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 B767998C for ; Thu, 23 Feb 2017 01:11:52 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from outmail148106.authsmtp.co.uk (outmail148106.authsmtp.co.uk [62.13.148.106]) by smtp1.linuxfoundation.org (Postfix) with ESMTP id 4A9EC19A for ; Thu, 23 Feb 2017 01:11:50 +0000 (UTC) Received: from mail-c232.authsmtp.com (mail-c232.authsmtp.com [62.13.128.232]) by punt22.authsmtp.com (8.14.2/8.14.2/) with ESMTP id v1N1BoKS030331 for ; Thu, 23 Feb 2017 01:11:50 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 v1N1Bmif028742 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) for ; Thu, 23 Feb 2017 01:11:49 GMT Received: from [127.0.0.1] (localhost [127.0.0.1]) by petertodd.org (Postfix) with ESMTPSA id 3232440123 for ; Thu, 23 Feb 2017 01:11:48 +0000 (UTC) Received: by localhost (Postfix, from userid 1000) id 746DC20245; Wed, 22 Feb 2017 20:11:47 -0500 (EST) Date: Wed, 22 Feb 2017 20:11:47 -0500 From: Peter Todd To: bitcoin-dev@lists.linuxfoundation.org Message-ID: <20170223011147.GB905@savin.petertodd.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="zx4FCpZtqtKETZ7O" Content-Disposition: inline User-Agent: Mutt/1.5.23 (2014-03-12) X-Server-Quench: 0daca1cf-f965-11e6-829f-00151795d556 X-AuthReport-Spam: If SPAM / abuse - report it at: http://www.authsmtp.com/abuse X-AuthRoute: OCd2Yg0TA1ZNQRgX IjsJECJaVQIpKltL GxAVJwpGK10IU0Fd P1hyKltILEZaQVBf Ri5dBBEKBAw1ADwr dVUTOktfYVU4Cld1 UkhIRUJSFQ9pABYF D1AbUAd3aQROfWBx Z0Z9XHVEXQo8dDh8 MBMNQ20EbWBkai4a V0RaOQMAIVAbf0wT dwZ2UnYQYGRSYGdo RV49emhpZGgGd3UI TlxQc0QucWFDFTk5 QhEeGDgpAVZNSyJ7 KRs6YlgREUpZOw0p NlU9HFwfKRxaFgoW FUwFHTRUKUQFLwAA 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: [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 01:11:52 -0000 --zx4FCpZtqtKETZ7O Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Something I've recently realised is that TXO commitments do not need to be implemented as a consensus protocol change to be useful. All the benefits t= hey provide to full nodes with regard to allowing for old UTXO data to be prune= d - and thus solving the UTXO bloat problem - can be implemented even without having miners commit to the TXO commitment itself. This has a significant deployment advantage too: we can try out multiple TXO commitment schemes, in production, without the need for consensus changes. # Reasoning 1) Like any other merkelized data structure, a TXO commitment allows a data= set - the TXO set - to be securely provided by an untrusted third party, allowi= ng the data itself to be discarded. So if you have a valid TXO commitment, you= can discard the TXO data itself, and rely on untrusted entities to provide you = that data on demand. 2) The TXO set is a super-set of the UTXO set; all data in the UTXO set is = also present in the TXO set. Thus a TXO commitment with spent TXO's pruned is equivalent to a UTXO set, doubly so if inner nodes in the commitment tree commit to the sum-unspent of their children. 3) Where a outpoint-indexed UTXO set has a uniform access pattern, an insertion-ordered TXO set has a delibrately *non-uniform* access pattern: n= ot only are new entries to the TXO set always appended to the end - an operati= on that requires a known, log2(n), sized set of merkle tips - but due to lost coins alone we can guarantee that older entries in the TXO set will be less frequently updated than newer entries. 4) Thus a full node that doesn't have enough local storage to maintain the = full UTXO set can instead keep track of a TXO commitment, and prune older UTXO's =66rom it that are unlikely to be spent. In the event those UTXO's are spen= t, transactions and blocks spending them can trustlessly provide the necessary data to temporarily fill-in the node's local TXO set database, allowing the next commitment to be calculated. 5) By *not* committing the TXO commitment in the block itself, we obsolete = my concept of delayed TXO commitments: you don't need to have calculated the T= XO commitment digest to validate a block anyway! # Deployment Plan 1) Implement a TXO commitment scheme with the ability to efficiently store = the last n versions of the commitment state for the purpose of reorgs (a reference-counted scheme naturally does this). 2) Add P2P support for advertising to peers what parts of the TXO set you've pruned. 3) Add P2P support to produce, consume, and update TXO unspentness proofs as part of transaction and block relaying. 4) Profit. # Bootstrapping New Nodes With a TXO commitment scheme implemented, it's also possible to produce serialized UTXO snapshots for bootstrapping new nodes. Equally, it's obviou= sly possible to distribute those snapshots, and have people you trust attest to= the validity of those snapshots. I argue that a snapshot with an attestation from known individuals that you trust is a *better* security model than having miners attest to validity: t= he latter is trusting an unknown set of unaccountable, anonymous, miners. This security model is not unlike the recently implemented -assumevalid scheme(1), in that auditing the validity of the assumed valid TXO commitmen= ts is something anyone can do provided they have a full node. Similarly, we co= uld ship Bitcoin nodes with an assumed-valid TXO commitment, and have those nod= es fill in the UTXO data from their peers. However it is a weaker security model, in that a false TXO commitment can m= ore easily be used to trick a node into accepting invalid transactions/blocks; assumed valid blocks requires proof-of-work to pull off this attack. A compromise may be to use assumed valid TXO commitments, extending my partial UTXO set(2) suggestion of having nodes validate the chain backwards, to eventually validate 100% of the chain. # References 1) https://github.com/bitcoin/bitcoin/pull/9484 2) [Bitcoin-development] SPV bitcoind? (was: Introducing BitcoinKit.framewo= rk), Peter Todd, Jul 17th 2013, Bitcoin development mailing list, https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2013-July/002917= =2Ehtml --=20 https://petertodd.org 'peter'[:-1]@petertodd.org --zx4FCpZtqtKETZ7O Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- iQEcBAEBCAAGBQJYrjbQAAoJECSBQD2l8JH7SI8H/1a/5zXlHpgLBE2/ujxQfh04 pJ9W2wlxw4l4LZLHF8Jtk6WeURMWPSrocoi46uq/MrbPjxq+ySbZQBVRskVwZGd6 5uD5cZjlUlFLZB6Yz5qP2A5dmj/7BDcOT12cSU7evXDEPh1iWIjB2f3Bo5Z6wmG9 QYkZt5dBKmcnRqyx4MyVTCfrrK3jBBoMUgIht4bF9+hwc7X+4M91KqwRg2XfrDMO 5tOBrfzIOpDajjZlwNB16fN1VZNy86UkuCtC7dSn8kvaela2FW6Cv9KPB4fSrBhq XXWttxuCoheD82C+LgojBwE1B6wEwi+6LOW9oJdm4sIgZe3e3+DOjnvs2pq4u/M= =h5w+ -----END PGP SIGNATURE----- --zx4FCpZtqtKETZ7O--