> It's a straight forward idea: there is a scriptpubkey bitmap per block
> which is committed. Users can request the map, and be SPV confident
> that they received a faithful one. If there are hits, they can request
> the block and be confident there was no censoring.

OK, I see now, thanks Gregory. You're right, the use of UTXO set in that
context was confusing me.

If I go back to when we first did Bloom filtering and imagine the same
proposal being made, I guess I would have been wondering about the
following issues. Perhaps they have solutions:

1. Miners have to upgrade to generate the per-block filters. Any block that
doesn't have such a filter has to be downloaded in full, still. So it would
have taken quite a while for the bandwidth savings to materialise.

2. If checking the filter isn't a consensus rule, any miner who builds a
wrong filter breaks the entire SPV userbase. With per-node filtering, a
malicious or wrong node breaks an individual sync, but if the wallet user
notices they don't seem to be properly synchronised they can just press
"Rescan chain" and most likely get fixed. In practice broken nodes have
never been reported, but it's worth considering.

3. Downloading full blocks is still a lot of data. If you have a wallet
that receives tips a couple of times per day, and you open your wallet once
per week, then with 1mb blocks you would be downloading ~14mb of data each
time. Pretty pokey even on a desktop. Much sadness if you're on mobile.

4. Header size is constant, but per-block filters wouldn't be. So even the
null sync would download more data as the network got busier. Of course
Bloom filtering has the same scaling problem, but only between hard disk ->
RAM -> CPU rather than across the network.

5. Is it really more private? Imagine we see a hit in block 100, so we
download the full block and find our transaction. But now we must also
learn when that transaction is spent, so we can keep the wallet-local UTXO
set up to date. So we scan forward and see another hit in block 105, so now
we download block 105. The remote node can now select all transactions in
block 105 that spend transactions in block 100 and narrow down which txs
are ours. If we are watching a wallet that does multi-sends then it feels
like this problem gets much worse.

I'd really like to find a solution that has O(1) scaling on sync. If we see
nodes just as sources of UTXO data then shovelling the client (tx, existing
merkle path) pairs keyed off script prefixes would (with one additional
index) be O(1) and provide the same security guarantees as Bloom filtering
today. It creates lots of other problems to solve, but at least it would
scale into the forseeable future.
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