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From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Wed, 17 Aug 2016 00:30:52 +0200
Message-ID: <CAPg+sBi30SgHHXCyipbNpiMRHYWPCRYz6ejQYKrDg3MLJp39EQ@mail.gmail.com>
To: "Russell O'Connor" <roconnor@blockstream.io>, 
	Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: multipart/alternative; boundary=94eb2c094e68d7fa34053a37e6da
Subject: Re: [bitcoin-dev] New BIP: Dealing with OP_IF and OP_NOTIF
 malleability in P2WSH
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On Aug 17, 2016 00:23, "Russell O'Connor via bitcoin-dev" <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> 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.

That implies that everyone will see both versions and be able to make that
choice. Unfortunately, those two versions will be definition be in conflict
with each other, and thus only one will end up paying a fee. We're can't
relay two transactions for the price of one, or we'd expose the p2p network
to a very cheap DDoS attack: just send increasingly small versions of the
same transaction.

Segwit's third party mallebility protection makes it not an issue for
dependent contracts if transactions are mauled (=apparently the verb
related to malleability), but there are still good reasons for senders not
to gratuitously make their transactions extensible in size or other
resources.

--
Pieter

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<p dir=3D"ltr">On Aug 17, 2016 00:23, &quot;Russell O&#39;Connor via bitcoi=
n-dev&quot; &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bi=
tcoin-dev@lists.linuxfoundation.org</a>&gt; wrote:</p>
<p dir=3D"ltr">&gt; If one&#39;s goal is to mess with an transaction to pre=
vent it from being mined, it is more effective to just not relay the transa=
ction rather than to mess with the witness.=C2=A0 Given two transactions wi=
th the same txid and different witness data, miners and good nodes ought to=
 mine/relay the version with the lower cost (smaller?) witness data.</p>
<p dir=3D"ltr">That implies that everyone will see both versions and be abl=
e to make that choice. Unfortunately, those two versions will be definition=
 be in conflict with each other, and thus only one will end up paying a fee=
. We&#39;re can&#39;t relay two transactions for the price of one, or we=
9;d expose the p2p network to a very cheap DDoS attack: just send increasin=
gly small versions of the same transaction.</p>
<p dir=3D"ltr">Segwit&#39;s third party mallebility protection makes it not=
 an issue for dependent contracts if transactions are mauled (=3Dapparently=
 the verb related to malleability), but there are still good reasons for se=
nders not to gratuitously make their transactions extensible in size or oth=
er resources.</p>
<p dir=3D"ltr">--<br>
Pieter<br>
</p>

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