Assuming Alice is paying Bob with an old style time-locked tx. Under your 
proposal, after the hardfork, Bob is still able to confirm the time-locked tx 
on both networks. To fulfil your new rules he just needs to send the outputs to 
himself again (with different tx format). But as Bob gets all the money on both 
forks, it is already a successful replay


> On 25 Jan 2017, at 15:15, Natanael <natanae...@gmail.com> wrote:
> 
> 
> Den 25 jan. 2017 08:06 skrev "Johnson Lau" <jl2...@xbt.hk 
> <mailto:jl2...@xbt.hk>>:
> 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.
> 
> Read it again. 
> 
> The validation algorithm would be extended so that the transaction can't be 
> replayed, because replaying it in the other network REQUIRES a child 
> transaction in the same block that is valid, a child transaction the is 
> unique to the network. By doing this policy change simultaneously in both 
> networks, old pre-signed transactions *can not be replayed by anybody but the 
> owner* of the coins (as he must spend them immediately in the child 
> transaction). 
> 
> It means that as soon as spent, the UTXO sets immediately and irrevocably 
> diverges across the two networks. Which is the entire point, isn't it? 

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