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

[Paul Sztorc]

Responses below.


On 5/23/2017 7:26 PM, ZmnSCPxj wrote:
> Good morning,
> 
> 
>>>
>>> How is OP_BRIBE superior to just using a <h*> OP_RETURN script? Cannot
>>> a sidechain scan the block for OP_RETURN attesting that the block hash
>>> is present in the block?
>>
>>The sidechain software can indeed, but the mainchain software cannot
>>(without making validation of both chains part of the mainchain, which
>>defeats the original purpose of sidechains).
>>
>>The purpose of OP_BRIBE is to allow "Sam" (on the sidechain) and "Mary"
>>(a mainchain miner) to work together. Sam would pay X BTC to Mary, if
>>Mary could provide Sam with some guarantee that Sam's sidechain block
>>[defined by h*] would make it into the largest chain.
> 
> Regarding "largest chain", do you mean mainchain or sidechain?

Sidechain. Sam is paying himself the revenues of the sidechain's block.
These are side:btc, that he gets, if this block is part of the largest
chain.
> 
> An OP_RETURN is still some guarantee that it will make it into the
> longest mainchain.  If OP_RETURN tx is in a shorter mainchain but not on
> the longer mainchain, then on the longer mainchain, the utxo's funding
> the OP_RETURN tx is still unspent and the OP_RETURN tx will still be
> mineable by any miner following the longer mainchain.  The X BTC would
> be the OP_RETURN transaction's fee, which Mary would still want to mine
> into the longest mainchain, as it is still money on the table if it is
> not mined on the longest mainchain.

As you say below, it is about the sidechain, not mainchain. (Anything
not in the longest mainchain is just discarded by everyone, as always.)
> 
> Or, does OP_BRIBE somehow assure that Sam's block goes onto the longer
> sidechain?  But then, do not side blocks refer to their previous side
> block to define the sidechain?

Yes, there is a new construction for this, which might be called "SPV
squared". Classical merged mining (ie Namecoin) does not require the
mainchain to do anything, except occasionally keep track of an ordered
list of hash commitments. BMM is different, it does require the
mainchain to keep track of a minimal amount of things. One is the total
number of sidechains, but the second is what you have hit on here.

In addition to a list of commitments (ideally, one commitment per
block), the mainchain also keeps track of the block number ( ..or, as
you'll see, perhaps "block number modulo x".. ) of recent sidechain
blocks (for the last x=65,536 mainchain blocks). It then subsets the
most recent y=4000 of these, and only allows new sidechain blocks to
appear if they have a blocknumber equal to a member of set Y', where Y'
is the set of all sidechain blocknumbers y blocks ago + 1.

Then, the sidechain nodes simply reject the block if the h* commitment
refers to a side:blockheader that has a different block number.

Perhaps these notes..

http://www.truthcoin.info/images/bmm-outline.txt

..would be helpful. Looking back, this is probably the hackiest part of
the entire system, by a wide margin, so it is good that you bring it up.

> 
> 
> Is there some predictable schedule for side->main withdrawals?  If a
> withdrawal is imminent, or if some actor can get "insider information"
> about whether a withdrawal is imminent, cannot some actor induce the
> above, with potentially shorter time to reach step 3?

Yes, these delays are parameters, which are defined per sidechain. So
once the sidechain has been 'added' to bitcoin, the schedule would be
fully predictable.

> 
> From my reading, Blockstream's sidechains proposal supports a reorg
> proof after a side->main withdrawal on the mainchain side, with a reorg
> proof burn window after the main:side->main withdrawal, preventing its
> utxo from being used.  If the reorg proof is published and shows that a
> sidechain reorg invalidates a particular side->main withdrawal, then the
> main:side->main withdrawal's utxo is burned.

In this, there is no reorg proof (the miners would simply prevent the
withdrawal from accumulating enough ACKs). A 51% miner coalition can
always filter any message they like from the blockchain, including the
reorg proofs.

> 
>>For extraordinary DAO-like situations, disinterested mainchain miners
>>merely need a single bit of information (per sidechain), which is
>>"distress=true", and indicates to them to temporarily stop ACKing
>>withdrawals from the sidechain. This alone is enough to give the reorg
>>an unlimited amount of time to work itself out.
> 
> Do you have some document containing these details?  I cannot find this
> in the blog posts I've read so far.

This specific detail is not documented. I feel it is comparable to, for
example, the March 2013 chain fork.

> 
>>>>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.
>>>
>>> I don't quite understand how Drivechain handles sidechain reorgs, while
>>> keeping Bitcoin miners blinded. It seems sidechains need to be known
>>> ("seen") by the miner, so I don't see what is being blinded by blinded
>>> merge mining.
>>
>>Mainchain miners do need to maintain some data about the sidechains, but
>>this is very minimal, and certainly does not include the transaction
>>data (or arbitrary messages) of the sidechain.
> 
> As above, do you have document containing what data mainchain needs to
> track?

