On Wed, Nov 16, 2016 at 04:43:08PM -0800, Eric Voskuil via bitcoin-dev wrote:
> > 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.

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 longer
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 more)
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; they are
    the maximums that thermodynamics will allow. And they strongly imply that
    brute-force attacks against 256-bit keys will be infeasible until computers 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

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