with cli is it possible to check the set priority?

thought stagenet fees were weirdly high, but duh the block reward is 13.8

set

Hi guys, i have a question concerning a 51% attack. I know that the longest chain always wins. But what happens if a 51% attack succeeds and the double spend chain is the longest chain. Are all nodes now forced on the protocol level to accept the new chain and delete the old one or do they save the old original chain as well?

The protocol doesn't know what data you keep locally cached

Correct answer. The standard implementation saves alt chains I believe

Although, maybe the protocol sort of implies it. Otherwise when you get a new block which makes the alt chain the longest chain, how do you know

seems like things would break if no one saved them

ok, i havent looked into the code:) So if a 51% attack happens honest miners can still continue to work on the true chain.

seems logical :)

miners dont detect 51% attacks, but they do keep forks for a period of time until there is a clear winner

What's the true chain?

The chain with the currently-observed highest cumulative work is the true chain, unless you adopt a different definition

(I assume that's what you meant by "longest chain")

With the true chain i meant the chain of the honest miners

who are not double spending

but having lower hashrate because of the 51%

How does the network know which miners are honest?

The consensus protocol can't detect honesty

It'd be pretty great if it could

But they can see the double spend or am i getting that wrong there?

If a block contains a conflict, nodes should judge it invalid

competing forks are bound to have the same transactions, so it's not possible to detect double spend there; after all, double spend occurs in meat space when a person or program decides a received output is legitimate

which is why it's often good to wait until a transaction is buried in blocks before making that determination

can each node decide by itself if it wants to act honest or not?

If it runs non-standard software, sure

The whole protocol runs on the assumption that there are non-honest network participants

and on the hope that there aren't too many of them working together

You can write whatever code you want to interact with the Monero network

this is where I shill ZtM2, which has a chapter on blockchain consensus (ch8) pdf-archive.com/2020/01/30/zerotomo…0-22/zerotomoneromaster-v1-0-22.pdf

no it's ch7 in that draft

ok thanks for the link

What happens if the attacker has the longest chain but most of the users are having the standard "honest" daemon software that is detecting double spends. Is the standard software going to throw away the malicious chain although it s longer under a continues attack or are they choosing the longest chain?

The protocol dictates that the valid chain with the most cumulative work is "the chain"

So it depends what view of the network you have, in part

Again, if you receive an invalid chain (e.g. an linking tag appears multiple times), that chain is... invalid

Nodes (using non-standard implementations) can decide to use whatever chain they wish... but then you're not using the same consensus protocol as others, presumably

Keep in mind that "an attempted double-spend on a single chain" is _not_ the same thing as "an output that is spent on separate forks"

those are not a single chain

If a rogue node somehow knows in its heart that a "shorter" chain is "more honest" for some reason, then it's not following the protocol, and presumably the bulk of other nodes will not agree with it

but in the btc world you could check if the output has been spent before right? Let 's say 100 blocks before the split. If it has already been spent you could say that this cant happen?

So he could know in his heart or not?

because it s not having the double spends

In Monero, you can use linking tags to detect an attempted double-spend

but you cannot link this to a particular output without additional information

Remember that Monero transactions specify _sets of possible spent outputs_ along with a tag

If you see (in a valid spend signature) a tag you have seen before (in a valid spend signature), then there is an attempt to double-spend

If you have not seen the tag before, there is not such an attempt

It's a question of transaction validity (which includes the tag), not "output validity"

ok but then you could reject the block?

if you can detect the double-spend.

Yes

The difference from BTC is that you cannot link this detection to a specific output, only to one of the outputs in the set specified by the transaction

But it doesn't matter... either there's a double-spend attempt or there isn't

(assuming you do not possess external information, of course)

51% attack refers to rewriting old blocks by replacing them with an apparently legitimate fork with a different record of events

Yes... I assumed kiwi_30 meant "detect the double-spend on a single chain"

If it's on multiple forks, you choose the one with highest total work

That's the rule

If your node doesn't like the rule, it can make its own rule, and hope/pray that other nodes also follow its rule

but this is unlikely

but if the forked chain is having the double spend which you can detect via the tag. Why are you choosing the longer fork chain with the highest cumulative work although you know it has been double spent?

Sry for my questions but i want to understand that

;)

Maybe #monero is a better place for this.

I'll try once more

If you have two forks

and one contains a conflict, and the other does not

the conflicting chain is invalid

The "longest" _valid_ chain wins

that's it

The moment a chain contains a conflict, it is not longer valid, and you seek the next "longest" valid chain

oh sry, then i already got it right. Understood sth wrong. Thanks for your great explanation sarang! Can i donate somewhere?

Happy to help kiwi_30

If you're interested in donating to support Monero-related projects, there are two open funding requests here: ccs.getmonero.org/funding-required

(and a general project donation link at the bottom)

Otherwise, pick a local charity that helps people, and go nuts :)

(Note: I am not the recipient of any of the requests at that link, FWIW)

ok then im going to donate for your next funding ;)

Why wait?

Pick your favourite local charity!

they are not going to accept monero ;)

Not with that attitude