From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id C4D2FE7A for ; Thu, 17 May 2018 02:56:14 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from ozlabs.org (ozlabs.org [203.11.71.1]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id E4BA76CE for ; Thu, 17 May 2018 02:56:13 +0000 (UTC) Received: by ozlabs.org (Postfix, from userid 1011) id 40mbXq4kC2z9s1d; Thu, 17 May 2018 12:56:11 +1000 (AEST) From: Rusty Russell To: Luke Dashjr , bitcoin-dev@lists.linuxfoundation.org In-Reply-To: <201805100227.42217.luke@dashjr.org> References: <87po25lmzs.fsf@rustcorp.com.au> <201805100227.42217.luke@dashjr.org> Date: Thu, 17 May 2018 12:14:53 +0930 Message-ID: <87vabnq9ui.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on smtp1.linux-foundation.org Subject: Re: [bitcoin-dev] Making OP_TRUE standard? X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 17 May 2018 02:56:14 -0000 Luke Dashjr writes: > An OP_TRUE-only script with a low value seems like a good example of where the > weight doesn't reflect the true cost: it uses a UTXO forever, while only > costing a weight of 4. > > I like Johnson's idea to have some template (perhaps OP_2-only, to preserve > expected behaviour of OP_TRUE-only) that when combined with a 0-value is > always valid only if spent in the same block. > > I wonder if it would make sense to actually tie it to a transaction version > bit, such that when the bit is set, the transaction is serialised with +1 on > the output count and 00000000000000000181 is simply injected into the > transaction hashing... But for now, simply having a consensus rule that a bit > MUST be set for the expected behaviour, and the bit may ONLY be set when the > last output is exactly 00000000000000000181, would allow us to code the > transaction serialisation up later. (Maybe it should be the first output > instead of the last... Is there any legitimate reason one would have multiple > such dummy outputs?) Your zero-val-OP_TRUE-can't-be-spent-after-same-block SF is interesting, but if we want a SF just give us SIGHASH_NOINPUT and we'll not need this at all (though others still might). It's nicer than the previous discussions on after-the-fact feebumping[1] though. Meanwhile, our best mitigation against UTXO bloat is: 1. Make the fees as low as possible[2] 2. Put a CSV delay on the to-remote output (currently there's asymmetry) 3. Attach more value to the OP_TRUE output, say 1000 satoshi. But turns out we probably don't want an OP_TRUE output nor P2SH, because then the spending tx would be malleable. So P2WSH is is. This brings us another theoretical problem: someone could spend our OP_TRUE with a low-fee non-RBF tx, and we'd not be able to use it to CPFP the tx. It'd be hard to do, but possible. I think the network benefits from using OP_TRUE (anyone can clean, and size, vs some only-known-to-me pubkey) outweighs the risk, but it'd be nice if OP_TRUE P2WSH spends were always considered RBF. Thanks, Rusty. [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-April/015864.html [2] Because bitcoin core use legacy measurements, this is actually 253 satoshi per kilosipa for us, see https://github.com/ElementsProject/lightning/commit/2e687b9b352c9092b5e8bd4a688916ac50b44af0