To: Tier Nolan <tier.nolan@gmail.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
References: <CABm2gDr2-MCiaFFjgUFP5Xc0fQfuqJ3=ZkrzjHqmOiwRZ50CBw@mail.gmail.com>
	<d58ee114-00fd-23c8-9ca7-9a4b28c26f27@voskuil.org>
	<CAE-z3OX5vak25UWcmBSe63OmoOVoGB394WmwyWwUcSxWeDOLhw@mail.gmail.com>
From: Eric Voskuil <eric@voskuil.org>
Message-ID: <e0e6679f-aec6-a579-667d-b5b58ea2360b@voskuil.org>
Date: Wed, 16 Nov 2016 16:43:08 -0800
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Subject: Re: [bitcoin-dev] BIP30 and BIP34 interaction (was Re: [BIP
 Proposal] Buried Deployments)
Precedence: list

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> This means that all future transactions will have different txids...
rules do guarantee it.

No, it means that the chance is small, there is a difference.

If there is an address collision, someone may lose some money. If there
is a tx hash collision, and implementations handle this differently, it
will produce a chain split. As such this is not something that a node
can just dismiss. If they do they are implementing a hard fork.

e

On 11/16/2016 04:31 PM, Tier Nolan via bitcoin-dev wrote:
>=20
>=20
> On Thu, Nov 17, 2016 at 12:10 AM, Eric Voskuil via bitcoin-dev
> <bitcoin-dev@lists.linuxfoundation.org
> <mailto:bitcoin-dev@lists.linuxfoundation.org>> wrote:
>=20
>     Both of these cases resulted from exact duplicate txs, which BIP34 =
now
>     precludes. However nothing precludes different txs from having the =
same
>     hash.
>=20
>=20
> The only way to have two transactions have the same txid is if their
> parents are identical, since the txids of the parents are included in a=

> transaction.
>=20
> Coinbases have no parents, so it used to be possible for two of them to=

> be identical.
>=20
> Duplicate outputs weren't possible in the database, so the later
> coinbase transaction effectively overwrote the earlier one.
>=20
> The happened for two coinbases.  That is what the exceptions are for.
>=20
> Neither of the those coinbases were spent before the overwrite
> happened.  I don't even think those coinbases were spent at all.
>=20
> This means that every activate coinbase transaction has a unique hash
> and all new coinbases will be unique.
>=20
> This means that all future transactions will have different txids.
>=20
> There might not be an explicit rule that says that txids have to be
> unique, but barring a break of the hash function, they rules do
> guarantee it.


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