Subject: Re: [bitcoin-dev] New BIP: Dealing with OP_IF and OP_NOTIF malleability in P2WSH

From: Luke Dashjr <[email-masked]>
Date: Wed, 17 Aug 2016 04:40:34 +0000

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