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>
	<e0e6679f-aec6-a579-667d-b5b58ea2360b@voskuil.org>
From: Eric Voskuil <eric@voskuil.org>
Message-ID: <b3e6473d-fd9c-2452-f673-930fc1322046@voskuil.org>
Date: Wed, 16 Nov 2016 16:53:45 -0800
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Subject: Re: [bitcoin-dev] BIP30 and BIP34 interaction (was Re: [BIP
 Proposal] Buried Deployments)
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Also, it's important to take note of the motivation behind not banning
duplicate tx hashes outright. Doing so would require that spent tx
hashes are retained forever. A pruning node will have no way of knowing
whether a new tx duplicates the hash of a preceding tx. Any
implementation that does retain such hashes and dismisses new txs on
that basis would fork against pruning nodes.

e

On 11/16/2016 04:43 PM, Eric Voskuil wrote:
>> This means that all future transactions will have different txids...
> rules do guarantee it.
>=20
> No, it means that the chance is small, there is a difference.
>=20
> 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.
>=20
> e
>=20
> On 11/16/2016 04:31 PM, Tier Nolan via bitcoin-dev wrote:
>>
>>
>> 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:
>>
>>     Both of these cases resulted from exact duplicate txs, which BIP34=
 now
>>     precludes. However nothing precludes different txs from having the=
 same
>>     hash.
>>
>>
>> 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.
>>
>> Coinbases have no parents, so it used to be possible for two of them t=
o
>> be identical.
>>
>> Duplicate outputs weren't possible in the database, so the later
>> coinbase transaction effectively overwrote the earlier one.
>>
>> The happened for two coinbases.  That is what the exceptions are for.
>>
>> Neither of the those coinbases were spent before the overwrite
>> happened.  I don't even think those coinbases were spent at all.
>>
>> This means that every activate coinbase transaction has a unique hash
>> and all new coinbases will be unique.
>>
>> This means that all future transactions will have different txids.
>>
>> 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.
>=20


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