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

From: Mark Friedenbach <[email-masked]>
Date: Thu, 13 Aug 2015 11:12:43 -0700

As per the rules of BIP 1, I hereby request that the BIP editor please
assign an official number to this work. The idea has been discussed before
on the bitcoin-dev mailing list:

http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-June/008452.html

And a reference implementation is available here:

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

On Thu, Aug 13, 2015 at 4:06 AM, Btc Drak via bitcoin-dev <
[email-masked]> wrote:

> 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.
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