In Bitcoin, a replay attack is the ability to re-use transactions from one chain on another. There are methods of preventing this, and Bitcoin Cash was able to implement them. Yet, for whatever reason, proponents of the Bitcoin New York Agreement coin, dubbed Segwit2X, do not want replay protection when their proposed hard fork takes place. According to Emil Oldenburg of Bitcoin.com, demanding a replay protection is an attack by the Bitcoin Core team:
If replay protection is added and it’s similar to Bitcoin Cash, the New York Agreement will die and the hard fork will never actually happen. This is the latest tactic from the Core camp to kill the hard fork.
The problem is relayed transactions is not new and has been well known by everyone who signed the agreement. The spirit of the agreement was to get a super majority support for the hard fork, enough to bring non-signers over to the hard forked chain as the legacy chain would quickly be abandoned. The difficulty of the bitcoin network is so high right now that if 90% hard forks, the legacy chain will in practice be unusable. Exchanges can only list activate and usable chains.
Bitcoin Core contributor Eric Lombrozo struck back with an inferred legal threat:
To be clear, it isn’t up to any of us. A good portion of the community wants to keep the legacy chain. NYA signers are free to create a hard fork, but not adding replay protection suggests intent to destroy the legacy chain. As long as a lot of people still want the legacy chain, attempts to destroy it will be treated as an attack on the property of all these people. It constitutes a serious cyberattack and decisive action against it, both technical and legal, has been prepared.
It’s hard to see how a lack of replay protection does not expose people to unwanted risks. It’s further hard to see why transactions made on a new chain should affect transactors on a legacy chain – at the very least, Bitcoin Cash were able to identify that this would be an unreasonable situation. Whatever accusations proponents of the New York agreement and the “2x” scaling solution have against Bitcoin Core contributors and Blockstream, it would seem that issues of contention are being raised which have no purpose other than to be contentious. Replay protection would seem a no-brainer, and resistance to implementing it is telling.