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 0D0272348 for ; Fri, 19 Apr 2019 13:24:08 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.7.6 Received: from mail-wm1-f68.google.com (mail-wm1-f68.google.com [209.85.128.68]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 389FC14D for ; Fri, 19 Apr 2019 13:24:07 +0000 (UTC) Received: by mail-wm1-f68.google.com with SMTP id a184so6228110wma.2 for ; Fri, 19 Apr 2019 06:24:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=eQa4eSATGZdb4StNCZA18HOJZZH6vSOdV4hZwbnDWsw=; b=O4Ucdsnly0rZ1d22sDyyP/IyLdiIFFPsAAwAEtUZXaI1eWYLfTJVGLyuBQ2LS+qD6a aShtn55uUnvaeoChuaeRktIKtYk2/F6ZWmh/FvyGuMqTZIsV+psH98W9zomEesX/rheC eKsYFVQfRN6VEC6H/lX4l7+FLzowrShbPXv5IlermvMd7+IOX+pdc8N9Ab5XBMOeydSW X4wdEW4TkLarHwLlVlIz5SM7STsGvqofGZohKw+WjGRYZUngbGrz/wQrpx+sljYa4LwT mm2i4aDhQNMbZAIb0zmSdnXw0x0pcnmrkJbanRoCwIP0S/PGmvzaZKTK7N4kF+IXZ60G L5FA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=eQa4eSATGZdb4StNCZA18HOJZZH6vSOdV4hZwbnDWsw=; b=N2zM/PJRfCIj3CMUkp+X6vUAyPkrmto6JjwZ+ojVZDvIEyLHsnjriE+Kg+DXevPz2U 0U9jODVmFZw40ZoU//8RODvSkP68ieVhSXqypn3aG35JrzdAUz+sj7BMbNoxubCeQW4s DC/DbL+h7aTYiDoQBztkp4tiPkbnUYh/LS6AKxSJfvz/iUjBlY9Oz42AO9KbzQ2OuP6Y yrL60NybvlrnnM8kW31g10pReTs5Ao0BsdHzKVQO5Sj1W9yvZzaweOGBmR5dfe5iXma5 RJrt2SqwSHQ2fi0B41ftmG2jbykXCmfMJocBVaFDzfLc5rW2OvZ/ydtR5bY2aSClpidL 3dkw== X-Gm-Message-State: APjAAAVJ/sTCklEpWW44borsWQ1cTrk05Jd/zdxAIrJhcvDKpfVGCgCS EGQr3IroVpyzd/1usYX5CAXKm7Ompf/4TvMK+Z88+Pbk X-Google-Smtp-Source: APXvYqwEkXDfwAIUxzEUgdeU2HWO9rsOKTA8z+uY43Ya9DjK4+i/Ke750a4aMCLn5k1WJnN15ji+cVo7ThCD181EQE8= X-Received: by 2002:a1c:415:: with SMTP id 21mr2806821wme.109.1555680245504; Fri, 19 Apr 2019 06:24:05 -0700 (PDT) MIME-Version: 1.0 References: <-tCD0qh97dAiz-VGkDQTwSbSQIm9cLF1kOzaWCnUDTI4dKdsmMgHJsGDntQhABZdE2_yBYpPAAdulm8EpdNxOB8o3lI6ZQJBJZWF1INzUrE=@protonmail.com> In-Reply-To: From: Ruben Somsen Date: Fri, 19 Apr 2019 15:23:50 +0200 Message-ID: To: Bitcoin Protocol Discussion Content-Type: text/plain; charset="UTF-8" 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 X-Mailman-Approved-At: Fri, 19 Apr 2019 13:57:18 +0000 Cc: tdryja@media.mit.edu Subject: Re: [bitcoin-dev] Improving SPV security with PoW fraud proofs 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: Fri, 19 Apr 2019 13:24:08 -0000 Hi ZmnSCPxj and Ethan, I apologize if my initial explanation was confusing, but it looks like you figured it out. For every fork, SPV clients only have to download one block. If there is a fork after block N, this means there are two blocks at N+1. You only download and verify N+1 from the longer chain. >Mining a block which will never be accepted is an expensive way to make SPV clients download validate and discard ~2-4 megabytes of data Absolutely, hence the name "PoW fraud proof". It gets naturally created by honest miners and is prohibitively expensive to forge. >SPV clients may not even learn about these splits because it requires that someone relay the split to them. Honest full nodes should not relay such splits. You could perform a fully valid repeated 1-block reorg from the top of the chain. So at least theoretically you could get an honest network to relay every split. >Having SPV clients slow down or become full nodes when a malicious miner with significant mining power is attempting to disrupt the network is probably a best case outcome. That is an excellent point. >As I understand it, this requires that UTXO commitments be mandatory. Perhaps UTXO sets can be made useful