Yes, I think the notes (above) may be helpful.

> 
>>>>>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.
>>>
>>> I see. However, block subsidies will drop far in the future, do you
>>> mean to say, that sidechains will be used only in the far future?
>>
>>In one sense, I mean "you have already endorsed this 'fees only will
>>work' premise, by endorsing Bitcoin".
> 
> I endorse this on the basis of Greg Maxwell's analysis that a block size
> limit is necessary to have a fee market.

Even were that the case, the limit does not need to be imposed by nodes.
Miners are likely to self-impose a limit, on nodes and each other, in
order to maximize their total revenues. In fact they will eventually be
required to do so (even though they do not now, beyond the naive
maximization of imposing minimum tx fee requirements).

In fact, in that case, the larger the range of possible blocksizes, the
greater the ability of miners to extract fees from users -- ie the less
limited the blocksize, the more hashpower security.

Simply, if revenue = R = f(demand, supply), and miners choose the supply
which argmax R , then R can only go down as supply is constrained.

And, the effect is even more beneficial to security than _that_ ...if
the sidechains are different from each other, such that blockspace in
one chain is _not_ a perfect substitute for blockspace in another. In
this case miners can choke up on some (or each) individual S_i, forcing
sum(R_i) to go even higher.


> 
> 
>>That mechanism is enforced by drivechain itself, not OP_BRIBE. (OP Bribe
>>is itself only ~half of BMM. I admit it is getting a little confusing.)
> 
> Can you provide the details of this mechanism?  For example, does h*
> actually include some information identifying the sidechain and OP_BRIBE
> is supposed to do some additional checking not shown in your current
> code, or ....?

Yes. Yes.

This is what we nicknamed the "ratchet", I cannot easily check the code
right now but will get back to you.

https://github.com/drivechain-project/bitcoin/commit/c4fe067e298f57252789c28161272db3d7483dca#diff-be5f6bb690c9898e44cbd7e78c465e43R83



> 
>>Drivechain requires a soft fork to add each new sidechain
> 
> Oh.
> 
> My understanding is that with Blockstream's zk-SNARKs, a new sidechain
> would not require a soft fork at all (or even miner voting on the
> validity of WT^: the validity of side:side->main transactions is assured
> by proof that the zk-SNARK checking that transaction was executed
> correctly, and the lack of a reorg proof during the burn window after
> the main:side->main).
> 
> Is your model then, that each sidechain maintainer has to maintain a
> patchset or some plugin system to Core?  And miners who want to support
> particular sidechains to modify their software, applying the patch for
> each sidechain they want to support?

Not necessarily, but I think "plugins" are a good metaphor.

> 
> It seems this is somewhat brittle and may cause sidechain coding
> problems to leak into mainchain.
> 
> I think, it is much less interesting to have to softfork in every
> sidechain, rather than to support a general mechanism (zk-SNARK) to
> allow sidechains to be launched without any modification to Core code.
> 

I completely disagree. Unrestrained smart contract execution will be the
death of most of the interesting applications of sidechains, and would
probably destabilize Bitcoin itself.

I would be as if your body "added" the ability to synthesize any
protein, including prions. Then you make one prion and your entire body
dies.

I thought this would come up, I have a presentation on this:
https://www.youtube.com/watch?v=xGu0o8HH10U&list=PLw8-6ARlyVciMH79ZyLOpImsMug3LgNc4&index=1

You don't need to watch the whole thing, maybe just parts 1 and 5.

> 
>>. It requires
>>this literally for a few good reasons...but the best is: there is an
>>implicit requirement that the miners not steal from the sidechain
>>anyway. In this way drivechain knows how to keep track of what it should
>>expect.
> 
> It seems to be, more of "completely sighted merged mining" than "blind
> merge mining".

You can call it microscope mining if you like..it attempts to address
the concerns that were raised earlier in the peer review process.

> 
> 
> 
>>> Perhaps the datacenter point is simply that your proposal suggests to
>>> reduce the size of the datacenter by removing surge suppressors and
>>> UPS's, to avoid some definition of "datacenter is a room with >$XXX of
>>> equipment".
>>
>>I hope that my replies above already help with these. If not, let me know.
> 
> I find this point now moot, as drivechains require a softfork for each
> sidechain, and the size of the datacenter is pointless if there is some
> need to softfork in every sidechain.

You might be confused...miners can soft fork in a sidechain that they
have *no* intention of merged mining, or BMM ing. I do not know why, as
they would be leaving money on the table, but it is possible. Perhaps
only 5% of miners want to bother with it. The point is that the other
90% hashpower has no inherent reason *not* to allow that the experiment
be run.

> 
> Regards,
> ZmnXCPxj