Subject: Re: [bitcoin-dev] Drivechain -- Request for Discussion

From: Paul Sztorc <[email-masked]>
Date: Mon, 22 May 2017 18:19:14 +0200

On May 22, 2017 10:39 AM, "ZmnSCPxj" <[email-masked]> wrote:

Good morning Paul,

I read only http://www.truthcoin.info/blog/blind-merged-mining/

>From just this document, I can't see a good justification for believing
that a main->side locking transaction can be safely spent into a side->main
unlocking transaction.  Do you have a better explanation?

Yes, a better explanation is in the drivechain spec, at:
http://www.truthcoin.info/blog/drivechain/

What you read is only an introduction of BMM. You may also consult the
notes (at the bottom of the BMM post) or the code, although this is time
consuming of course.

If I attempt to spend a main->side locking transaction on the basis of a
"mistaken" side block #49, what prevents me from this sequence:

The literal answer to your question is that mainchain Bitcoin will notice
that, for the second withdrawal, the sum of the inputs is less than the sum
of the outputs and they the transaction is therefore invalid.

1.  Put a side:side->main transaction into a block together with TheDAO's
hacked money.

So far, the only good side->main transfer I know of is in Blockstream's
original sidechains paper, with the main:side->main transaction ... Is your
proposal at the technical level actually similar, or does it truly seem to
be riskier?

I feel that my proposal is more secure, as it can operate healthily and
quickly while using spv proofs which are much slower and much much easier
to audit.

seems to me that your OP_is_h_in_coinbase should scan a series of sidechain
block headers backed by mainchain (meaning at the minimum that sidechains
should have some common header format prefix), rather than just mainchain
depth as your article seems to imply.

How would security be improved as a result? In either case, 51% of hashrate
can cause a reorg. The sidechain software itself does scan block headers,
of course.

Blind merged mining seems strictly inferior ... a rich attacker can simply
reorg the sidechain outright without playing such games.

In the future, when there is no block subsidy, a rich attacker can also do
that in mainchain Bitcoin.

Or is your proposal strictly for centralized sidechains, where only one
entity creates side blocks?

Not at all.

How does your proposal handle multiple side block creators on the same
sidechain, with the possibility that chain splits occur?

The side block is only "mined" if it is committed to in a mainchain Bitcoin
blog, and each mainchain block can only contain one block per sidechain. In
this way, drivechain sidechains are different from classical Namecoin
merged mining (where one _could_ run the entire system, mining included,
without interfacing with Bitcoin at all).

Regarding your dig about people who dislike data centers, the main issue
with miners blindly accepting sidechain commitments is that it violates
"Don't trust, verify", not that allows datacenters to be slightly smaller
by not including side:nodes.

As I explain early on, in earlier rounds of peer review, the focus was on
harms the sidechain technology might do to mainchain Bitcoin, and the
"datacenter point" was specifically the chief objection raised. So I am
afraid you are entirely incorrect.

In point of fact, the transactions *are* validated...by sidechain full
nodes, same as Bitcoin proper.

Paul

Regards,
ZmnSCPxj

Sent with ProtonMail Secure Email.

-------- Original Message --------
Subject: [bitcoin-dev] Drivechain -- Request for Discussion
Local Time: May 22, 2017 6:17 AM
UTC Time: May 22, 2017 6:17 AM
From: [email-masked]
To: Bitcoin Dev <[email-masked]>

Dear list,

I've been working on "drivechain", a sidechain enabling technology, for
some time.

* The technical info site is here: www.drivechain.info
* The changes to Bitcoin are here:
https://github.com/drivechain-project/bitcoin/tree/mainchainBMM
* A Blank sidechain template is here:
https://github.com/drivechain-project/bitcoin/tree/sidechainBMM

As many of you know, I've been seeking feedback in person, at various
conferences and meetups over the past year, most prominently Scaling
Milan. And I intend to continue to seek feedback at Consensus2017 this
week, so if you are in NYC please just walk up and start talking to me!

But I also wanted to ask the list for feedback. Initially, I was
hesitant because I try not to consume reviewers' scarce time until the
author has put in a serious effort. However, I may have waiting too
long, as today it is actually quite close to a working release.

Scaling Implications
---------------------

This upgrade would have significant scaling implications. Since it is
the case that sidechains can be added by soft fork, and since each of
these chains will have its own blockspace, this theoretically removes
the blocksize limit from "the Bitcoin system" (if one includes
sidechains as part of such a system). People who want a LargeBlock
bitcoin can just move their BTC over to such a network [1], and their
txns will have no longer have an impact on "Bitcoin Core". Thus, even
though this upgrade does not actually increase "scalability" per se, it
may in fact put an end to the scalability debate...forever.

This work includes the relatively new concept of "Blind Merged Mining"
[2] which I developed in January to allow SHA256^2 miners to merge-mine
these "drivechains", even if these miners aren't running the actual
sidechain software. The goal is to prevent sidechains from affecting the
levelness of the mining "playing field". BMM is conceptually similar to
ZooKeeV [3] which Peter Todd sketched out in mid-2013. BMM is not
required for drivechain, but it would address some of the last remaining
concerns.

Total Transaction Fees in the Far Future
-----------------------------------------

Some people feel that a maximum blocksize limit is needed to ensure that
future total equilibrium transaction fees are non-negligible. I
presented [4] on why I don't agree, 8 months ago. The reviewers I spoke
to over the last year have stopped bringing this complaint up, but I am
not sure everyone feels that way.

Juxtaposition with a recent "Scaling Compromise"
-------------------------------------------------

Recently, a scalability proposal began to circulate on social media. As
far as I could tell, it goes something like "immediately activate
SegWit, and then HF to double the nonwitness blockspace to 2MB within 12
months". But such a proposal is quite meager, compared to a "LargeBlock
Drivechain". The drivechain is better on both fronts, as it would not
require a hardfork, and could *almost immediately* add _any_ amount of
extra blockspace (specifically, I might expect a BIP101-like LargeBlock
chain that has an 8 MB maxblocksize, which doubles every two years).

In other words, I don't know why anyone would support that proposal over
mine. The only reasons would be either ignorance (ie, unfamiliarity with
drivechain) or because there are still nagging unspoken complaints about
drivechain which I apparently need to hear and address.

Other Thoughts
---------------

Unfortunately, anyone who worked on the "first generation" of sidechain
technology (the skiplist) or the "second generation" (federated /
Liquid), will find that this is very different.

I will admit that I am very pessimistic about any conversation that
involves scalability. It is often said that "talking politics lowers
your IQ by 25 points". Bitcoin scalability conversations seem to drain
50 points. (Instead of conversing, I think people should quietly work on
whatever they are passionate about until their problem either is solved,
or it goes away for some other reason, or until we all agree to just
stop talking about it.)

Cheers,
Paul

[1] http://www.drivechain.info/faq/#can-sidechains-really-help-with-scaling
[2] http://www.truthcoin.info/blog/blind-merged-mining/
[3] https://s3.amazonaws.com/peter.todd/bitcoin-wizards-13-10-17.log
[4]
https://www.youtube.com/watch?v=YErLEuOi3xU&list=PLw8-
6ARlyVciNjgS_NFhAu-qt7HPf_dtg&index=4

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