On Tue, Aug 16, 2016 at 3:43 PM, Peter Todd via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

On Tue, Aug 16, 2016 at 07:37:19PM +0000, Luke Dashjr via bitcoin-dev wrote:
> On Tuesday, August 16, 2016 5:53:08 PM Johnson Lau via bitcoin-dev wrote:
> > A new BIP is prepared to deal with OP_IF and OP_NOTIF malleability in
> > P2WSH:
> > https://github.com/jl2012/bips/blob/minimalif/bip-minimalif.mediawiki
> > https://github.com/bitcoin/bitcoin/pull/8526
>
> I am not sure this makes sense. SegWit transactions are already non-malleable
> due to skipping the witness data in calculating the transaction id. What is
> the benefit to this?

SegWit txids aren't malleable, but segwit transactions as a whole still are.
For instance, I could mess with a segwit transaction by replacing part of the
witness that is used as an argument to an OP_IF with a much larger push,
potentially making the transaction larger, thus making it not get mined due to
the higher fee. There are also potential legal issues if someone replaces a
push with data where posession in your jurisdiction is illegal.

If one's goal is to mess with an transaction to prevent it from being mined, it is more effective to just not relay the transaction rather than to mess with the witness. Given two transactions with the same txid and different witness data, miners and good nodes ought to mine/relay the version with the lower cost (smaller?) witness data.

Worries about "illegal data" appearing in the blockchain is not an issue worth writing a soft-fork over.

There may be good reasons for this BIP, but I don't think the reasons give above are good.