Re: [bitcoin-dev] assumeutxo and UTXO snapshots

[Jim Posen <jim.posen@gmail.com>]

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Big Concept ACK. I think this would be one of the biggest usability
improvements for Bitcoin and I see no security issues with the assumevalid
approach. I also agree that it's important to start work on this even
before the ultimate, perfect accumulator has been designed/tested and the
commitment scheme can always be upgraded later on. assumeutxo syncing
actually seems pretty orthogonal to the accumulator research.

I have a few questions

- So any nodes that do an initial sync will stop at the assumeutxo height,
serialize a snapshot of the chain state and store it? How many nodes are
expected to do this? Any idea how long this takes? Should it be enabled by
default?
- Would pruned nodes still download all historic blocks to double-check the
snapshot or only full nodes that intend to serve block data?
- How long are old snapshots retained? Presumably during a new release
nodes should keep at least a version back. Without P2P signalling of which
snapshots are available, they maybe have to keep all old snapshots or even
download old ones.

and comments

- The snapshot should probably be chunked up to minimize the amount of
bandwidth/IO/memory a malicious node could waste before you realize. Also,
it would make parallel downloading easier.

On Tue, Apr 2, 2019 at 4:43 PM James O'Beirne via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> 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
>
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