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Haraldulf
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May 29, 2014, 07:40:51 PM
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Hi everybody ! I'm a computer engineer and I have some questions in my mind. I read several research paper which were very interesting about bitcoin.

After those reading I was wondering: suppose we have half the computing power of the bitcoin network would it be possible to double-spend our money?

If we can control the traffic in the network (break connections between any of peers). How could we disrupt the network ? Thus, can we make money?

Thank you very much  Smiley !
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DannyHamilton
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May 29, 2014, 07:52:44 PM
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Hi everybody ! I'm a computer engineer and I have some questions in my mind. I read several research paper which were very interesting about bitcoin.

After those reading I was wondering: suppose we have half the computing power of the bitcoin network would it be possible to double-spend our money?

Yes.  This is a well known and much discussed attack vector.  You'll often see it called a 51% attack, or a > 50% attack.

If we can control the traffic in the network (break connections between any of peers). How could we disrupt the network ?

Please explain your question.

If you can break the connections that a peer has, then you can control which transactions they see, and whether or not they see new blocks (for as long as you can control their connections).

Thus, can we make money?

Thank you very much  Smiley !

Make money?

You mean steal money?

Perhaps, but it will cost you a lot of money to acquire half of the hashing power of the entire network.  You may find it difficult to steal more than you spend.
Haraldulf
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May 29, 2014, 09:57:05 PM
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Please explain your question.

If you can break the connections that a peer has, then you can control which transactions they see, and whether or not they see new blocks (for as long as you can control their connections).
Make money?
You mean steal money?

Perhaps, but it will cost you a lot of money to acquire half of the hashing power of the entire network.  You may find it difficult to steal more than you spend.


Thank you for your answer !  Smiley

For the question to explain:

1. Disrupt the network means to take advantage of the control of the network, by any way possible. Make money, prevent peers to make money... What is it possible to do if we have the control of the network?

2. Steal money yes and no, the question is related to the question with the control of the network, if we can control the network (and by that I mean breaking connections between peers) is it possible to make (by any way) or steal money from the peers/from the network (which then we can control).
DannyHamilton
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May 29, 2014, 10:18:36 PM
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Thank you for your answer !  Smiley

For the question to explain:

1. Disrupt the network means to take advantage of the control of the network, by any way possible. Make money, prevent peers to make money... What is it possible to do if we have the control of the network?

2. Steal money yes and no, the question is related to the question with the control of the network, if we can control the network (and by that I mean breaking connections between peers) is it possible to make (by any way) or steal money from the peers/from the network (which then we can control).

If you can fool someone into running a fake version of the bitcoin software instead of a version that follows the rules of the real protocol, then you can trick them into believing that they have received a bitcoin transaction from you even though they really haven't.

You could then offer to send bitcoins in exchange for things of value (such as cash, gold, or electronics).  By the time the victim realizes that you have not really sent them bitcoins, you will have stolen the items.  This is of course fraud, and theft, so if you can be identified you will be prosecuted.

If you have complete control over all connections between nodes, then you can split the network isolating your victim to a subset of the network where the total hash power of all the miners on that portion of the network is less than 50% of the total hash power across the entire network.  You could then simultaneously broadcast a transaction on the subnetwork with you victim that pays them some bitcoins in exchange for something of value AND broadcast a transaction that spends at least 1 of the same inputs and sends the bitcoins to an address that you control.  The victim's subnetwork will build up confirmations slowly, and when it reaches enough confirmations, your victim will provide you with the thing of value that you have "purchased".  Meanwhile the larger subnetwork with most of the hash power will build up confirmations rapidly and will exceed the number of blocks solved by the victims subnetwork.  Once you have the thing of value, you can allow the two networks to communicate with each other again.  The transaction that you broadcast on the larger subnetwork will make the transaction on the smaller subnetwork invalid.  The bitcoins will vanish from the victims wallet, and you will have your bitcoins back. is is of course fraud, and theft, so if you can be identified you will be prosecuted.  Additionally, there is a reasonable chance that those who are on your victim's subnetwork will realize that the network has been split and that they are unable to communicate with many nodes.  If you victim realizes this, they may not be willing to provide the thing of value.

Controlling the network doesn't allow you to create money, and it doesn't allow you to spend someone else's bitcoins.  All it allows you to do is to control which transactions and blocks the victim sees and doesn't see.

Controlling more hash power than the entire combined hash power of the rest of the network allows you to mine all the new blocks (and therefore receive all newly created bitcoins) for as long as you can maintain that majority.
Haraldulf
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May 29, 2014, 10:29:49 PM
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Wow ! Thank you for your answer again !  Smiley It's more clear now ! Don't worry I will not try anything of that on anybody, it's just theoretical!

I'm a Phd student in university and I was just asking myself these questions after some reading on bitcoin  Smiley.

Thanks again !
DannyHamilton
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May 29, 2014, 10:34:27 PM
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Wow ! Thank you for your answer again !  Smiley It's more clear now ! Don't worry I will not try anything of that on anybody, it's just theoretical!

I'm a Phd student in university and I was just asking myself these questions after some reading on bitcoin  Smiley.

Thanks again !

The sort of control over the network that would be required to pull off one of these attacks is beyond what the typical hacker has access to.

It could probably, however, be perpetrated by a large government, or a large internet service provider.  As I mentioned though, there's a good chance that if they tried it very many times, or for very long, it would be noticed.
ondratra
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May 31, 2014, 11:47:49 AM
 #7

Hi everybody ! I'm a computer engineer and I have some questions in my mind. I read several research paper which were very interesting about bitcoin.

After those reading I was wondering: suppose we have half the computing power of the bitcoin network would it be possible to double-spend our money?

Yes.  This is a well known and much discussed attack vector.  You'll often see it called a 51% attack, or a > 50% attack.


And there is no need to actually have 51% of network - it is possible to attack with minor part too with decreased probability of success.
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