Could Innopolis Tech launch a 51% attack on the testnet?

So, Innopolis Tech has got a whopping 51.76% of the Bitcoin testnet's hash rate, sitting at 118.88 TH/s. Does this mean they could pull off a 51% attack? What are the implications for the testnet's security?

Considering testnet coins are supposed to have ZERO value, a 51% attack is not only meaningless but also pointless. The tBTC coins are only needed to test code, once the testing is done no one should care about them and in the past the coins were usually returned to their source for others to use. The only folks who would get hurt are the assholes that 'mine' it for-profit by getting tBTC & then selling/exchanging it for other coins and the idjits that buy them. Seeing their 'holdings' disrupted certainly would not break my heart

It was possible in the past, and it is possible today. Currently, there is no chain I know of, which can trace block headers from other chains, and use that information, to properly calculate the global difficulty. Which means, that if testnet has a small fraction of mainnet hashrate (for example 1%, or even less than that), then any mining pool, at any time, can launch 51% attack, given enough incentive. It was done in the past on many altcoins, and it can be done on testnet, too, if test coins will be too valuable. Testnet was never secure. As long as the difficulty is forced to drop to the minimal value, when nothing is mined for 20 minutes, it is a flaw, which is constantly abused. And recently it turned out, that even signet is somewhat insecure, because it has even lower difficulty, and signet nodes can be flooded with valid blocks, when miners will tweak nonces from the original blocks, produced by signet creators (see: fork.observer at block height 243473). In general, the problem of making a good test network is still unsolved.

100% agreeing with NotFuzzyWarm on this one... Testnet isn't created to be secure, or a store of value... It is created to test. And if you use it for the purpose it was created (testing), it does a good job. Use testnet to test out new wallets, new scripts, spend, receive, create paper wallets, create an airgapped setup, test out the payment gateway on your new service, test out my new bitcoin core v29.0 docker image . Afterwards, either keep the tBTC or give it away, as soon as your tests are done, there's no real use for the tBTC anymore. Hey, if you ever want to test how a 51% attack would work, testnet would probably be the chain you'd use to do so Do realise that having zero value does not mean it's free to mine the tBTC... I'm always a bit grumpy when i see company's throwing relatively modern ASIC's towards tBTC mining. I always wonder why... In my mind, tBTC should be something that it basically thrown your way whenever you need it (be it faucets, or just cpu mining). Throwing "good" ASIC's towards tBTC mining makes it rare. I know i used to ask for collateral whenever i loaned out tBTC v3 in the past, because it was allmost impossible to mine any in meaningfull quantity's, and nobody was giving away meaningfull quantity's on faucets either so i asked for collateral so i'd get most of the tBTC back for the next person that needed some.

Aside from what other member say, it means Signet is better choice for some people. Not if the 51% attacker mine empty block, which prevent everyone TX from getting confirmed.

It works only when signet creators produce blocks, where changing the nonce won't lead to flooding the network. Which means for example using at least "1d00ffff" as difficulty (because if it is significantly lower, then nodes can hear the same block headers, with the same merkle root, and they don't know, which chain is the real one, when block hashes are slightly different. In case of signet, if network creators don't sign anything, then absolutely nobody can extend the chain, no matter how big hashrate will be used. While in testnet, it is at least possible in theory, to mine the next block. Which also means, that once you start a signet, you have to always maintain it, because there is no way to change signet challenge on-the-fly, and preserve the existing chain, without re-mining, and re-signing the whole chain (which is costly). It is still not implemented, as far as I know. However, if nodes can be tricked to switch from one chain tip to another at the same height, then it could be done today (but would cause other issues as well). It would be much less profitable if testnet wouldn't start from 50 tBTC per block. Which is why for example testnet3 miners are forced to include transactions, if they want to deal with bigger amounts. There are only 2^32 possible outcomes, so it may be possible to grind the challenge, and make a collision with existing signets.

The question is of course: which incentive could work here? You need a target to attack if you launch an 51%. And this target has to be valuable more than the resources spent on the 51% attack. Just for fun, I googled "sell testnet bitcoin" and there were indeed some results. But the first Google listing, sidesift.ai, has the trading pair "paused". Then there is a site "Buy Testnet", which seems to be an one-way exchange of some miner, selling each tBTC at 0.0000045 BTC. However, the attacker needs to have access to the sell side, i.e. he has to be the one selling or depositing the tBTC, otherwise the attack would not work. On "Buy Testnet" this isn't the case, it seems. According to CoinPaprika there is an exchange called "AltQuick.com" which allows to buy and sell tBTC. Indeed this one would allow the attack, it even accepts coins of two different testnet versions. Price for testnet3 coins is at 0.00000272, but orderbooks are very thin. So let's say you 51% attack AltQuick.com after a deposit of 10.000 tBTC and expect to get 0.0027 Bitcoins, you only would get about half of them. That would be about $110-120. And a much higher deposit doesn't make sense because there are only 0.004 BTC offered for this pair. So in the best case you would get $300 with the attack. Would really $300 be worth the hassle? Perhaps it would be worth it as a call to AltQuick.com to delist that pair.

There is no valid reason and benefit to attack Bitcoin testnet because the Bitcoin testnet can be stopped and move to a new Testnet version. Historically Bitcoin developers did it many times when there were abusement on tBTC like selling it for money while the real usecase of tBTC is for development and testing. Bitcoin testnet.Migration from testnet3 to testnet 4.Testnet Bitcoin. Activate, experience it, but don't trade and get scammed.

