Date: Wed, 17 Aug 2016 20:00:38 -0700
From: Peter Todd <pete@petertodd.org>
To: Sergio Demian Lerner <sergio.d.lerner@gmail.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Cc: Gregory Maxwell <gmaxwell@gmail.com>
Subject: Re: [bitcoin-dev] New BIP: Dealing with OP_IF and OP_NOTIF
 malleability in P2WSH
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On Wed, Aug 17, 2016 at 09:33:24PM -0300, Sergio Demian Lerner via bitcoin-=
dev wrote:
> If I send a transaction to an IoT device (say to an OpenDime or to the old
> Firmcoin), and the OpenDime must verify that the transaction has been min=
ed
> (SPV verification), then it may expect the witness program to be of certa=
in
> maximum size (an implementation-imposed  limit). If a Miner modifies the
> witness size and makes it too large, then the device may not be able to
> accept the transaction and the bitcoins may be lost. Lost because the
> private key is in the device, and because the device cannot accept that
> cloned transaction, never ever.
>=20
> The same is true (although less strict) for side-chains and drive-chains:
> they may have certain restrictions on the size of transactions they accept
> to lock bitcoins.
>=20
> That's why I'm proposing that a transaction becomes INVALID if the witness
> size is higher than the expected size (by the sender).

An important part of the design of segwit is that resource constained devic=
es
doing lite-client verification don't need to get witness data at all to ver=
ify
lite-client merkle-path proofs.

Remember that lite-clients can't verify anything useful in witnesses anyway=
, so
for them to have witness data is useless (unless they're doing some kind of
embedded consensus protocol with data published in witnesses, but few people
here care about that use-case).

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

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