Return-Path: <rusty@ozlabs.org>
Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org
	[172.17.192.35])
	by mail.linuxfoundation.org (Postfix) with ESMTPS id F0134C1B;
	Tue,  1 Mar 2016 02:06:01 +0000 (UTC)
X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6
X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6
Received: from ozlabs.org (ozlabs.org [103.22.144.67])
	by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 55DFF109;
	Tue,  1 Mar 2016 02:06:01 +0000 (UTC)
Received: by ozlabs.org (Postfix, from userid 1011)
	id 277A4140BB2; Tue,  1 Mar 2016 13:05:58 +1100 (AEDT)
From: Rusty Russell <rusty@rustcorp.com.au>
To: Joseph Poon <joseph@lightning.network>,
	bitcoin-dev@lists.linuxfoundation.org
In-Reply-To: <20160226010746.GB10295@lightning.network>
References: <20160226010746.GB10295@lightning.network>
User-Agent: Notmuch/0.20.2 (http://notmuchmail.org) Emacs/24.5.1
	(x86_64-pc-linux-gnu)
Date: Mon, 29 Feb 2016 10:55:53 +1030
Message-ID: <87oab0e50u.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
X-Spam-Status: No, score=-2.9 required=5.0 tests=BAYES_00, DATE_IN_PAST_24_48, 
	RCVD_IN_DNSWL_MED,RP_MATCHES_RCVD autolearn=ham version=3.3.1
X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on
	smtp1.linux-foundation.org
Cc: lightning-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] SIGHASH_NOINPUT in Segregated Witness
X-BeenThere: bitcoin-dev@lists.linuxfoundation.org
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: Bitcoin Development Discussion <bitcoin-dev.lists.linuxfoundation.org>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/bitcoin-dev>,
	<mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/bitcoin-dev/>
List-Post: <mailto:bitcoin-dev@lists.linuxfoundation.org>
List-Help: <mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev>,
	<mailto:bitcoin-dev-request@lists.linuxfoundation.org?subject=subscribe>
X-List-Received-Date: Tue, 01 Mar 2016 02:06:02 -0000

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.