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From: Luke Dashjr <luke@dashjr.org>
To: bitcoin-dev@lists.linuxfoundation.org,
Johnson Lau <jl2012@xbt.hk>
Date: Wed, 17 Aug 2016 04:40:34 +0000
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Subject: Re: [bitcoin-dev] New BIP: Dealing with OP_IF and OP_NOTIF
malleability in P2WSH
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On Wednesday, August 17, 2016 3:02:53 AM Johnson Lau via bitcoin-dev wrote:
> To completely replicate the original behaviour, one may use:
> "DEPTH TOALTSTACK IFDUP DEPTH FROMALTSTACK NUMNOTEQUAL IF 2DROP {if script}
> ELSE DROP {else script} ENDIF"
This is much uglier than expected. IMO if that's the best workaround for the
current behaviour, people should just use "OP_1 OP_EQUAL OP_IF" when/if they
need to avoid malleability issues.
I suspect most cases OP_IF would be used, you really want to accept any non-
zero value. For example, the HTLC script I posted on the list about not long
ago (OP_IF operates on the result from OP_SIZE). Counter-examples would be BIP
124, the examples in BIP 65 and BIP 112, but I note all of these could be just
as easily done without the explicit boolean being fed to the OP_IF (you'd need
an OP_DUP to keep the value, so it wouldn't reduce the byte-size).
Of course, as long as we're talking about a softfork activating together with
segwit, and only having effect in segwit scripts... there's no reason we can't
add whatever opcodes we need so long as it gets done before 0.13.1. I suggest
OP_CASTTOBOOL and OP_DUPASBOOL would be two good candidates if we make OP_IF
stricter. There's also the possibility of adding an OP_RETAINIF which behaves
as the current OP_IF, except not popping the conditional value off the stack.
But perhaps this is getting too complicated for testing in time for segwit...
Luke
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