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 75ED5CD3 for ; Tue, 2 Apr 2019 20:43:25 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.7.6 Received: from mail-vs1-f41.google.com (mail-vs1-f41.google.com [209.85.217.41]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 614E36E0 for ; Tue, 2 Apr 2019 20:43:24 +0000 (UTC) Received: by mail-vs1-f41.google.com with SMTP id e2so7924286vsc.13 for ; Tue, 02 Apr 2019 13:43:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:from:date:message-id:subject:to; bh=B75u/vmKLaG1WYCxUAos9WqPGUZ+WE096JVBIalX0iA=; b=tNEtAUZRId2NdK/ikLn28rHtVStQZuktLb4DoI+tSzNhTdb/TYzTwxwuh41bnIh+HH WHvpaQgAj7OMp+y1KYFTGsgxVnRfPFbP545ALJcwbQ64wIUiQqlmC9CpeUhnlwbppAWc rpXQAwP7AnHjobw3s8hT7aCSgA7gdwa/raPpY6H+SiDJAdMixaXSsw15sV/ZZA6Vtx8w 9BYUA0M3DKgw0otjezJ8lKpp3Em0T0Ez0ufYJB1u24LduG/uTNXPzIrjsYzxlmEQFLaW yvn6mA4ppP1ZMGYlftsM/zHqIM1gfPJqkSpDY4t/QMFe7t/GeYD6aFqs/vBT9cuVOV6B HiwA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:from:date:message-id:subject:to; bh=B75u/vmKLaG1WYCxUAos9WqPGUZ+WE096JVBIalX0iA=; b=hfZtKdy8uK5EAMHRkFZ/pQMv0KHWNoC6Pg4byEqVqDc2KgBp8aPKFTfwXuu+E0oe93 BuaJlZRHIsfFJ2whMRsjkFmPS3sTrsj7W9clqknlFHRVJ8/YaKXqtzl2mJWiEHSogrrS nnxILL/mE+sQ2ZWUCgWtajRDj9EZFXqJ5BVzrm4wUVTFvhNFH30vFOOuiClSgAvuAxFO CEWCROqJaxkTeHRc89wygo6aWGkNHDsUgK013915/TNzOU/cJ8IW1k7ZtxPPO7dtF53f Du1g180MUnGAFxLAMStDJuEE4i4BzMuhhqw5aeuiGMAEFc0mprVQHJU+HQFa4LatodDV T4Cg== X-Gm-Message-State: APjAAAV+2hKAVI95S/seFak8XqI4vbAMznueKYYULJD1Pp8PXlUbFYZQ 1Htrotjx/xXPWmWCwhU0BQn3iqPoHvFf7eZWeLRhLXOVx0A= X-Google-Smtp-Source: APXvYqwpuy6h/+K4yz072pfD9O9FCnceCvycJ6O0rAOjnbK4lTJqE/uF73qXJ/LR1fpdu8UpuZ6dJH4a7660NcjlByA= X-Received: by 2002:a67:7a4b:: with SMTP id v72mr44457981vsc.79.1554237802507; Tue, 02 Apr 2019 13:43:22 -0700 (PDT) MIME-Version: 1.0 From: "James O'Beirne" Date: Tue, 2 Apr 2019 16:43:11 -0400 Message-ID: To: bitcoin-dev@lists.linuxfoundation.org Content-Type: multipart/alternative; boundary="0000000000001d72890585923051" X-Spam-Status: No, score=-2.0 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, FREEMAIL_FROM, HTML_MESSAGE, 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: Tue, 02 Apr 2019 23:42:52 +0000 Subject: [bitcoin-dev] assumeutxo and UTXO snapshots 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: Tue, 02 Apr 2019 20:43:25 -0000 --0000000000001d72890585923051 Content-Type: text/plain; charset="UTF-8" Hi, I'd like to discuss assumeutxo, which is an appealing and simple optimization in the spirit of assumevalid[0]. # Motivation To start a fully validating bitcoin client from scratch, that client currently needs to perform an initial block download. To the surprise of no one, IBD takes a linear amount time based on the length of the chain's history. For clients running on modest hardware under limited bandwidth constraints, say a mobile device, completing IBD takes a considerable amount of time and thus poses serious usability challenges. As a result, having fully validating clients run on such hardware is rare and basically unrealistic. Clients with even moderate resource constraints are encouraged to rely on the SPV trust model. Though we have promising improvements to existing SPV modes pending deployment[1], it's worth thinking about a mechanism that would allow such clients to use trust models closer to full validation. The subject of this mail is a proposal for a complementary alternative to SPV modes, and which is in the spirit of an existing default, `assumevalid`. It may help modest clients transact under a security model that closely resembles full validation within minutes instead of hours or days. # assumeutxo The basic idea is to allow nodes to initialize using a serialized version of the UTXO set rendered by another node at some predetermined height. The initializing node syncs the headers chain from the network, then obtains and loads one of these UTXO snapshots (i.e. a serialized version of the UTXO set bundled with the block header indicating its "base" and some other metadata). Based upon the