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It seems like a man in the middle attack could be performed in a Lightning Network.

Let's say A opens a channel with B. If A manages to control all the connects B has to the bitcoin network (becoming a man in the middle), A can prevent B's attempts to propagate or publish refund transactions. In this case, A can send some money through B to another address A controls, then publish the original balance transactions. If A can continue to MITM B for the 10 days (or whatever) it takes for that transaction to go through, A would have successfully stolen all the funds A was able to convince B to transfer.

B could potentially become aware that either the blockchain data he's getting isn't valid or simply become aware the balance transaction has been published, but if he can't get on the bitcoin network to publish the refund, he's screwed.

Is this scenario possible?

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Such an attack is not limited to just Lightning; it could happen with just normal transactions and normal Bitcoin usage. However performing such an attack is very expensive and unlikely. It would require A to control as much hashrate as the entire network has. Otherwise blocks will come slowly for B and eventually warn him if too few blocks are received within a given time frame. Furthermore, if blocks were invalid, B will ban A's nodes and find other nodes to connect to. A would also have to control B's internet connection in order to prevent him from connecting to other people and opening channels with people other than A.

Lastly, the lightning network has a minimum balance requirement (minimum must be kept for both parties after it has been met, but both parties do not need to start with the minimum in the channel) so B would not lose all of his money. There is also currently a maximum amount that can be put in a channel so not a lot of money is risked.

  • Well, a large attacker A could trick B into accepting an already-spent transaction (ie a double spend), but couldn't actively steal any of B's bitcoins. So I don't think its true that this kind of attack is possible on pure Bitcoin. But it sounds like what you're saying i essentially, "yes, an attacker like this could steal up to the amount of bitcoins committed to the channel". Is that right? – B T Jun 18 '17 at 3:50
  • Yes. I think this attack would work in theory, but in practice, it would basically be impossible, or at the very least, be prohibitively expensive to do for a very very small reward. – Andrew Chow Jun 18 '17 at 4:30
  • I understand that surrounding someone's network with MITM isn't trivial. But could you elaborate what makes this basically impossible, or what about it would be inherently expensive? – B T Jun 18 '17 at 5:17
  • In order for A to attack B, A needs to fully control B's internet connection. Otherwise B will be able to make a connection to another node and get the actual blockchain. That in itself is pretty hard to do. Then, if you were to get that somehow, you would need to be able to mine blocks at the same difficulty and rate that the network currently does. In order to do that, you have to have the same hashrate as the network, otherwise B's full node will warn him that blocks are too slow. That is prohibitively expensive. – Andrew Chow Jun 18 '17 at 5:33
  • Ok, yes, if we're talking about the blockchain I agree. But this question is about the lightning network, not the blockchain. – B T Jun 18 '17 at 6:04

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