Re: [bitcoin-dev] Anti-transaction replay in a hardfork

[Johnson Lau <jl2012@xbt.hk>]

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What you describe is not a fix of replay attack. By confirming the same tx in both network, the tx has been already replayed. Their child txs do not matter.

> On 25 Jan 2017, at 09:22, Natanael <natanael.l@gmail.com> wrote:
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> Den 24 jan. 2017 15:33 skrev "Johnson Lau via bitcoin-dev" <bitcoin-dev@lists.linuxfoundation.org <mailto:bitcoin-dev@lists.linuxfoundation.org>>:
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> B. For transactions created before this proposal is made, they are not protected from anti-replay. The new fork has to accept these transactions, as there is no guarantee that the existing fork would survive nor maintain any value. People made time-locked transactions in anticipation that they would be accepted later. In order to maximise the value of such transactions, the only way is to make them accepted by any potential hardforks.
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> This can be fixed. 
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> Make old-format transactions valid *only when paired with a fork-only follow-up transaction* which is spending at least one (or all) of the outputs of the old-format transaction. 
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> (Yes, I know this introduces new statefulness into the block validation logic. Unfortunately it is necessary for maximal fork safety. It can be disabled at a later time if ever deemed no longer necessary.)
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> Meanwhile, the old network SHOULD soft-fork in an identical rule with a follow-up transaction format incompatible with the fork. 
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> This means that old transactions can not be replayed across forks/networks, because they're not valid when stand-alone. It also means that all wallet clients either needs to be updated OR paired with software that intercepts generated transactions, and automatically generates the correct follow-up transaction for it (old network only). 
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> The rules should be that old-format transactions can't reference new-format transactions, even if only a softfork change differ between the formats. This prevents an unnecessary amount of transactions pairs generated by old wallets. Thus they can spend old outputs, but not spend new ones. 
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