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 2824E94F for ; Thu, 23 Feb 2017 07:23:16 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from outmail148098.authsmtp.com (outmail148098.authsmtp.com [62.13.148.98]) by smtp1.linuxfoundation.org (Postfix) with ESMTP id 13AC314C for ; Thu, 23 Feb 2017 07:23:14 +0000 (UTC) Received: from mail-c232.authsmtp.com (mail-c232.authsmtp.com [62.13.128.232]) by punt21.authsmtp.com (8.14.2/8.14.2/) with ESMTP id v1N7NDgo038055 for ; Thu, 23 Feb 2017 07:23:13 GMT Received: from petertodd.org (ec2-52-5-185-120.compute-1.amazonaws.com [52.5.185.120]) (authenticated bits=0) by mail.authsmtp.com (8.14.2/8.14.2/) with ESMTP id v1N7NB6o090555 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) for ; Thu, 23 Feb 2017 07:23:12 GMT Received: from [127.0.0.1] (localhost [127.0.0.1]) by petertodd.org (Postfix) with ESMTPSA id D7BD940123 for ; Thu, 23 Feb 2017 07:23:10 +0000 (UTC) Received: by localhost (Postfix, from userid 1000) id 115DA20245; Thu, 23 Feb 2017 02:23:10 -0500 (EST) Date: Thu, 23 Feb 2017 02:23:10 -0500 From: Peter Todd To: Bitcoin Protocol Discussion Message-ID: <20170223072310.GA3122@savin.petertodd.org> References: <20170223011147.GB905@savin.petertodd.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="17pEHd4RhPHOinZp" Content-Disposition: inline In-Reply-To: <20170223011147.GB905@savin.petertodd.org> User-Agent: Mutt/1.5.23 (2014-03-12) X-Server-Quench: ef2585af-f998-11e6-829f-00151795d556 X-AuthReport-Spam: If SPAM / abuse - report it at: http://www.authsmtp.com/abuse X-AuthRoute: OCd2Yg0TA1ZNQRgX IjsJECJaVQIpKltL GxAVJwpGK10IU0Fd P1hyKltILEZaQVBf Ri5dBBEKBAw1ADwr dVUTOktfYVU4Cld1 UkhIRUJSEw9qAhYA BFAbUAd3aQROfWBx Z0Z9XHVEXQo8dDh6 PDxSSm0PZWJlbS4X UEhfOVYCcgZCe04T dwZ2XXsQYGRSYGdo RV49emhpZGgGd3UI TlxQc0QoTBRDLTQ9 WxsFHDNqEUAbcm0P HztuIVkZGUcNN0g0 LUBpVVVQNRgOQgtT Ek0FCWdCIFcdAiQs FwASQUlWGjA/CTpH DxM1Jne4 X-Authentic-SMTP: 61633532353630.1037:706 X-AuthFastPath: 0 (Was 255) X-AuthSMTP-Origin: 52.5.185.120/25 X-AuthVirus-Status: No virus detected - but ensure you scan with your own anti-virus system. X-Spam-Status: No, score=-2.6 required=5.0 tests=BAYES_00,RCVD_IN_DNSWL_LOW 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] TXO commitments do not need a soft-fork to be useful 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, 23 Feb 2017 07:23:16 -0000 --17pEHd4RhPHOinZp Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Feb 22, 2017 at 08:11:47PM -0500, Peter Todd via bitcoin-dev wrote: > 5) By *not* committing the TXO commitment in the block itself, we obsolet= e my > concept of delayed TXO commitments: you don't need to have calculated the= TXO > commitment digest to validate a block anyway! Thinking about this a bit more, by not being forced to calculate a TXO commitment for every block, we may be able to do significantly better than delayed TXO commitments by lazily hashing. Suppose we have the following perfect merkle tree, which we're using as a key-value map. We'll represent inner nodes for which we've calculated diges= ts with "0"'s to represent what version of the tree they correspond too: 0 / \ / \ / \ / \ / \ 0 0 / \ / \ / \ / \ 0 0 0 0 / \ / \ / \ / \ a b c d e f g h If a value is updated, digests above it become out of date and need to be recalculated: 1 / \ / \ / \ / \ / \ 0 1 / \ / \ / \ / \ 0 0 0 1 / \ / \ / \ / \ a b c d e f g H 2 / \ / \ / \ / \ / \ 0 2 / \ / \ / \ / \ 0 0 2 1 / \ / \ / \ / \ A b c d e F g H 3 / \ / \ / \ / \ / \ 0 3 / \ / \ / \ / \ 0 0 2 3 / \ / \ / \ / \ a b c d e F G H Suppose however that your implementation does lazy hashing; after