[bitcoin-dev] assumeutxo and UTXO snapshots

Wed Apr 3 09:55:26 UTC 2019

This would lead to users trusting third parties (like developers) way too 
much.

Furthermore, removing the ability for users to easily set it removes the one 
reasonable use case: where the user has already verified the state at some 
point previously, and saved the hash (ie, as backup of the UTXO set).

Luke

On Tuesday 02 April 2019 20:43:11 James O'Beirne via bitcoin-dev 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-L1
>61 [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