From rusty at rustcorp.com.au  Mon Feb 29 00:25:53 2016
From: rusty at rustcorp.com.au (Rusty Russell)
Date: Mon, 29 Feb 2016 10:55:53 +1030
Subject: [Lightning-dev] [bitcoin-dev] SIGHASH_NOINPUT in Segregated
	Witness
In-Reply-To: <20160226010746.GB10295@lightning.network>
References: <20160226010746.GB10295@lightning.network>
Message-ID: <87oab0e50u.fsf@rustcorp.com.au>

Joseph Poon via bitcoin-dev <bitcoin-dev at 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.