Subject: Re: [bitcoindev] Re: SwiftSync - smarter synchronization with hints

From: Ruben Somsen <[email-masked]>
Date: Fri, 2 May 2025 12:59:22 +0200

Hi Greg,

I appreciate your thoughts.

>excellent idea that preserves the assume valid-like security properties

Thanks. Yeah, the assumevalid version is particularly efficient. Without
assumevalid there's a bandwidth tradeoff, but that also clearly seems worth
it.

>Also I like the lack of basically anything being normative, it's easier to
feel comfortable with something when you won't be stuck with it forever

Yes, that is a very pleasant property. There is essentially no difference
between the end state that you reach with or without SwiftSync.

>the hints themselves are not security relevant, if they're wrong you'll
just fail

I agree.

>sha256 you should place the salt first so that it can be absorbed then the
midstate reused

Nice. That is a very good point that I had not yet considered.

>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

I don't see a practical way to do this without compromising the benefits of
SwiftSync because of the "later find them being spent" part. For one, it
would require doing a lookup for each input to see if it's in your UTXO
set, something we currently don't need to do at all. Secondly, it would
require blocks to be processed in order, limiting parallelization. The
space savings would also be negligible, considering the hint data is
already <100MB (compressed).

I think most of the remaining optimization potential (other than the
engineering work to enable parallel validation) is in the hash
aggregate, like the midstate reuse. Is there a faster alternative to
sha256? Can we get away with 16 byte hashes? I could be mistaken, but in
theory it seems we could even get away with 4 byte hashes if we allowed for
a 1 in 4 billion chance of accidentally accepting an invalid chain. Leaving
consensus to chance feels philosophically improper, but it's a pretty safe
bet considering it also involves PoW to trigger and even then would only
affect one or two random unlucky people on Earth.

Thanks for chiming in.

Cheers,
Ruben

On Fri, May 2, 2025 at 8:48 AM Greg Maxwell <[email-masked]> wrote:

> This sounds like an excellent idea that preserves the assume valid-like
> security properties but with more performance and more optimization
> oppturnities.
>
> I like particularly -- if I understand it correctly-- that the hints
> themselves are not security relevant, if they're wrong you'll just fail
> rather than end up with something incorrect.  Also I like the lack of
> basically anything being normative, it's easier to feel comfortable with
> something when you won't be stuck with it forever... the whole scheme could
> just be reworked every version with no particular harm because its effects
> are all ephemeral.  At worst you might have problems if you started IBD
> with one version and tried to complete it with another.
>
> I haven't seen any code,  but if hash() is a MD style hash function such
> as sha256 you should place the salt first so that it can be absorbed then
> the midstate reused. For sha256 you could get potentially double the
> hashing performance and I assume/hope that hash is actually fairly high in
> the profile cost.. maybe more like a 1/3 improvement considering the size
> of the entry, care should be taken to try to minimize compression function
> runs. ... but at least the utxo representation there is entirely
> implementation defined.
>
> 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
>>
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