snapshot, the node is able to quickly reconstruct its chainstate, and compares a hash of the resulting UTXO set to a preordained hash hard-coded in the software a la assumevalid. This all takes ~23 minutes, not accounting for download of the 3.2GB snapshot[2]. The node then syncs to the network tip and afterwards begins a simultaneous background validation (i.e., a conventional IBD) up to the base height of the snapshot in order to achieve full validation. Crucially, even while the background validation is happening the node can validate incoming blocks and transact with the benefit of the full (assumed-valid) UTXO set. Snapshots could be obtained from multiple separate peers in the same manner as block download, but I haven't put much thought into this. In concept it doesn't matter too much where the snapshots come from since their validity is determined via content hash. # Security Obviously there are some security implications due consideration. While this proposal is in the spirit of assumevalid, practical attacks may become easier. Under assumevalid, a user can be tricked into transacting under a false history if an attacker convinces them to start bitcoind with a malicious `-assumevalid` parameter, sybils their node, and then feeds them a bogus chain encompassing all of the hard-coded checkpoints[3]. The same attack is made easier in assumeutxo because, unlike in assumevalid, the attacker need not construct a valid PoW chain to get the victim's node into a false state; they simply need to get the user to accept a bad `-assumeutxo` parameter and then supply them an easily made UTXO snapshot containing, say, a false coin assignment. For this reason, I recommend that if we were to implement assumeutxo, we not allow its specification via commandline argument[4]. Beyond this risk, I can't think of material differences in security relative to assumevalid, though I appeal to the list for help with this. # More fully validating clients A particularly exciting use-case for assumeutxo is the possibility of mobile devices functioning as fully validating nodes with access to the complete UTXO set (as an alternative to SPV models). The total resource burden needed to start a node from scratch based on a snapshot is, at time of writing, a ~(3.2GB + blocks_to_tip * 4MB) download and a few minutes of processing time, which sounds manageable for many mobile devices currently in use. A mobile user could initialize an assumed-valid bitcoin node within an hour, transact immediately, and complete a pruned full validation of their assumed-valid chain over the next few days, perhaps only doing the background IBD when their device has access to suitable high-bandwidth connections. If we end up implementing an accumulator-based UTXO scaling design[5][6] down the road, it's easy to imagine an analogous process that would allow very fast startup using an accumulator of a few kilobytes in lieu of a multi-GB snapshot. --- I've created a related issue at our Github repository here: https://github.com/bitcoin/bitcoin/issues/15605 and have submitted a draft implementation of snapshot usage via RPC here: https://github.com/bitcoin/bitcoin/pull/15606 I'd like to discuss here whether this is a good fit for Bitcoin conceptually. Concrete plans for deployment steps should be discussed in the Github issue, and after all that my implementation may be reviewed as a sketch of the specific software changes necessary. Regards, James [0]: https://bitcoincore.org/en/2017/03/08/release-0.14.0/#assumed-valid-blocks [1]: https://github.com/bitcoin/bips/blob/master/bip-0157.mediawiki [2]: as tested at height 569895, on a 12 core Intel Xeon Silver 4116 CPU @ 2.10GHz [3]: https://github.com/bitcoin/bitcoin/blob/84d0fdc/src/chainparams.cpp#L145-L161 [4]: Marco Falke is due credit for this point [5]: utreexo: https://www.youtube.com/watch?v=edRun-6ubCc [6]: Boneh, Bunz, Fisch on accumulators: https://eprint.iacr.org/2018/1188 --0000000000001d72890585923051 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi,

