Re: [bitcoin-dev] assumeutxo and UTXO snapshots

[Jonas Schnelli <dev@jonasschnelli.ch>]

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Thanks James for the post.

I proposed a similar idea [1] back in 2016 with the difference of signing the UTXO-set hash in a gitian-ish way.

While the idea of UTXO-set-syncs are attractive, there are probably still significant downsides in usability (compared to models with less security), mainly:
* Assume the UTXO set is 6 weeks old (which seems a reasonable age for providing enough security) a peer using that snapshot would still require to download and verify ~6048 blocks (~7.9GB at 1.3MB blocks,… probably CPU-days on a phone)
* Do we semi-trust the peer that servers the UTXO set (compared to a block or tx which we can validate)? What channel to we use to serve the snapshot?

If the goal is to run a full node on a consumer device that is also been used for other CPU intense operations (like a phone, etc.), I’m not sure if this proposal will lead to a satisfactory user experience.

The longer I think around this problem, the more I lean towards accepting the fact that one need to use dedicated hardware in his own environment to perform a painless full validation.

/jonas

[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-February/012478.html

> Am 02.04.2019 um 22:43 schrieb James O'Beirne via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>:
> 
> 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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