Since the sidechain has the sidechain BMM headers that they want the LD
(bribe) data for, I think that they could specifically request LD data
relevant only to that sidechain by providing a list of hashes to the
mainchain, and the mainchain can return a list of boolean values telling
the sidechain if the LD data exists. That way the data never even has to
go over the network, just a verification that it exists on the mainchain
and

Since you seem to be talking about the initial block download process for the drivechain. It seems that we might as well request the set of *valid* blocks from a bitcoin peer first, since at the end of the day they are in control of the mining process on the sidechain. Here is what I envision:

1. Request all hashes for sidechain from a bitcoin peer
2. Request all sidechain block header's for the hashes the bitcoin peer gave us
3. Order the set of sidechain block headers by looking at hashPrevBlock.
   
4. Request full sidechain blocks and start validating against the consensus rule set of the sidechain

we can drop the sidechain_id from the script.

I agree the 'sidechain_id' is unneeded in the coinbase tx output. We should just fix these based on index. This should be reflected in my latest commit if we are talking about the same thing: https://github.com/Christewart/bitcoin/blob/c355e39fbe2ea48856ea86b25cb8a97710feb032/src/script/script.cpp#L254

and have the sidechain handle filtering out invalid LD data /
only requesting the verification of LD data that they know to be valid
as far as chain order goes.

 I agree, the whole point of BMM is to have bitcoin miners indifferent to what happens in the sidechain (although Paul argues in a wonky way they do care sort of). If there is an invalid (in the sense the block it represents does *not* follow the sidechain's consensus rule set) OP_BRIBEVERIFY that pays *more* money than a valid OP_BRIBEVERIFY output, we need to assume that a 'blind' bitcoin miner will choose the one that pays them the most money.

>I might be wrong but I thought that OP_RETURN outputs do not go into the
UTXO set. Anyone else want to chime in?

I'm fairly certain you are right. It just feels like we should be able to exploit the fact that *only* miners can claim these OP_BRIBEVERIFY outputs. I'll have to think about this more, maybe some one else can come up with something clever that exploits that fact.

On Sun, Jun 18, 2017 at 4:27 PM, CryptAxe via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

> >OP_RETURN <sidechain_id> <critical hash>
>
> I think it is redundant here to have the <sidechain id>, we can
> implicitly assume what the sidechain_id is since we have a fixed set
> of drivechains. I.e. mining reward is index 0, mimblewimble sidechain
> is index 1, etc. CryptAxe has specific indexes defined already in his
> implementation:
> https://github.com/drivechain-project/bitcoin/blob/mainchainBMM/src/sidechain.h#L26-L30.
>

Since the sidechain has the sidechain BMM headers that they want the LD
(bribe) data for, I think that they could specifically request LD data
relevant only to that sidechain by providing a list of hashes to the
mainchain, and the mainchain can return a list of boolean values telling
the sidechain if the LD data exists. That way the data never even has to
go over the network, just a verification that it exists on the mainchain
and we can drop the sidechain_id from the script.


> I think it would be wise to include a version byte to allow us to
> upgrade this commitment structure in the future. Similar to how
> witness program's are now versioned.

Agreed, we need that.


>
> ><block height> <sidechain_id> <critical hash> OP_BRIBE_VERIFY
>
> If <block height> is an argument that OP_BRIBE_VERIFY takes, doesn't
> that mean the mainchain miner has to validate this *is* the actual
> block height on the sidechain? Does that take the 'blindness' away
> from BMM?

No, OP_BRIBE (the old version) didn't care about the block height. Where
the blockheight was relevant is when bribe data is added to LD. In order
to be added to LD, the bribe needed to either be a fork (block height
less than the sidechain tip height) or extending the current side-chain
(block height = sidechain tip height + 1). The goal of this was to allow
for reorgs, and also make it so that people cannot skip forward (which
would never be valid so it's a waste of time and space) so that the
sidechain makes progress. With the new design that we have been talking
about, I think that we will need to drop this requirement from the
mainchain, and have the sidechain handle filtering out invalid LD data /
only requesting the verification of LD data that they know to be valid
as far as chain order goes.


>
> Overall, I think we need to work on the commitment structure to the
> coinbase tx.

Agreed. It might be helpful if you outlined the idea you had on IRC to
the mailing list.


> If I understand the current implementation correctly we can have up to
> 256 OP_RETURNs embedded in the coinbase tx signifying new blocks mined
> on drivechains.. this seems less than ideal. It might be prudent to
> make these outputs ANYONECANSPEND, and then have miners spending these
> outputs to themselves for every block mined.. maybe this doesn't have
> a benefit over using OP_RETURNs though?
>
> The structure could be something like:
> <version> <critical hash>
>
> and then in a subsequent block the miner spends that output to
> themselves. I will admit I'm not super familiar with how OP_RETURNs
> work with the UTXO set -- maybe this scheme doesn't have any benefit.
>
> -Chris

I might be wrong but I thought that OP_RETURN outputs do not go into the
UTXO set. Anyone else want to chime in?

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