Re: [bitcoin-dev] Purge attacks (spin on sabotage attacks)

[ZmnSCPxj <ZmnSCPxj@protonmail.com>]

Good morning M,


> I don't see how the scenario you outline here has anything to do with the mechanism I proposed. An empty block doesn't contain any transactions (by definition) so it wont contest any transactions in any given node's mempool. The aim isn't to prevent empty nodes, it's to discourage miners from including transactions in their block that conflict with the eventually-consistent state of consensus in the mempool.
>  

What?

From the original post:

> TLDR
> * An attacker replaces the most recent blocks full of transactions with empty blocks.

Are you sure you are solving the same problem?

The mempool **has no consensus**.
It is strictly an optimization, preventing a node from needlessly broadcasting transactions.

Making consensus dependent on the state of the mempool requires that you record the state of the mempool at the point at which the block snapshot was taken.
Otherwise, newly-started nodes can be fooled into taking the "wrong" consensus branch leading to persistent chainsplits.

>
> > Always avoid violating that principle in any consensus code.
> > If it is not committed to in the block and is not provable using only data you provide with the block, you cannot use it safely without risking chainsplit.
> >
> > (and no, banning or even disincentivizing SPV mining will not work, different nodes have different views of the mempool and temporary chainsplits can occur by chance where one chainsplit has transactions that are not confirmed in the other chainsplit, which again is just another short-term inadvertent Purge attack on the network.)
> >
> > >
> > > > Purge attacks can still be defended against and does not require mass cooperation.
> > > > If there is a transaction that is economically beneficial to me, it does so by paying some Bitcoins to me.
> > > > If it pays Bitcoins to me, I can spend those Bitcoins in a transaction that just offers to pay mining fees and transfers it back to me (i.e. child pays for parent) to convince miners to mine the purged transaction.
> > > > As the Purge attack is "just" a censorship attack (i.e. a censorship of all transactions in the block under attack), the increased mining fees for the transactions being censored (i.e. offered via child-pays-for-parent in this case) is an economic counterattack on the censoring miner (i.e. it forgoes the mining fees).
> > >
> > > > With enough self-interested users, the fee offered to confirm the transactions can be substantial enough that non-censoring miners can be convinced to mine those transactions.
> > > > No coordination necessary, as is typical for all defenses against censorship (and the basis of the censorship-resistance of Bitcoin).
> > >
> > > The attack itself is better classified as a form of sabotage than censorship. The goal is to demonstrate the ongoing mutability of transactions beyond any inherent heuristic for “finality”. iow it is a demonstration that will damage the network’s future ability to offer settlement assurances.
> > >
> > > Trying to use Child Pays For Parent to defend in a bidding war against an opportunist attacker retrieving spent Bitcoin via RBF is a losing game for the defender. There’s no opportunity cost for the attacker, any amount retrieved is profit. The defender, on the other hand, is always losing value. This is exactly the kind of conflict and discoordination the attack is intended to induce.
> >
> > Your defender, in this attack, should avoid the Sunk Cost Fallacy here.
> > If the defender has been so foolish as to provide a product or service based on only a *few* confirmations, like 1 or 2, then that product or service has been Sunk, and it should ignore the Sunk Cost here.
> >
> > From that point of view, the attacker and the defender are simply bidding up from the *same* value, i.e. the value of the UTXO that is being removed by the purge attack.
> > As the same value is under contest on both sides, they are equally matched and both censoring and non-censoring miners will get the same incentive, splitting up the network into two nearly equal halves, and then chance (lucky block discovery) decides between which is the winner or the loser.
> >
> > The difference here is that the chainsplit in this case is in a metastable state, and once a string of lucky block discoveries occurs, it falls into a stable state and now everybody agrees again on who won and who lost.
> > Your solution risks *persistent* *stable* chainsplits.
> > Worse, this occurrence without your solution would only happen if some miners actually attack the blockchain.
> > With your solution, persistent chainsplits can occur without malice, simply chance.
>
> How would this mechanism produce a chainsplit by chance?

I already described it in the previous post.

Purge attacks happen all the time, when two miners mine blocks at nearly the same time, but with different sets of transactions in their blocks.
And as I pointed out, any mechanism which uses non-block data (such as mempool data) *will* lead to persistent chainsplits.

>  
>
> > And as in many things in life, the only winning move is not to play.
> > Just wait for more than a small number of confirmations (e.g. 6 is generally considered safe), and the chance that a Purge attack on your transactions succeeds is low enough that worse force majeur (a rogue asteroid hitting your datacenter, for example) is more likely.
>
> I got to thinking about "purge attacks" and mitigations because I was red teaming how G20 states that have seized the major mining operations could most effectively destroy value and confidence in Bitcoin. This scenario is _a lot_ more likely than rogue asteroids.
>
> What happens if the G20 decide to reorg deeper 6 - say 10, or even 20?
>
> If the Bitcoin continues to offer replace by fee I think this will be their first attack with seized majority hashrate;
>
> - mine offline
> - reach > 10 deep empty block reorg as heaviest chain 
> - announce it
> - semi-honest mine with a preference for RBF'ed "root" txns, ignoring any profitable child pays for parent.
> - repeat above, until some goal reached (eg. $ value of Bitcoin reaching x)
> - switch to "DoS mode" where you empty block reorg the chain tip
>
> If we got rid of RBF, their only option would be DoS mode. Once it stops, honest mining could resume and the blocks will fill back up again with transactions out of the mempool preserved in the right order.#

You ***cannot*** get rid of RBF.
The incentives of miners mean they will actually want to implement RBF and ignore any "convention" of RBF-flagging.
My understanding is that there are claims that a minority of miners already do this (possibly Peter Todd has more information, but I am uncertain), and will accept "full" RBF i.e. ignore the RBF flag and always apply RBF to all transactions regardless.
Nothing in consensus prevents this, and this is why we always wait for confirmation.


Regardless of however many blocks are attacked, always remember that in the end, this is still a *censorship* attack: it is attempting to censor Bitcoin completely.
As such, this page applies: https://github.com/libbitcoin/libbitcoin-system/wiki/Censorship-Resistance-Property


Regards,
ZmnSCPxj