Replies inline.
Matt
On 3/7/19 3:03 PM, Russell O'Connor wrote:
* OP_CODESEPARATOR in non-BIP 143 scripts fails the script validation.
This includes OP_CODESEPARATORs in unexecuted branches of if
statements,
similar to other disabled opcodes, but unlike OP_RETURN.
OP_CODESEPARATOR is the only mechanism available that allows users to
sign which particular branch they are authorizing for within scripts
that have multiple possible conditions that reuse the same public key.
This is true, and yet it does not appear to actually be practically
usable. Thus far, despite a ton of effort, I have not yet seen a
practical use-case for OP_CODESEPARATOR (except for one example of it
being used to make SegWit scripts ever-so-slightly more effecient in
TumbleBit, hence why this BIP does not propose disabling it for SegWit).
Because of P2SH you cannot know that no one is currently using this
feature. Activating a soft-fork as describe above means these sorts of
funds would be permanently lost. It is not acceptable to risk people's
money like this.
(1) It has been well documented again and again that there is desire to
remove OP_CODESEPARATOR, (2) it is well-documented OP_CODESEPARATOR in
non-segwit scripts represents a rather significant vulnerability in
Bitcoin today, and (3) lots of effort has gone into attempting to find
practical use-cases for OP_CODESEPARATOR's specific construction, with
no successes as of yet. I strongly, strongly disagree that the
highly-unlikely remote possibility that someone created something before
which could be rendered unspendable is sufficient reason to not fix a
vulnerability in Bitcoin today.
I suggest an alternative whereby the execution of OP_CODESEPARATOR
increases the transactions weight suitably as to temper the
vulnerability caused by it. Alternatively there could be some sort of
limit (maybe 1) on the maximum number of OP_CODESEPARATORs allowed to be
executed per script, but that would require an argument as to why
exceeding that limit isn't reasonable.
You could equally argue, however, that any such limit could render some
moderately-large transaction unspendable, so I'm somewhat skeptical of
this argument. Note that OP_CODESEPARATOR is non-standard, so getting
them mined is rather difficult in any case.
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