without committing them. I have some very loose thoughts on the subject, I consider it an open question. > More difficult is: how can an SPV node acquire the UTXO set at a particular block? I think you are asking fair questions about how the UTXO set commitments would work in practice, and how viable that makes it. I'm not sure. The most comprehensive work I have seen on this topic has been the utreexo proposal by Tadge Dryja: https://www.youtube.com/watch?v=edRun-6ubCc Actually, now that I think about it... As an alternative to UTXO set commitments, the old fraud proofs idea for segwit can be applied here. We get miners to commit to the location of the UTXOs that are being spent (e.g. transaction 5 in block 12). This allows full nodes to succinctly prove invalidity to SPV clients in the following ways: - a committed location does not contain the stated UTXO - the UTXO has already been spent in a prior block If no fraud proofs are given, then the inputs can be assumed to be valid. As you may recall, these kinds of fraud proofs were abandoned mainly because the data unavailability claim could only be verified by downloading the data, resulting in a DoS vector where all blocks had to be downloaded. This problem does not seem to apply here, because we are only interested in blocks which have forks, so it's more doable to download them. -- Ruben Somsen On Fri, Apr 19, 2019 at 6:48 AM ZmnSCPxj wrote: > > Good morning Ethan, > > > My above email contains an error. The SPV client needs to only > > download S+1, not S+1 and S+2. > > > > I agree with you that a weakness of this approach is a miner can make > > SPV clients do substantially more work. However: > > > > 1. Mining a block which will never be accepted is an expensive way to > > make SPV clients download, validate and discard ~2-4 megabytes of > > data. There are far less expensive ways of wasting the resources of > > SPV clients. Its unclear why someone would want to do this instead of > > just packeting full nodes or SPV servers like we saw with the recent > > DDoS attacks against electrum servers. > > > > 2. SPV clients may not even learn about these splits because it > > requires that someone relay the split to them. Honest full nodes > > should not relay such splits. To their bitcoin's worth the attacker > > must also connect to lots of SPV clients. > > > > 3. Having SPV clients slow down or become full nodes when a malicious > > miner with significant mining power is attempting to disrupt the > > network is probably a best case outcome. I would prefer this failure > > mode to the current SPV behavior which is to just go with the > > "longest" chain. > > > I understand. > It seems a reasonable point to do so. > > As I understand it, this requires that UTXO commitments be mandatory. > In particular, if UTXO commitments were not mandatory, it would be trivial to force chainsplits at heights where a UTXO commitment was not made, and force an SPV node to download more blocks backwards until a block with a UTXO commitment is found. > > More difficult is: how can an SPV node acquire the UTXO set at a particular block? > Fullnodes automatically update their UTXO set at each block they accept as tip. > Reversing the blocks to update the UTXO set at a particular past time would require a good amount of CPU and memory. > Thus any service that can provide the actual UTXO set at each block would potentially be attackable by simply requesting enough past blocks. > > > Regards, > ZmnSCPxj