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Date: Fri, 26 Feb 2016 01:32:34 +0000
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From: Gregory Maxwell <greg@xiph.org>
To: Joseph Poon <joseph@lightning.network>
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Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] SIGHASH_NOINPUT in Segregated Witness
Precedence: list

On Fri, Feb 26, 2016 at 1:07 AM, Joseph Poon via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> I'm interested in input and in the level of receptiveness to this. If
> there is interest, I'll write up a draft BIP in the next couple days.

The design of segwit was carefully constructed to make it maximally
easy and safe to soft-fork in future script enhancements after its
deployment with the specific goal of avoiding indefinite delays in its
deployment from inevitable scope creep from additional things that are
"easy" to deploy as part of segwit.  I think to be successful we must
be absolutely ruthless about changes that go in there beyond the
absolute minimum needed for the safe deployment of segwit... so I
think this should probably be constructed as a new segwit script type,
and not a base feature.

The exact construction you're thinking of there isn't clear to me...
one thing that comes to mind is that I think it is imperative that we
do not deploy a without-inputs SIGHASH flag without also deploying at
least a fee-committing sighash-all. The reason for this is that if
hardware wallets are forced to continue transferring input
transactions to check fees or to use without-inputs, they may choose
the latter and leave the users needlessly exposed to replay attacks.

When you do write a BIP for this its imperative that the vulnerability
to replay is called out in bold blinking flaming text, along with the
necessary description of how to use it safely. The fact that without
input commitments transactions are replayable is highly surprising to
many developers... Personally, I'd even go so far as to name the flag
SIGHASH_REPLAY_VULNERABLE. :)