[bitcoin-dev] Alternative name for CHECKSEQUENCEVERIFY (BIP112)

Dave Scotese dscotese at litmocracy.com
Fri Nov 27 04:08:35 UTC 2015

I was curious about there being only 10 single-byte opcodes left.  There
are ten single-byte OP_NOPx opcodes defined, but there are 15 opcodes that
"simply *do not exist anymore* in the protocol" because they are scary (had
bugs that "could crash any Bitcoin node if exploited" or "allowed anyone to
spend anyone's bitcoins").  There are also 66 single-byte values that are
currently reserved, 186 - 252 (0xba - 0xfc).

If the name OP_CHECKSEQUENCEVERIFY should not be changed, each of us has a
single best reason not to change it.  Finding other reasons suggests that
one's top reason isn't good enough.  See Nassim Taleb's book, Antifragile,
if that claim makes you curious.  The same goes for changing it.  In any
case, it is 178 (0xb2) and app developers can call it whatever they want.

It seems trivial to me since the following, in script.h, would neither slow
compilation nor confuse anyone, but could lead the curious to explore the
history and expand their knowledge:
OP_NOP3 = 0xb2,
OP_CHECKMATURITYVERIFY = OP_NOP3, // A comment defending the alternative

I don't know the consensus here on leaving breadcrumbs in code comments
(and enum/variable names) for curious coders to use as inspiration for
studying the history, but I advocate it, since modern IDEs are fairly
well-equipped to make skipping or hiding comments easy.

On Wed, Nov 25, 2015 at 3:05 PM, Mark Friedenbach via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> Looks like I'm the long dissenting voice here? As the originator of the
> name CHECKSEQUENCEVERIFY, perhaps I can explain why the name was
> appropriately chosen and why the proposed alternatives don't stand up.
> First, the names are purposefully chosen to illustrate what they do:
> What does CHECKLOCKTIMEVERIFY do? It verifies the range of tx.nLockTime.
> What does CHECKSEQUENCEVERIFY do? It verifies the range of txin.nSequence.
> Second, the semantics are not limited to relative lock-time / maturity
> only. They both leave open ranges with possible, but currently undefined
> future consensus-enforced behavior. We don't know what sort of future
> behavior these values might trigger, but the associated opcodes are generic
> enough to handle them:
> CHECKLOCKTIMEVERIFY will pass an nSequence between 1985 and 2009, even
> though such constraints have no meaning in Bitcoin.
> CHECKSEQUENCEVERIFY is explicitly written to permit a 5-byte push operand,
> while checking only 17 of the available 39 bits of both the operand and the
> nSequence. Indeed the most recent semantic change of CSV was justified in
> part because it relaxes all constraints over the values of these bits
> freeing them for other purposes in transaction validation and/or future
> extensions of the opcode semantics.
> Third, single-byte opcode space is limited. There are less than 10 such
> opcodes left. Maybe space won't be so precious in a post-segwitness world,
> but I don't want to presume that just yet.
> As for the alternatives, they capture only the initial use case of
> nSequence. My objection would relax if nSequence were renamed, but I think
> that would be too disruptive and unnecessary. In any case, the imagined use
> cases for CHECKSEQUENCEVERIFY has to do with sequencing execution pathways
> of script, so it's not a stretch in meaning. Previously CHECKMATURITYVERIFY
> was a hypothicated opcode that directly checked the minimum age of inputs
> of a transaction. The indirect naming of CHECKSEQUENCEVERIFY on the other
> hand is due to its indirect behavior. RELATIVELOCKTIMEVERIFY was also a
> hypothicated opcode that would check a ficticious nRelativeLockTime field,
> which does not exist. Again my objection would go away if we renamed
> nSequence, but I actually think the nSequence name is better...
> On Tue, Nov 24, 2015 at 2:30 AM, Btc Drak via bitcoin-dev <
> bitcoin-dev at lists.linuxfoundation.org> wrote:
>> BIP68 introduces relative lock-time semantics to part of the nSequence
>> field leaving the majority of bits undefined for other future applications.
>> BIP112 introduces opcode CHECKSEQUENCEVERIFY (OP_CSV) that is
>> specifically limited to verifying transaction inputs according to BIP68's
>> relative lock-time[1], yet the _name_ OP_CSV is much boarder than that. We
>> spent months limiting the number of bits used in BIP68 so they would be
>> available for future use cases, thus we have acknowledged there will be
>> completely different usecases that take advantage of unused nSequence bits.
>> For this reason I believe the BIP112 should be renamed specifically for
>> it's usecase, which is verifying the time/maturity of transaction inputs
>> relative to their inclusion in a block.
>> Suggestions:-
>> We could of course softfork additional meaning into OP_CSV each time we
>> add new sequence number usecases, but that would become obscure and
>> confusing. We have already shown there is no shortage of opcodes so it
>> makes no sense to cram everything into one generic opcode.
>> TL;DR: let's give BIP112 opcode a name that reflects it's actual usecase
>> rather than focusing on the bitcoin internals.
>> [1]
>> https://github.com/bitcoin/bitcoin/pull/6564/files#diff-be2905e2f5218ecdbe4e55637dac75f3R1223
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