Testnet's hashrate is closer to 0.0001% of mainnet, right? It's really low. If I understand correctly, this would be possible with much less than 51%: I think this should work: wait for a block storm. Wait for hundreds to thousands of low difficulty blocks to be mined. Meanwhile, turn on your ASIC, ignore all those CPU blocks, and mine a single block on top of the previous ASIC block. If I understand correctly, this one block will orphan thousands of other blocks.

It means nothing at all... If they decide to perform the 51% attack then it will be just for testing reasons because they can't do an economic harm since we are talking about a testnet network. To understand this we must understand the 51% attack and what's the real danger behind it. Let's say a user send 100 bitcoins to an exchange, sell them and then send the profit to his wallet in a coin like USDT, then he can use the 51% attack to get the 100 btc back just by re-writing the transaction to the explorer and sending those 100 btc to a new address. This is why this can affect the main net but not the test net.

^^ Exactly. By design testnet coins are worthless so who the fu** cares? The only folks affected are the idjits that think or insist they should have value. If some folks strive to burst their bubble - works for me!

Folks, you DO realize that anyone with a SINGLE current high-end miner can easily do a Testnet 51% attack right? The Bitmain S21 XP hydro does 473TH/s.... Just checked and that greatly exceeds the current total pointed there. https://mempool.space/testnet/graphs/mining/pools#24h At worst, 2 of them should do it in a heartbeat.

According to ASICMinerValue.com, this thing mines €18.53 worth of Bitcoin per day. So the cost (in missed revenue) of this 51% attack to anyone who owns this ASIC would only be $21 per day. I'd say the fact that nobody does it confirms there's no point in doing this

Yep. It also reinforces the point that the only reasons there are folks hitting the testnet so hard is because:  A. They can  B. They enjoy screwing with testnet and breaking what it is intended to be used for  C. Sad to say, there is a market for selling tBTC to idjits Heavens forbid that they make a Faucet to distribute at least some of their coins for free to developers that actually need it and can't mine it themselves because of these attacks...

OP's post refers to testnet3 (not even testnet4), which is even worse since it has a lot less value anyway. It seems like some pools are doing some testing on testnet3, and given that testnet4 is being hijacked by that same asshole who is abusing the rules of testnet4, they just didn't bother to compete with him and overthrow them. Which makes you wonder -- do these guys even know how to code shit? If yes, what stops them from taking testnet4? They don't want to fight evil with evil?

Good question. The current 'leader' is Terra Pool which has 1.82EH behind them per the website (if you dig deep enough). Their peak on testnet-3 was >300TH/s in the last 24 hrs so thank God more isn't coming from them. Odd thing is - all many Google searches come up with them being an altcoin validator pool - LUNA it be exact. So why the F is one or more of their users pointing hash rate at the testnet? I do see that most of their blocks contain tx's so as you said, maybe someone is using pooled resources to actually (and very quickly) test code? All that said, it would be amusing to see someone hammer back against the idjit bombarding tnet-4 for a while - and distribute the coins for free to devs through a Faucet....

Consensus rules in testnet4 are more restrictive, than they are in testnet3. In the older testnet, you can mine 2016 blocks, and then bring down the difficulty into one, and mine a lot of blocks, just like Jameson Lopp did. However, in testnet4, after 2016 blocks, you have to mine at least one block with ASIC difficulty, and there are no shortcuts (so, if you have only a CPU, and if you mine offline with yourself, then you can do that forever in testnet3, but in testnet4, you will be stuck after 2016 blocks or sooner). I guess many people are unaware, when it comes to coding, because otherwise, they would flood the official signet with a lot of valid block headers, because it can be done on just a CPU, and you can mine hundreds of valid signet block headers, for each testnet CPU-mined block. And because nonces are unsigned in signets, nodes would have no idea, which chain is the best one, until signet creators would produce the next block, so all P2P clients would be forced to download everything.

This got me thinking, and correct me if I'm wrong: someone with enough ASIC power could not just do a 51% attack, they could build a completely new chain for testnet 4 from the very first block! There are currently 78970 blocks, many of them are CPU blocks. If I understand "chainwork" correctly, someone could start from the first block, keep quadrupling the difficulty every 2016 blocks, and reach a high enough difficulty to have the chain with most work in it at much less than 78970 blocks. Is this correct?

Yes, if you produce a chain with a bigger chainwork from scratch, then you can overwrite the original chain. And it is true on all networks, including mainnet (but you need a bigger chainwork there, than in any testnets). And if you for example produce ASIC-only blocks, on top of the Genesis Block, then it is possible to soft-fork the network into testnet5, where the delay between any two blocks will never be bigger than 20 minutes, and then it will be backward-compatible, if miners will keep reorging all CPU-mined blocks, and simply mark them as "always invalid", and enforce that rule with their hashrate. But practically speaking, if you compare chainworks from testnet3 and testnet4, then still, testnet3 has a bigger chainwork, so testnet4 isn't strong enough yet, to even override the older version (probably because it isn't profitable, when you still have 250 satoshis per 1 tBTC3, and 20 satoshis per 1 tBTC4 on centralized exchanges; and of course, technically, the chain will never be reorged, because of the different Genesis Block).

It's probably pointless to launch the attack, unless they want to proceed with a 'griefing attack, that screw up network operation. The 51% attack would help improvise enough testnet to generate a great number of blocks in a little time - James Lopp created 165000 blocks in a week using this attack and slowed down the network. The good part of it is that it hurts the functionality of testnet marketplaces, and the delay it causes the workflow of developers who are running their applications on the testnet, is certainly the bad side of the attack, although the action has no benefits for the attacker, it really pisses off developers.