the 3rd update your state will be: . / \ / \ / \ / \ / \ 0 . / \ / \ / \ / \ 0 0 . . / \ / \ / \ / \ a b c d e F G H Basically all the digests on the right side is out of date and need to be recalculated. Now, first of all it's obviously possible for your implementa= tion to keep updating values in the tree given their keys - you've essentially regressed to a bog standard binary tree. But what happens if you discard part of your dataset? Let's suppose you've discarded the left half: . / \ / \ / \ / \ / \ 0 . / \ / \ . . / \ / \ e F G H Note how you still have sufficient information to calculate the current mer= kle tip commitment: the left side hasn't changed yet. But what happens when som= eone gives you an update proof? Specifically, suppose they want to change b -> B. That requires them to provide you with the part of the merkle tree proving = that position #1 is b. Now you might think that's this data: 3 / \ / \ / \ / \ / \ 0 3 / \ / \ 0 0 / \ a b But the inner node digests marked "3" are useless to you: you haven't calculated those digests yet so you can't compare them to anything. What you can compare is the following: 0 / \ / \ 0 0 / \ a b With that extra data your local knowledge is now: . / \ / \ / \ / \ / \ 0 . / \ / \ / \ / \ 0 0 . . / \ / \ / \ a b e F G H Allowing you to apply the update: . / \ / \ / \ / \ / \ . . / \ / \ / \ / \ . 0 . . / \ / \ / \ a B e F G H If you want to again prune that data, simply recalculate the digests so you can verify a copy given to you by a peer in the future: . / \ / \ / \ / \ / \ 4 . / \ / \ / \ / \ 4 0 . . / \ / \ / \ a B e F G H And prune, leaving you with: . / \ / \ / \ / \ / \ 4 . / \ / \ . . / \ / \ e F G H So tl;dr: the reason this works is that we can substitute commitments for pointers: our merkle tree can also be viewed as a binary tree. So a reasona= ble real-world implementation would be to delay computation of digests for anyt= hing we have in RAM, and only compute digests as in-RAM data is flushed to disk. Equally, on disk we can use standard time-space tradeoffs to only store a subset of the digests, recalculating the rest on the fly. Given that'd we c= ould effectively combine both a cryptographic data structure and a standard pointer-based data structure in one, I suspect we can get good performance = out of this. The main subtlety of this approach will be how exactly to handle the proofs: the level of verification possible depends on what digests a given node has calculated, and we want to avoid making network splitting attacks possible = by attackers deliberately giving nodes proofs with upper digests that are incorrect, something only some nodes can detect. Not sure yet exactly what's the right approach there. Finally, notice how this entire approach depends on schemes like MMR's where the overall structure of the tree does not change as nodes are added and updated; it would be much harder to implement this idea for something like a merklized red-black tree where the structure changes as the tree is rebalan= ced. --=20 https://petertodd.org 'peter'[:-1]@petertodd.org --17pEHd4RhPHOinZp Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- iQEcBAEBCAAGBQJYro3ZAAoJECSBQD2l8JH7JgMH/RA54bEamAkLnuFrfMwU4W8T 0V9PvFPx1H3Lb6a4aT+Hqzui+llIqRtP8gEdPd4pQneZcFoui2hIwh8UEkoEZLHj MI7Gd0bKTWxlAJHRwEoLPi0w+jrLfFHsBVMecvscfLvOHVQokT1NZDZ4OogP30NG DxWEttho1hFJWwZnrCrlzhm7lKBnDGtOn30IiQjQKLAWAat5aExp9Y+Whi/pEDiY zUyOEZVxMjEaJuAgz+IAbNiZFiLBOCuXqElNJtrToml2psv2PVgQxQc1oTmxZCuO 4J0VwRgOnW8Vsrtwxix5MdDEdOcW81aIc9bCFazS3xfQR+GSwFqEwx9Ct1cziVE= =1cic -----END PGP SIGNATURE----- --17pEHd4RhPHOinZp--