Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id C46354A3 for ; Thu, 13 Apr 2017 00:43:53 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.7.6 Received: from mail-pf0-f176.google.com (mail-pf0-f176.google.com [209.85.192.176]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 38E7E159 for ; Thu, 13 Apr 2017 00:43:53 +0000 (UTC) Received: by mail-pf0-f176.google.com with SMTP id s16so21020058pfs.0 for ; Wed, 12 Apr 2017 17:43:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:content-transfer-encoding:mime-version:subject:message-id:date :to; bh=+NV6MI8tWdJ6QRRVhDvwZhUw/js4QKsVQ+6AooCYNSE=; b=o9OGdiBVFNaC2Q38hm2bxFHkvy1xCoeN0djUfindH2WEXPQRrM4BXikpfIJD6xf3Bq od0ItlbnAHUe+7Ncd81it3LUF7Rl4y3FU8VefIPOTF6Opiv0IPwZDAnBlOD1RLGS4ZX4 nwlychC1f07mYbCH6x1X1CXDuA90gRwLsfPFeg/fPNQXsYjKQZScWcYDNNt2ABXNuILF 0oIMCHrTug+EDEc/e0+PkahKH6hfIBBxKPnJkGjDElfFL2eirzjdHnWCWxj1gHthX7EA G1ydPoSVuBxIe9+ZkYlUH+Z/ljO7GwlskQMu5ph7ZcjtMOEaEB4OBeAh7d/NlWxrDfyP zRTQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:content-transfer-encoding:mime-version :subject:message-id:date:to; bh=+NV6MI8tWdJ6QRRVhDvwZhUw/js4QKsVQ+6AooCYNSE=; b=jU27dNZOWgKvpZZrD7ya102MySnAk+/J7WVqigv68a3IyYt+iARNTzcNQDmKDzoBLC pQ1c10Jur3uXoIi6+xHsMwCcGoqTZr5EOlFA1KQlK/nvEOz8BtEYoRl1OYUPOwf1TGXs xjxxICvo2MLPUKrMtiSwwGlnHwPlXOO5dAKmF2Lb9m0fhulhCJdle8PrwM4DNZ9qugUi ethny0Tk0cbtcOI+4K7Y8zuHQ6I6p08od931yzUO1N88d0DYHy2uTS07xX7to8hx6/NP eHHMDxpwxXGkAo1ML9m0rr2KdsX9cy2HQkZO6eMyru8al8qoSi+6ICPfDfmKKnknD11J oSxQ== X-Gm-Message-State: AN3rC/4z16UT92bvcQ6l8oNqCT27EhSl8ZKWBIcmTLpAjBveZ7Pw4xOX Dt4Xm889LWDH3XUPY0w= X-Received: by 10.98.59.9 with SMTP id i9mr521584pfa.50.1492044232416; Wed, 12 Apr 2017 17:43:52 -0700 (PDT) Received: from [192.168.1.126] ([52.119.113.96]) by smtp.gmail.com with ESMTPSA id 11sm38571702pgf.28.2017.04.12.17.43.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 12 Apr 2017 17:43:51 -0700 (PDT) From: Oleg Andreev Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Mime-Version: 1.0 (Mac OS X Mail 10.3 \(3273\)) Message-Id: <6DF303E6-E379-45FD-947B-BB5F938177E5@gmail.com> Date: Wed, 12 Apr 2017 17:43:50 -0700 To: "bitcoin-dev@lists.linuxfoundation.org" X-Mailer: Apple Mail (2.3273) X-Spam-Status: No, score=-2.0 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE 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] Deploying CT in Bitcoin without extension blocks? 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, 13 Apr 2017 00:43:53 -0000 (This is a sketch, not a fully-formed proposal, just to kick off the = discussion.) Confidential Transactions (by GMaxwell & Poelstra) require a new = accounting model,=20 new representation of numbers (EC points as Pedersen commitments) and = range proofs=20 per number. Setting aside performance and bandwidth concerns (3-4Kb per = output,=20 50x more signature checks), how would we deploy that feature on Bitcoin = network=20 in the most compatible manner? I'll try to present a sketch of the proposal. I apologize if this = discussion already happened somewhere, although I couldn't find anything on this subject, = apart from Elements=20 sidechain proposal, of course. At first glance we could create a new extblock and transaction format, = add a protocol to "convert" money into and from such extblock, and commit to that extblock = from the=20 outer block's coinbase transaction. Unfortunately, this opens gates to a = flood of debates such as what should be the block size limit in such block, = should we=20 take opportunity to fix over 9000 of pet-peeve issues with existing = transactions and blocks, should we adjust inflation schedule, insert additional PoW, = what would Satoshi say etc. Federated sidechain suffers from the same issues, plus = adds=20 concerns regarding governance, although it would be more decoupled, = which is useful. I tried to look at a possibility to make the change as compatible as = possible, sticking confidential values right into the existing transaction = structure and see how that would look like. As a nice bonus, confidential transactions = would have=20 to fit into the hard-coded 1 Mb limit, preserving the drama around it = :-P We start with a segwit-enabled script versioning and introduce 2 new = script versions: version A has an actual program concatenated with the commitment, while = version B=20 has only the commitment and allows mimblewimble usage (no signatures, = non-interactive=20 cut-through etc). Legacy cleartext amount can nicely act as "min value" = to minimize the range proof size, and range proofs themselves are provided = separately in the segregated witness payload. Then, we soft fork additional rules: 1. In non-coinbase tx, sum of commitments on inputs must balance with = sum of commitments on the outputs plus the cleartext mining fee in the witness. 2. Range proof can be confidential, based on borromean ring signature. 3. Range proof can be non-confidential, consisting of an amount and raw = blinding factor. 4. Tx witness can have an excess value (cf. MW) and cleartext amount for = a miner's fee. 5. In coinbase tx, total plaintext reward + commitments must balance = with subsidy,=20 legacy fees and new fees in the witness. 6. Extra fees in the witness must be signed with the excess value's key. The confidential transactions use the same UTXO set, can be co-authored = with plaintext inputs/outputs using legacy software and maybe even improve scalability by compressing = on-chain transactions using mimblewimble cut-through. The rules above could have been made more complicated with export/import = logic to allow users converting their coins to and from confidential ones, but that would = require more complex support from miners to respect and merge outputs = representing "plaintext value bank", mutate export transactions, which in turn requires introduction of a = non-malleable TxID that excludes miner-adjustable export/import outputs. The rules above have a nice side effect that miners, being the minters = of confidential coins,=20 can sell them at a premium, which creates an incentive for them to = actually support that feature and work on improving performance of rangeproof validation = (e.g. in GPUs). Would love to hear comments and criticism of that approach. Thanks! Oleg.