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From: Gregory Maxwell <greg@xiph.org>
Date: Mon, 15 May 2017 23:59:58 +0000
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To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Rolling UTXO set hashes
Precedence: list

On Mon, May 15, 2017 at 11:04 PM, ZmnSCPxj via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> transactions is in the header, which would let lite nodes download a UTXO
> set from any full node and verify it by verifying only block headers
> starting from genesis.

Ya, lite nodes with UTXO sets are one of the the oldest observed
advantages of a commitment to the UTXO data:

https://bitcointalk.org/index.php?topic=21995.0

But it requires a commitment. And for most of the arguments for those
you really want compact membership proofs.  The recent rise in
interest in full block lite clients (for privacy reasons), perhaps
complements the membership proofless usage.

Pieter describes some uses for doing something like this without a
commitment.  In my view, it's more interesting to first gain
experience with an operation without committing to it (which is a
consensus change and requires more care and consideration, which are
easier if people have implementation experience).

> rather than merkle tree root of transactions is in the header,

For audibility and engineering reasons it would need to be be in
addition to rather than rather than, because the proof of work needs
to commit to the witness data (in that kind of flip, the transactions
themselves become witnesses for UTXO deltas) or you get trivial DOS
attacks where people provide malleated blocks that have invalid
witnesses.