If an attacker gets more than 50 % of mining power

[theymos]

So you are essencially admitting that such an attack would render the system useless, as long as the attack is sustained.

Yes, but only for as long as the attack is sustained. It would be a DoS attack, not the End Of Bitcoin.

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[ThiagoCMC]

WOW! Thank you!!

If miners cash-out someday, this will leave the network unprotected.. But why?!?

But when a lot of miners "cashes-out", then the bitcoin client will automatically lower the mining difficultly level based on a hard-coded protocol.  Since it will now be easier to mine, then that means that other people, including noobs like yourself, would be incentivized to mine at the easier difficultly level, since the bitcoin payment for solving a block would remain the same (50 BTC/block solved currently).

 Reading the wiki page about scalability (https://en.bitcoin.it/wiki/Scalability), they are talking about a "supernode client", this supernode can assure the safety of the network for themselves without mining? If the miners go away abruptly...

 Or the purpose of the supernode is just to save disk space for regular clients?

I'm very curious and concerned!

Thanks!
Thiago

No problem.  Again, I had similar concerns when I was a noob as well.  Anyway you get a gold star in my book for using Ubuntu Linux and for asking simple & clear, but non-trolling questions/concerns.

 AWESOME! THANKS!!!  

[bcearl]

To reverse a 1000 blocks and to catch up then to make the longest block chain takes way more than 50 %, if you want to get it done in a decade.

If you have more than the rest of the network combined (so more than 50% of total power including you), you can grow the alternative chain indefinitely as long as you are ahead. If you have 1% more than the network, you will be 1.44 block ahead per day. If you have 10% more, you will have 14.4 blocks per day. If you have twice the current Bitcoin network, you will reverse 1000 blocks in less than a week.

If you want to do as much of a mess, grow your chain, and wait until the rest of the network caches up with the speed. And then release the alternative chain.

If you want to do that in secret, you will have your own difficulty, which means that the others are twice as fast.

If you don't do that in secret, and you reject blocks of others, they will discover you very early.

[bcearl]

To reverse a 1000 blocks and to catch up then to make the longest block chain takes way more than 50 %, if you want to get it done in a decade.

Not really. With 67%, you can reverse 1000 past blocks in a week. Reversing 1000 future blocks takes a week with 52%.

You can't do it faster, because the difficulty will increase. If you generate a lot more than 6 blocks an hour nobody would accept that chain because the difficulty does not match.

Forget that, of course you can generate as fast as you like blocks with timestamps from the past.

[Gabi]

But spending billions of dollars on a ton of AMD GPUs simply so that you can destroy the bitcoin network is a negative sum game.

15 millions of $ aren't "billions" 

[bcearl]

If you control 52% of the network, you must use one of your blocks to negate a legitimate block 48% of the time. So 48% of the network is producing legitimate blocks, 48% of the network is negating those blocks, and only 4% is left producing new blocks. The network would only produce 5.76 blocks per day.

So you are essencially admitting that such an attack would render the system useless, as long as the attack is sustained.

Only until difficulty is recalculated.

[VPoro]

And once the attacker gets sick of wasting so much money, everything can be fixed without much loss by blacklisting their chain.

Isn't blacklisting blocks that have been mined per the bitcoin protocol (albeit being fraudulent) kind of like... central banks who don't like people that make counterfeit money? Bitcoin is peer-to-peer, so which central authority in bitcoin would decide what blockchain to blacklist? I really hope there isn't one, because if there is, that central authority could use their power to "blacklist" any blockchain they want, in principle...

[bcearl]

But spending billions of dollars on a ton of AMD GPUs simply so that you can destroy the bitcoin network is a negative sum game.

15 millions of $ aren't "billions" 

Billions of Yen.

[em3rgentOrdr]

And once the attacker gets sick of wasting so much money, everything can be fixed without much loss by blacklisting their chain.

Isn't blacklisting blocks that have been mined per the bitcoin protocol (albeit being fraudulent) kind of like... central banks who don't like people that make counterfeit money? Bitcoin is peer-to-peer, so which central authority in bitcoin would decide what blockchain to blacklist? I really hope there isn't one, because if there is, that central authority could use their power to "blacklist" any blockchain they want, in principle...

Well there wouldn't be a centralized authority using violence to enforce the blacklisting.  I'm guessing it would be something like bitcoin.org putting up an alert to block a certain range of questionable ip addresses or if there was a way to identify the questionable blocks then send out a quick patch for miners to voluntarilly install.  Note that you wouldn't need everyone to comply, but instead just enough miners to shift the advantage from the forces of evil to the forces of good.

[Meni Rosenfeld]

Reversing 1000 future blocks takes a week with 52%.

This is certainly wrong, and your other numbers are probably wrong, too. If you control 52% of the network, you must use one of your blocks to negate a legitimate block 48% of the time. So 48% of the network is producing legitimate blocks, 48% of the network is negating those blocks, and only 4% is left producing new blocks. The network would only produce 5.76 blocks per day.

My numbers are not wrong, they just relate to a different attack from what you have in mind. I was talking about building up an alternative branch starting from the current block, without releasing blocks during construction. After about a week, the honest network will have on average 1000 blocks while the attacker will have on average 1083 blocks, and with high probability he will have more blocks.

[bcearl]

Reversing 1000 future blocks takes a week with 52%.

This is certainly wrong, and your other numbers are probably wrong, too. If you control 52% of the network, you must use one of your blocks to negate a legitimate block 48% of the time. So 48% of the network is producing legitimate blocks, 48% of the network is negating those blocks, and only 4% is left producing new blocks. The network would only produce 5.76 blocks per day.

My numbers are not wrong, they just relate to a different attack from what you have in mind. I was talking about building up an alternative branch starting from the current block, without releasing blocks during construction. After about a week, the honest network will have on average 1000 blocks while the attacker will have on average 1083 blocks, and with high probability he will have more blocks.

What does an attacker get from having the longer blockchain? Waste of energy?

[Meni Rosenfeld]

Reversing 1000 future blocks takes a week with 52%.

This is certainly wrong, and your other numbers are probably wrong, too. If you control 52% of the network, you must use one of your blocks to negate a legitimate block 48% of the time. So 48% of the network is producing legitimate blocks, 48% of the network is negating those blocks, and only 4% is left producing new blocks. The network would only produce 5.76 blocks per day.

My numbers are not wrong, they just relate to a different attack from what you have in mind. I was talking about building up an alternative branch starting from the current block, without releasing blocks during construction. After about a week, the honest network will have on average 1000 blocks while the attacker will have on average 1083 blocks, and with high probability he will have more blocks.

What does an attacker get from having the longer blockchain? Waste of energy?

The Bitcoin protocol specifies that the longest branch is the valid one. Thus an attacker having the longer chain can decide which transactions are to be included in it.

And I thought we already said that this allows disrupting Bitcoin commerce, for:
1. Damaging Bitcoin for political reasons.
2. Profiting from shorting.
3. Other stuff.