to
achieve relative lock-time, and wrote the reference implementation of
CHECKSEQUENCEVERIFY.

The reference implementation and this BIP was based heavily on work
done by Peter Todd for the closely related BIP 65.

BtcDrak authored this BIP document.

==References==

BIP 68: Consensus-enforced transaction replacement signalled via
sequence numbers
https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki

BIP 65: OP_CHECKLOCKTIMEVERIFY
https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki

BIP XX: Median past block time for time-lock constraints
https://github.com/bitcoin/bips/blob/master/bip-00XX.mediawiki

HTLCs using OP_CHECKSEQUENCEVERIFY/OP_LOCKTIMEVERIFY and
revocation hashes
http://lists.linuxfoundation.org/pipermail/lightning-dev/2015-July/000021.html

==Copyright==

This document is placed in the public domain.