I&#= 39;d like to discuss assumeutxo, which is an appealing and simple=C2=A0
optimization in the spirit of assumevalid[0].

# Motivation

To start a fully validating bitcoi= n client from scratch, that client currently
needs to perform an = initial block download. To the surprise of no one, IBD=C2=A0
take= s a linear amount time based on the length of the chain's history. For= =C2=A0
clients running on modest hardware under limited bandwidth= constraints,=C2=A0
say a mobile device, completing IBD takes a c= onsiderable amount of time=C2=A0
and thus poses serious usability= challenges.

As a result, having fully validating = clients run on such hardware is rare and
basically unrealistic. C= lients with even moderate resource constraints
are encouraged to = rely on the SPV trust model. Though we have promising
improvement= s to existing SPV modes pending deployment[1], it's worth
thi= nking about a mechanism that would allow such clients to use trust
models closer to full validation.

The subject of= this mail is a proposal for a complementary alternative to SPV
m= odes, and which is in the spirit of an existing default, `assumevalid`. It = may
help modest clients transact under a security model that clos= ely resembles
full validation within minutes instead of hours or = days.

# assumeutxo

The ba= sic idea is to allow nodes to initialize using a serialized version of the<= /div>
UTXO set rendered by another node at some predetermined height. T= he
initializing node syncs the headers chain from the network, th= en obtains and
loads one of these UTXO snapshots (i.e. a serializ= ed version of the UTXO set
bundled with the block header indicati= ng its "base" and some other metadata).

= Based upon the snapshot, the node is able to quickly reconstruct its chains= tate,
and compares a hash of the resulting UTXO set to a preordai= ned hash hard-coded
in the software a la assumevalid. This all ta= kes ~23 minutes, not accounting for
download of the 3.2GB snapsho= t[2].=C2=A0

The node then syncs to the network tip= and afterwards begins a simultaneous
background validation (i.e.= , a conventional IBD) up to the base height of the
snapshot in or= der to achieve full validation. Crucially, even while the
backgro= und validation is happening the node can validate incoming blocks and
=
transact with the benefit of the full (assumed-valid) UTXO set.
<= div>
Snapshots could be obtained from multiple separate peers= in the same manner as
block download, but I haven't put much= thought into this. In concept it doesn't
matter too much whe= re the snapshots come from since their validity is
determined via= content hash.

# Security

Obviously there are some security implications due consideration. While th= is
proposal is in the spirit of assumevalid, practical attacks ma= y become easier.
Under assumevalid, a user can be tricked into tr= ansacting under a false history
if an attacker convinces them to = start bitcoind with a malicious `-assumevalid`
parameter, sybils = their node, and then feeds them a bogus chain encompassing
all of= the hard-coded checkpoints[3].=C2=A0

The same att= ack is made easier in assumeutxo because, unlike in assumevalid,
= the attacker need not construct a valid PoW chain to get the victim's n= ode into
a false state; they simply need to get the user to accep= t a bad `-assumeutxo`
parameter and then supply them an easily ma= de UTXO snapshot containing, say, a
false coin assignment.
<= div>
For this reason, I recommend that if we were to implemen= t assumeutxo, we not
allow its specification via commandline argu= ment[4].

Beyond this risk, I can't think of ma= terial differences in security relative to
assumevalid, though I = appeal to the list for help with this.

# More full= y validating clients

A particularly exciting use-c= ase for assumeutxo is the possibility of mobile
devices functioni= ng as fully validating nodes with access to the complete UTXO
set= (as an alternative to SPV models). The total resource burden needed to sta= rt a node
from scratch based on a snapshot is, at time of writing= , a ~(3.2GB
+ blocks_to_tip * 4MB) download and a few minutes of = processing time, which sounds
manageable for many mobile devices = currently in use.
=C2=A0=C2=A0
A mobile user could init= ialize an assumed-valid bitcoin node within an hour,
transact imm= ediately, and complete a pruned full validation of their
assumed-= valid chain over the next few days, perhaps only doing the background
=
IBD when their device has access to suitable high-bandwidth connection= s.

If we end up implementing an accumulator-based = UTXO scaling design[5][6] down
the road, it's easy to imagine= an analogous process that would allow very fast
startup using an= accumulator of a few kilobytes in lieu of a multi-GB snapshot.
<= br>
---

I've created a related issue= at our Github repository here:

and have submitted a draft implementati= on of snapshot usage via RPC here:

I'd like to discuss here whether thi= s is a good fit for Bitcoin conceptually. Concrete
plans for depl= oyment steps should be discussed in the Github issue, and after all=C2=A0
that my implementation may be reviewed as a sketch of the specific= software
changes necessary.

Regards,
James


[0]: http= s://bitcoincore.org/en/2017/03/08/release-0.14.0/#assumed-valid-blocks<= /div>
[2]: as tested at height 569895, on a 12 core Intel Xeon S= ilver 4116 CPU @ 2.10GHz
[4]= : Marco Falke is due credit for this point
[6]: Boneh, Bunz, Fisch on accumulators= : https://eprint.iacr.org/201= 8/1188

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