Date: Thu, 17 Nov 2016 03:44:05 -0500
From: Peter Todd <pete@petertodd.org>
To: Eric Voskuil <eric@voskuil.org>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20161117084405.GA12334@savin.petertodd.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>
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Subject: Re: [bitcoin-dev] BIP30 and BIP34 interaction (was Re: [BIP
 Proposal] Buried Deployments)
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On Wed, Nov 16, 2016 at 04:43:08PM -0800, Eric Voskuil via bitcoin-dev wrot=
e:
> > 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.

If there is a tx hash collision it is almost certainly going to be because
SHA256 has become weak through advances in cryptography, much like MD5. If =
that
is the case, Bitcoin is fundementally broken because the blockchain no long=
er
can be relied upon to commit to a unique transaction history: miners would =
be
able to generate blocks that have SHA256 collisions in transactions and even
the merkle tree itself, making it possible to simultaneously mine two (or m=
ore)
contradictory transaction histories at once.

Meanwhile the probability of SHA256 _not_ being broken and a collision being
found is low enough that we should be more worried about earth-killing
asteroids and mutant sharks, among other things.

Quoting Bruce Schneier:

    These numbers have nothing to do with the technology of the devices; th=
ey are
    the maximums that thermodynamics will allow. And they strongly imply th=
at
    brute-force attacks against 256-bit keys will be infeasible until compu=
ters are
    built from something other than matter and occupy something other than =
space.

-https://www.schneier.com/blog/archives/2009/09/the_doghouse_cr.html

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

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