> On August 16, 2016 at 8:27 PM Russell O'Connor via bitcoin-dev
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> Okay.
>
> I'm not really opposed to this BIP, but I am worried that fighting script
> malleability is a battle that can never be won; even leaving one avenue of
> malleability open is probably just as bad as having many avenues of
> malleability, so it just doesn't seem worthwhile to me.
Not really. I think the goal is to protect as many common scripts as possible.
For example:
1) BIP146 (Low S values signatures) will eliminate all malleability for P2WPKH
2) BIP146 + null dummy value for CHECKMULTISIG ("NULLDUMMY") will eliminate all
malleability for simple multi-sig in P2WSH. This is particularly interesting
since without NULLDUMMY, attackers are able to replace the dummy value with
anything.
3) BIP146 + NULLDUMMY + minimal IF argument ("MINIMALIF") will eliminate
malleability for any Lightening Network scripts that I'm aware of.
With 3), 99.99% of segwit transactions in foreseeable future should be fully
protected.
The plan is to implement MINIMALIF as a relay policy first, and enforce the
softfork after further risks assessment. This BIP serves as a warning to users
for not using incompatible script.
Peter Todd:
> Having said that, a better approach may be a separate CHECKBOOLVERIFY opcode
> that fails unless the top item on the stack is a minimally encoded true or
> false value, to allow script writers to opt into this behavior; it's not
> always ideal.
I believe all Lightening Network scripts (the only real users of IF/NOTIF in
foreseeable future) are already compatible with MINIMALIF. It may not be a good
idea for them to spend 1 more byte to get protected.
If people want to have the original OP_IF behaviour, a simple way would be
using "0NOTEQUAL IF". However, this works only if the argument is a valid
number (also beware of MINIMALDATA rule in BIP62).
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 because we don't have a simple OP_CASTTOBOOL, and IFDUP is 1 of the 4
codes that perform CastToBool on top stack item (the others are VERIFY, IF, and
NOTIF; and VERIFY can't be used here since it terminates the script with a
False).
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