What are transaction replays?

Hi it’s Keir Finlow-Bates here, and today I
want to talk about transaction replay attacks, and give a specific example
in the history of Ethereum where, when there was the Ethereum roll-back after
the DAO attack and the Ethereum classic branch and the Ethereum proper branch
came into existence, it turned out that neither branch put in a marker to
indicate that they were different from the other branch, and this had two up
shots – the first is that if you held ether before the branching then you
ended up with ether on both branches, so you kind of doubled your money (except
that ether classic is not worth anywhere near as much as ether), and the
second thing is that if you submited a transaction after the branch on one of
the branches of Ethereum then the same transaction could be submitted on
to the other one, and it would also take place, and so what happened initially
before the exchanges cottoned on is that people would deposit ether into the
exchange and then request that it be sent back, and it would get sent back to
them, and then they would take that transaction and play it on the other
branch and get free ether classic, and then they would do that again and
again, so there were cases of large quantities of ether classic
basically being drained from exchanges that hadn’t caught on to the
fact that they had this extra ether. And the other case in which this
happened, and it’s still going on today is the split between Bitcoin Cash ABC
and Bitcoin Satoshi Vision – again there’s a bit of acrimony between these two
branches, and as a result neither of them put a special marker in the transaction
to say this is a transaction for this chain only. That did happen when
Bitcoin and Bitcoin Cash split. Bitcoin Cash put a new marker in. But it didn’t
happen for Bitcoin Cash and Bitcoin SV. And again as a result, if you
serve submit a transaction for Bitcoin Cash and you have the same quantity of
Bitcoin SV and you don’t spend that, then somebody could trigger that payment in
any case, so it’s a bit of a problem, but it’s also kind of interesting – this sort
of cloning of blockchains can end up with this situation happening.
Hope you found that interesting. See you in the next video soon. Bye for now!

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