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