From: Rusty Russell <rusty@rustcorp.com.au>
To: Gregory Maxwell <greg@xiph.org>, Pieter Wuille <pieter.wuille@gmail.com>
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	<20170516110104.GA5564@fedora-23-dvm>
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Date: Tue, 23 May 2017 14:17:48 +0930
Message-ID: <8760gs2n7v.fsf@rustcorp.com.au>
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Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Rolling UTXO set hashes
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Gregory Maxwell via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> writes:
> On Tue, May 16, 2017 at 6:17 PM, Pieter Wuille <pieter.wuille@gmail.com> wrote:
>> just the first - and one that has very low costs and no normative
>> datastructures at all.
>
> The serialization of the txout itself is normative, but very minimal.

I do prefer the (2) approach, BTW, as it reuses existing primitives, but
I know "simpler" means a different thing to mathier brains :)

Since it wasn't explicit in the proposal, I think the txout information
placed in the hash here is worth discussing.

I prefer a simple txid||outnumber[1], because it allows simple validation
without knowing the UTXO set itself; even a lightweight node can assert
that UTXOhash for block N+1 is valid if the UTXOhash for block N is
valid (and vice versa!) given block N+1.  And miners can't really use
that even if they were to try not validating against UTXO (!) because
they need to know input amounts for fees (which are becoming
significant).

If I want to hand you the complete validatable UTXO set, I need to hand
you all the txs with any unspent output, and some bitfield to indicate
which ones are unspent.

OTOH, if you serialize more (eg. ...||amount||scriptPubKey ?), then the UTXO
set size needed to validate the utxohash is a little smaller: you need
to send the txid, but not the tx nVersion, nLocktime or inputs.  But in a
SegWit world, that's actually *bigger* AFAICT.

Thanks,
Rusty.

[1] I think you could actually use txid^outnumber, and if that's not a
    curve point SHA256() again, etc.  But I don't think that saves any
    real time, and may cause other issues.