Is it true that any UTXO's that are unspent prefork of BCH AUG 2017, has a risk of a replay attack? Say I have 1 BTC in my wallet which has never moved since 2016. Now I load that private key into BCH wallet and now I have 1 BCH. I make a transaction on BCH to a vendor, but now he takes that same transaction and broadcasts it on BTC chain. Now I lost my BTC too. Is this correct?


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No, BCH and BTC transactions were never replayable to the other network. Starting with the very first BCH block, all BCH transactions were required to use a different SIGHASH_FORKID than used in BTC. This mandatory replay-protection causes BCH transactions to appear invalid to BTC clients, and vice versa, BTC transactions to appear invalid to BCH clients. However, (pre-segwit) addresses remained compatible between the two network, which caused some user confusion.

I assume that the fact that the replay protection was always mandatory may have been lost on some observers, because BCH's replay protection was only made mandatory a couple days before BCH forked off, so a lot of the prior discussion and worries became obsolete last minute. Alternatively, the source of this worry may have been confused by BSV forking off from BCH without any replay-protection whatsoever.

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