You can roll back transactions for whatever duration of time you were able to outspend the network. Critically, that can include your own transactions.
You mine quietly, outpacing the regular network, creating your own longer chain. Perhaps you do this for an hour, several hours, or a day. The only transactions on this chain are those in which you send your own coins to yourself -- maybe plus some random transactions from the mempool you throw in to help cover your own tracks. Meanwhile you spend those same coins (UTXOs) on the "honest" chain.
Once you are satisfied, you publish your longer chain. By the rules of the network the longest chain is the true one. Honest miners immediately begin building their blocks on top of your dishonest version of events. Since you are now working together with them, blocks are produced quickly. The honest chain dies.
Chaos would ensue.
Anybody who received coins on the dead chain can try to republish the transaction to the mempool and hope it gets mined again, so they get their money again. But if the sender is fast, they might realize they have a double-spend opportunity here, and submit a higher-fee transaction in which they sent those coins to themselves. It's a race. Of course, for your own UTXOs that were already spent in the dishonest chain, the race has already been won. Those double spends are successful.
if you split the network, you can add your own machines, that can rewrite the history arbitrarily from the future of that split.
As for "a few minutes", it depends on which one we're talking about. BTC, yes -- but disrupting the global BTC network requires only a few minutes of time. Just dump everything into unusable addresses, and watch the reaction.
No, you cannot do that. Transactions would still need to be signed by the corresponding owners. All you could do is reorder (and thus invalidate) some transactions, drop some transactions, or add some that weren't originally included.
> Just dump everything into unusable addresses...
Nope; that's not possible. You wouldn't have the signatures to do that even with 51% hashing power.
You're right about the dumping; I was thinking more about the case where you controlled the protocol on the machines of a partitioned network (eg., so you release a new version of btc which uses exploitable crytography, etc.).
In the case of a mere 51% at scale, hijinks are still quite possible from replays, reorderings, etc.
I was more preoccupied by the case where the state's acting in its own borders with control over the network, major miners, most machines on the network -- at this point basically no gaurentees remain
> the case where you controlled the protocol on the machines of a partitioned network (eg., so you release a new version of btc which uses exploitable crytography
Such a change would be treated as a "monopoly money" fork by those wanting to transact, and ignored. "No I don't want your fake money; send me real BTC or GTFO". This is why miners cannot arbitrarily change the protocol in their favour even today.
You mine quietly, outpacing the regular network, creating your own longer chain. Perhaps you do this for an hour, several hours, or a day. The only transactions on this chain are those in which you send your own coins to yourself -- maybe plus some random transactions from the mempool you throw in to help cover your own tracks. Meanwhile you spend those same coins (UTXOs) on the "honest" chain.
Once you are satisfied, you publish your longer chain. By the rules of the network the longest chain is the true one. Honest miners immediately begin building their blocks on top of your dishonest version of events. Since you are now working together with them, blocks are produced quickly. The honest chain dies.
Chaos would ensue.
Anybody who received coins on the dead chain can try to republish the transaction to the mempool and hope it gets mined again, so they get their money again. But if the sender is fast, they might realize they have a double-spend opportunity here, and submit a higher-fee transaction in which they sent those coins to themselves. It's a race. Of course, for your own UTXOs that were already spent in the dishonest chain, the race has already been won. Those double spends are successful.