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From: Rusty Russell <rusty@rustcorp.com.au>
To: Joseph Poon <joseph@lightning.network>,
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Subject: Re: [bitcoin-dev] SIGHASH_NOINPUT in Segregated Witness
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Joseph Poon via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> writes:
> Ideally, a 3rd-party can be handed a transaction which can encompass all
> prior states in a compact way. For currently-designed Segregated Witness
> transactions, this requires storing all previous signatures, which can
> become very costly if individuals to thousands of channel state updates
> per day.
AFAICT we need more than this. Or are you using something other than
the deployable lightning commit tx style?
If each HTLC output is a p2sh[1], you need the timeout and rhash for
each one to build the script to redeem it. In practice, there's not
much difference between sending a watcher a tx for every commit tx and
sending it information for every new HTLC (roughly a factor of 2).
So we also need to put more in the scriptPubKey for this to work; either
the entire redeemscript, or possibly some kind of multiple-choice P2SH
where any one of the hashes will redeem the payment.
Cheers,
Rusty.
[1] eg. from https://github.com/ElementsProject/lightning/blob/master/doc/deployable-lightning.pdf
OP_HASH160 OP_DUP # Replace top element with two copies of its hash
<R-HASH> OP_EQUAL # Test if they supplied the HTLC R value
OP_SWAP <COMMIT-REVOCATION-HASH> OP_EQUAL OP_ADD
# Or the commitment revocation hash
OP_IF # If any hash matched.
<KEY-B> # Pay to B.
OP_ELSE # Must be A, after HTLC has timed out.
<HTLC-TIMEOUT> OP_CHECKLOCKTIMEVERIFY Ensure (absolute) time has passed.
<DELAY> OP_CHECKSEQUENCEVERIFY # Delay gives B enough time to use revocation if it has it.
OP_2DROP # Drop the delay and htlc-timeout from the stack.
<KEY-A> # Pay to A.
OP_ENDIF
OP_CHECKSIG # Verify A or B's signature is correct.
Cheers,
Rusty.
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