Subject: Re: [bitcoin-dev] A Better MMR Definition

From: Bram Cohen <[email-masked]>
Date: Thu, 23 Feb 2017 16:49:01 -0800

On Thu, Feb 23, 2017 at 3:51 PM, Peter Todd <[email-masked]> wrote:

> On Thu, Feb 23, 2017 at 03:13:43PM -0800, Bram Cohen wrote:
> >
> > I can't speak to MMRs (they look a bit redundant with the actual
> blockchain
> > history to my eye) but circling back to utxo commitments, the benefits
> are
>
> In what way do you see MMRs as redundant?
>

You can readily prove something is in the TXO or STXO set using the actual
blockchain, and the proofs will be nice and compact because even light
nodes are expected to already have all the historical headers.

What you can't do with MMRs or the blockchain is make a compact proof that
something is still in the utxo set, which is the whole point of utxo
commitments.

It's totally reasonable for full nodes to independently update and
recalculate the utxo set as part of their validation process. The same
can't be done for a balanced version of the txo set because it's too big.
Relying on proofs as a crutch for using the full txo set would badly
exacerbate the already extant problem of miners doing spv mining, and
increase the bandwidth a full validating node had to use by a multiple.

This whole conversation is badly sidetracked. If people have comments on my
merkle set I'd like to engage further with them, but mmrs need to be argued
independently on their own merits before being used as a counterpoint to
utxo commitments.