[Bitcoin-segwit2x] Strong 2-Way Replay Protection

Alex Bosworth alex.bosworth at gmail.com
Sun Oct 8 20:00:19 UTC 2017

In June of this year I asked about the provisions for strong 2-way
replay protection. Strong 2-way replay protection means transactions
valid on one chain should be invalid on the other, and vice-versa. I
also asked about wipe-out protection, and that has since been
implemented, which is great.

I talked about the possibility of a contentious hard fork, where
significant value persists across multiple chains. Support for the NYA
hard fork is still lacking from Bitfinex, Local Bitcoins (the largest
volume peer to peer exchange), Slush Pool, Trezor, Ledger, Electrum,
and many others in the industry and individuals in the development and
broader community.

I know there has been some fervent hope that the contentious
possibility of this hard fork would abate or become minimal in the
remaining time, and that indeed was a laudable goal. But since June,
contention has only risen. Instead of adding significant support to
the agreement, important and longstanding Bitcoin companies such as
BitWala, F2Pool, and others have pulled out of the New York Agreement
that prompted this project. A number of signers have since decided to
devote their energies to other currency projects, partially or in

Early results from futures trading shows significant market demand for
both sides of the fork.

I can now say that preparations have begun in earnest to support both
chains. The possibility of a contentious hard fork now looks like the
probable future reality. Thus, this hard fork should provide strong
2-way replay protection. This means that transactions valid on one
chain should be invalid on the other, and vice-versa. Without this
protection, users would only be able to safely transact on the chains
separately by using explicit splitting techniques, which puts
excessive burden on the end user.

I can restate suggestions from this list from Sergio Lerner and
WuJihan who have pointed to encouraging multiple visions to coexist,
potentially using a simple sighash modification that will fix the very
simple technical problem that a valid signature authorizing a
transaction of one currency should not be also used as a valid
signature authorizing the transaction of another currency.

Strong 2-way replay protection will help all businesses regardless of
their position, and help regular users as well. I can quote from an
industry statement regarding the previous contentious hard fork
proposal BTU, which also proposed a hard fork coordinated around 3/4
hash power signaling:

"However, none of the undersigned can list BTU unless we can run both
chains independently without incident. Consequently, we insist that
the Bitcoin Unlimited community (or any other consensus breaking
implementation) build in strong two-way replay protection. Failure to
do so will impede our ability to preserve BTU for customers and will
either delay or outright preclude the listing of BTU."


This quote specifically calls out "or any other consensus breaking
implementation". The statement was signed by companies such as
Bitstamp, Bitfinex, bitso, bitsquare, bittrex, btcc, btcchina,
coinfloor, itbit, Kraken, QuadrigaCX, and ShapeShift.

Sent from my iPhone

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