SegWit2x got canceled, and with it in all probability the most heated “battle” in the Bitcoin place therefore far attracts to a close. There are numerous lessons to be learned from this ordeal as properly as some other forks that are happening about Bitcoin – what constitutes “consensus” in the local community, how will the future forks be taken care of and so on. 1 essential factor I haven’t witnessed mentioned as considerably presently is the ongoing situation of fork-evidence replay security – a characteristic that brought about some controversy by its absence in SegWit2x and created Bitcoin Gold a laughing stock when they created a bounty for it very close to their forking day.

Powerful vs decide-in replay security

There is an essential difference to be created between potent and decide-in replay security. When a fork occurs, the former is often on and prevents any transaction on a single facet of the fork from staying legitimate on the other facet of the chain. Bitcoin Hard cash has potent replay security for instance.

Opt-in replay security, on the other hand, is optional – you can build a transaction that won’t be legitimate on the other facet of the fork, but you are also ready to have a transaction that is legitimate on equally sides.

Powerful replay security is beneficial when you want to break up from the principal chain and continue to be an impartial venture. Having said that, it can be harmful when the alterations you are proposing are intended to be an enhance to the current code relatively than forking off into a new venture. This is why SegWit2x did not decide to have a replay security – it was intended to be an enhance to the Bitcoin venture and be the only edition made use of. The only way to obtain that was to make it difficult for equally sides of the fork to coexist.

Issue with decide-in replay security

When decide-in replay security sounds like the suitable way to go, there is a single essential challenge to look at – how do you put into action it in a way that will use to all future forks?

In a ideal globe, each fork would be diligently maintained and it would make confident to make its decide-in replay security build transactions only legitimate on its individual chain. Having said that, as Bitcoin Gold and other tasks have confirmed – we cannot depend on forks staying managed competently. As a result, we want a common decide-in replay security, a single that is agnostic to any future forks (even individuals that don’t honor any replay security whatsoever) and generates transactions that will be only legitimate on a single chain.

Common decide-in replay security

A fork that splits off from the principal venture can be brought about by any alteration to the protocol. There is no common way to differentiate between equally sides of the fork forward of time save for a single – the blockchain heritage. You can mimic anything at all about the code, even faux to be some various shopper, but for specified at some factors the blockchains will diverge – if not, we wouldn’t be dealing with a fork. When that is finished, the info can be made use of to put into action the replay security.

If a single could flag a transaction to only be legitimate if a specified block hash is existing in the blockchain, that would be more than enough to make sure it will often be possible to securely go coins about on any fork that might arise in the future.

If equally sides of the fork honor that flag, only a single facet will involve the transaction in the block. Do this on equally sides and the coins will be protected to shell out on two sides of the fork.

If only a single facet of the fork honors the flag however, the replay security could still operate, albeit with some limitations. You would want to build a transaction with the flag on the chain that doesn’t honor it. This way the chain will involve the transaction in its blockchain, but the principal chain will reject it, considering the fact that it would not realize the block hash. After the to start with transaction is confirmed, it would be protected to shell out the coins on the other facet of the fork.


It is possible to put into action a common decide-in replay security that will still be productive even if only a single facet of a specified fork will regard its principles. This must be adequate to safeguard one’s bitcoins in an event of a possible future bitcoin fork.

The proposed implementation is relatively uncomplicated and classy. I came up with this plan when considering SegWit2x awhile back, but then turned pleasantly stunned when I uncovered out someone else already proposed it as a BIP115 a yr before :). It truly is not part of the principal codebase nonetheless, but maybe by the time future fork rolls about we are going to have anything to safeguard our bitcoins with…


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