[Bitcoin-segwit2x] Notes on segwit2x and anti-replay methods

Jeff Garzik jeff at bloq.com
Thu Jul 27 12:52:49 UTC 2017

Given recent threads, it seemed useful to summarize the status of segwit2x
vis anti-replay.

1) Thanks to all for some healthy debate on btc1 Slack's #debate channel,
as well as in public.

2) anti-replay is currently an open issue:
https://github.com/btc1/bitcoin/issues/34  Being an open issue implies that
we are actively soliciting suggestions and solutions.

3) In informal discussions, I've termed segwit2x a "node HF", meaning that
it is somewhat of a hybrid fork, somewhere between soft fork and hard fork.

3a) In a soft fork, nodes and SPV wallets auto-accept new rules (*opt-out*).
At most a "new rules!" warning is emitted, but operation otherwise
continues under the new ruleset.

3b) In a "pure" hard fork, nodes and SPV wallets do not auto-accept new
rules, and must upgrade to adopt (*opt-in*).

3c) In segwit2x, nodes and SPV wallets split the difference.  nodes do not
auto-accept new rules, but many wallets, notably SPV wallets, will.

4) It is good to provide some sort of facility such that SPV wallets may
also emit a "new rules" warning for the segwit2x 2M HF upgrade, to match
the experience of an opt-out soft fork upgrade.  James Hilliard's PR was
re-opened for this purpose: https://github.com/btc1/bitcoin/pull/46

5) It is *not* good to include a change that breaks all wallets (meaning,
requires upgrade to continue working post-2M HF).  The likely case is that
the NYA participants and 80+% hashpower will upgrade to 2M BBSI.  Thus, in
the the likely "one chain" outcome, a break-all-wallets change would be
unnecessarily disruptive to users (to make a large understatement).

6) This leaves us with a range of solutions that include opt-in replay
protection (#34) and HF visibility via block version bit (#46).  Gavin's
suggestion is in #34.  Sergio had another suggestion on twitter last
night.  Both seem viable, because they are opt-in.

Jeff Garzik
CEO and Co Founder
Bloq, Inc.
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