You may be able to optimize the size of the hints further with the observation that false positives are harmless as long as they're rare, as in you can save some extra txouts and if you later find them being spent, then just go ahead and remove them. So for example ribbon filters give you a memory space and read efficient hash table that is constructed by solving a linear system to make sure all the inputs match. One could solve for successively larger filters to target a specific false positive rate. (I mean just a 2,4 cuckoo filter or similar would also work, but that's two memory accesses required and you don't actually need runtime modification).

On Wednesday, April 9, 2025 at 10:11:29 AM UTC Ruben Somsen wrote:

Hi everyone,

SwiftSync is a new validation method that allows for near-stateless, fully parallelizable validation of the Bitcoin blockchain via hints about which outputs remain unspent (<100MB total). All other inputs/outputs are efficiently crossed off inside a single hash aggregate that only reaches zero if validation was successful and the hints were correct.

The main observation is that it can be much easier to validate that a given UTXO set is correct than to compute it yourself. It allows us to no longer require a stateful moment-to-moment UTXO set during IBD and allows everything to be order independent. I'll briefly summarize the protocol, before sharing the link to the full write-up.

Each output gets a boolean hint (e.g. committed into Bitcoin Core) about whether or not it will still be in the UTXO set after validation completes. If it does, we write it to disk (append-only - it won't be used until SwiftSync finishes). If it does not, we hash the UTXO data and add it to an aggregate. For each input, we once again hash the UTXO data and remove it from the aggregate.

At the end, for every added output there should have been exactly one removed input, bringing the end total of the aggregate to zero. If this is indeed the case, we will have validated that the hints, and the resulting UTXO set, were correct.

E.g. For spent outputs A, B and inputs C, D we calculate hash(UTXO_A||salt) + hash(UTXO_B||salt) - hash(UTXO_C||salt) - hash(UTXO_D||salt) == 0 (proving (A==C && B==D) || (A==D && B==C)).

There is one missing step. The UTXO data is only available when processing the output, but not when processing the input. We resolve this by either downloading the outputs that were spent for each block (equivalent to the undo data, maybe 10-15% of a block), or we lean on assumevalid, making it sufficient to only hash the outpoints (which are available in both the output and input) rather than the full UTXO data.

Ignoring bandwidth, the expectation is that the speedup will be most significant on either RAM limited devices and/or devices with many CPU cores. Initial PoC benchmarks (thanks to theStack) show a 5.28x speed-up, while currently still being largely sequential.

Many more details are in the full write-up:
https://gist.github.com/RubenSomsen/a61a37d14182ccd78760e477c78133cd

It will answer the following questions (and more):

- How the hash aggregate can be made secure against the generalized birthday problem
- How exactly assumevalid is utilized and what the implications are
- How we can still check for inflation when we skip the amounts with assumevalid
- How we can validate transaction order while doing everything in parallel
- How we can perform the BIP30 duplicate output check without the UTXO set
- How this all relates to assumeutxo

To my knowledge, every validation check involving the UTXO set is covered, but I'd be curious to hear if anything was overlooked or if you spot any other issues.

Thanks for reading, and thanks to everyone who provided invaluable feedback while the idea was coming together.

-- Ruben Somsen