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Date: Mon, 15 Jun 2015 05:11:49 +0100
From: Peter Todd <pete@petertodd.org>
To: Eric Lombrozo <elombrozo@gmail.com>
Message-ID: <20150615041149.GA13286@muck>
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] comments on BIP 100
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On Sun, Jun 14, 2015 at 05:53:05PM -0700, Eric Lombrozo wrote:
> I think the whole complexity talk is missing the bigger issue.
>=20
> Sure, per block validation scales linearly (or quasilinearly=E2=80=A6ther=
e=E2=80=99s an O(log n) term in there somewhere but it=E2=80=99s probably d=
ominated by linear factors at current levels=E2=80=A6asymptotic limits don=
=E2=80=99t always apply very well to finite systems). And there=E2=80=99s a=
n O(n^2) bandwidth issue.
>=20
> The real issue, though, is validation cost. The n in O(n) here does not r=
epresent block size - it represents the size of the entire block chain for =
every new validator that must be synchronized! It means we have no way to c=
onstruct short proofs (or at least arguments that are computationally *hard=
* to forge) without requiring the validator to maintain the complete system=
 state. And currently, there is no mechanism for directly compensating vali=
dators.

=2E..and can there be? The goal of validation after all is finding if a
mistake has been made, and current production cryptography doesn't have
any way to prove you have done that honestly. You need "moon math" like
recursive SNARKS to do that, and it's unknown when they'll be available
for production usage.

When we say "compensating validators", if we're being honest with
outselves what we really mean is the much more boring task of
compensating servers who are giving us blockchain data. That has nothing
to do with validation.

A useful task would be to make an SPV archival node implementation that
did no validation at all, while distributing the blockchain data linked
to the longest chain. Such an implementation can and should serve SPV
clients, as this is what their actual security model usually is given
the lack of authentication of the identity of the server they're
connecting too. Actually implementing this would be a simple matter of
patching Bitcoin Core to turn off block validation.

> A full validator that goes offline even for a short period of time takes =
a while to fully catch up to the rest of the network - and starting up a ne=
w validator from scratch will continue to be painful=E2=80=A6even for those=
 of us who=E2=80=99ve turned this into routine by now, let alone new nontec=
hnical users.

Concretely, 20MB blocks lead to 20GB/week of blocks. On my 1MB/second
down internet, turning on my node after a week away would take five
hours; starting up a new node after two years of 20MB blocks would take
23 days - likely longer in practice.

There's serious unsolved and undiscussed devops and development issues
with this. For instance, after changes to the validation code, it's
routine to resync/reindex Bitcoin Core to ensure starting up a new node
actually works. Even now we haven't really come to grips with what
consistent 1MB blocks looks like from this point of view after a few
years of usage, let another order of magnitude longer sync times.

> Satoshi=E2=80=99s SPV is not a real solution - it=E2=80=99s a mere sugges=
tion that wasn=E2=80=99t fully thought out at the time of the Bitcoin white=
 paper. Besides lacking a good validation security model, practical impleme=
ntations of it weaken privacy and complicate client implementations substan=
tially=E2=80=A6and the worst part, it still doesn=E2=80=99t scale all that =
well. The validator still has to query every single block (even if filtered=
) back to the first transaction (which cannot be determined without doing a=
 blockchain scan anyway).

Note how with 20MB blocks it would take up to 1TB of IO per year-synced
for a bloom-filter-using wallet to sync the blockchain. We already have
a bloom IO DoS attack issue - what are the consequences of making that
issue 20x worse? Nobody has analysed it yet.

--=20
'peter'[:-1]@petertodd.org
0000000000000000127ab1d576dc851f374424f1269c4700ccaba2c42d97e778

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