How to spend Bitcoin Cash without the corresponding transaction being mirrored?

[Beetkoin]

So, Bitcoin Cash and Bitcoin both use the same address system?

I personally got into Bitcoin long after Segwit, so I have no BCH.

But let's say someone owns both BTC and BCH after the hard fork.

And then, he wants to sell BCH and buy BTC.

However, when he broadcasts a transaction on BCH, what is to stop someone from broadcasting the same transaction on the BTC network?

Worse, since Segwit essentially creates a "anyone can spend" UTXO as interpreted by pre-Segwit nodes, if he transfers BTC into a Segwit address, what is to prevent someone from stealing his coins on BCH?

[NeuroticFish]

what is to prevent someone from stealing his coins on BCH?

All the proper tutorials about claiming fork coins tell this:
before claiming the fork coin - no matter if it means to send the fork coins to an exchange or other operations - the very first step is to move your bitcoin into a new safe wallet.
This is necessary not only because of mirroring tx. This is also necessary because the fork coin wallet may be malicious, for example.

[BlackHatCoiner]

However, when he broadcasts a transaction on BCH, what is to stop someone from broadcasting the same transaction on the BTC network?

Nothing. That's why we say they are separate chains. This is what hard fork means. If you've found two people willing to buy your coin, but they both want different chains and you had the opportunity to obtain some before the split happens, then what you do is, essentially, double-spending.

You've doubled your belongings. (Assuming 1 BTC and 1 BCH have the same value, which is never the case)

what is to prevent someone from stealing his coins on BCH?

What's the association with SegWit? If they transfer BTC to an address, whether that's SegWit or not, they've spent the UTXO. But, this happens on the Bitcoin network; there's no relation with the Bitcoin Cash network since the split happens.

[Beetkoin]

what is to prevent someone from stealing his coins on BCH?

All the proper tutorials about claiming fork coins tell this:
before claiming the fork coin - no matter if it means to send the fork coins to an exchange or other operations - the very first step is to move your bitcoin into a new safe wallet.
This is necessary not only because of mirroring tx. This is also necessary because the fork coin wallet may be malicious, for example.

But when I move my BTC to a safe wallet, someone is going to mirror that transaction on Bitcoin Cash?
So isn't it an endless game of cat and mouse?

If I move my BTC to a SegWit address, then on BCH it will appear as a "anyone can spend" UTXO therefore immediately stolen?

[NeuroticFish]

If I move my BTC to a SegWit address, then on BCH it will appear as a "anyone can spend" UTXO therefore immediately stolen?

Wouldn't it "steal" those BCH.. by moving them to your (new, safe) bitcoin address?  

[PrimeNumber7]

But let's say someone owns both BTC and BCH after the hard fork.

And then, he wants to sell BCH and buy BTC.

However, when he broadcasts a transaction on BCH, what is to stop someone from broadcasting the same transaction on the BTC network?

Bcash split from bitcoin as of block 478559. If you want to spend coin on the bitcoin blockchain that is invalid on the bcash blockchain, you must use a UTXO that can be traced to a block reward from 478560 or later. This will guarantee your transaction will not be valid on the bcash blockchain.

Alternatively, you can include a UTXO that can be traced back to a transaction whose 'sister' UTXO was spent to another address on the bcash blockchain.
For example. UTXO ABC:2 is for 2 BTC, was spent on the bitcoin blockchain via txid xyz with one output to bitcon address xx. The same UTXO ABC:2 was spent on the bcash blockchain via txid wxy that had one output to bcash address zz. Any UTXO that can be traced back to the utxo xyz:0 will not be valid on the bcash blockchain.

[DannyHamilton]

If you don't have access to any UTXO that only exist on one chain, they you can try the following.

Create address A on BTC chain.
Create address B on BCH chain.

Send full balance on BTC chain to address A.
Immediately (as close to simultaneously as you can) send full balance on BCH chain to address B.

If someone does manage to rebroadcast one of the transactions from one chain to the other before the miners have confirmed the transactions, and your transaction gets bumped by the rebroadcast transaction, then at least you still have access to the private keys for both transactions, so you haven't lost anything more than the transaction fees. You can re-try or you can look for another solution.

However, there is a high likelihood that your transaction to address A will only confirm on the BTC chain and the transaction to address B will only confirm on the BCH chain. If that happens, then you have now blocked the ability for anyone to rebroadcast your transactions from one chain to another anymore. since the two UTXO are completely different.

[DaveF]

But let's say someone owns both BTC and BCH after the hard fork.

And then, he wants to sell BCH and buy BTC.

However, when he broadcasts a transaction on BCH, what is to stop someone from broadcasting the same transaction on the BTC network?

Bcash split from bitcoin as of block 478559. If you want to spend coin on the bitcoin blockchain that is invalid on the bcash blockchain, you must use a UTXO that can be traced to a block reward from 478560 or later. This will guarantee your transaction will not be valid on the bcash blockchain.

Alternatively, you can include a UTXO that can be traced back to a transaction whose 'sister' UTXO was spent to another address on the bcash blockchain.
For example. UTXO ABC:2 is for 2 BTC, was spent on the bitcoin blockchain via txid xyz with one output to bitcon address xx. The same UTXO ABC:2 was spent on the bcash blockchain via txid wxy that had one output to bcash address zz. Any UTXO that can be traced back to the utxo xyz:0 will not be valid on the bcash blockchain.

More or less this with one more addition. There are several forks (BCash, BSV, BCHA, eCash) that are worth some money if you have 1BTC all of the ones I mentioned will get you about another $900. If you only have .1BTC it's probably not worth the time for $90 YMMV. But anyway, if you send on 1 of the forks you send on all so you have to taint your coins with a bit of BCash, BSV, and such so when you move them you only move them on the 1 blockchain.

-Dave

[HCP]

Isn't this all rather moot considering that BCH implemented 2-way replay protection?

As far as I'm aware, you can't broadcast a BTC transaction on the BCH chain and you can't broadcast a BCH transaction on BTC chain. The replay issue was only really a thing when the BCH/BCHA/BSV forks happened with no replay protection.

[DaveF]

Isn't this all rather moot considering that BCH implemented 2-way replay protection?

As far as I'm aware, you can't broadcast a BTC transaction on the BCH chain and you can't broadcast a BCH transaction on BTC chain. The replay issue was only really a thing when the BCH/BCHA/BSV forks happened with no replay protection.

Nope, if you have an input that predates the forks and send it to another legacy address it can be broadcast on the other chains. I don't know how common it is for the other chains to pick up the transaction but it is the same. *Unless something has changed since the BSV fork. I saw a bunch of drama back then pop up with people trying to move older coins and moving other chains with them by accident. Didn't really care, just something I read.

-Dave

[HCP]

Nope, if you have an input that predates the forks and send it to another legacy address it can be broadcast on the other chains.

So, what happened to the whole SIGHASH_FORKID thing that the Bitcoin Cash devs implemented at the eleventh hour (it went in like a day or 2 before the fork)?

https://bitcoin.stackexchange.com/a/102405

Addresses between BTC and BCH are "compatible" yes... but transactions are not... again, the replay attack stuff was purely with BCH and it's forks (BCHA BSV etc)

[NeuroticFish]

Isn't this all rather moot considering that BCH implemented 2-way replay protection?

* I didn't follow BCash hence I didn't know whether that was implemented. Possibly OP also didn't, hence it's good that you've mentioned that.
* it can be seen as a hypothetical question for other forks, especially if by chance OP knew that this was implemented in BCash