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Question: How Exactly does Bitcoin Deal with Sybil Attacks

I was wondering what happens if a malicious actor is somehow able to create fake full nodes on the Bitcoin network, which total to more than half of Bitcoin‘s real full nodes. In this way, the attacker would be able to put fake transactions on these nodes (By forking Bitcoin core and modifying its code, for example) and make the entire network agree on them since he has 51% of all nodes (not hashing power) in the network. I know that having more than 51% of the hashing power is practically impossible and is referred to as a 51% attack, but I don’t think it is impossible for someone to create tens/hundreds of thousands of Bitcoin full nodes. In the end, it is just disk space.

I searched this question up on the internet and found out that such an attack is referred to as a Sybil attack. However, all the answers I found are quite unclear to me… For example, [this answer](https://bitcoin.stackexchange.com/questions/50922/whats-a-sybil-attack) from stack exchange:

>Sybil attacks are avoided in Bitcoin by requiring block generation ability to be proportional to computational power available through the proof-of-work mechanism. That way, an adversary is limited in how many blocks they can produce. This provides strong cryptographic guarantees of Sybil resilience.

I don’t really understand this sentence and more specifically, what is meant by “block generation ability”. I noticed that some answers bring 51% of attacks into the mix But I don’t understand what 51% of attacks have to do with Sybil attacks as a 51% attack concerns the miners whereas a Sybil attack concerns the nodes.

One solution that I could think of is that the network would reject so many nodes being created all at once and since everyone is able to verify the date of creation of these nodes, it is clear that it is an attack. However, since I am not skilled enough to read the code, I don’t know if such a check is implemented anywhere.

I hope all this makes any sense…

I thank you all for your help in advance!



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4 Comments

  1. You only need to connect to 1 honest node to find the best chain (i.e. the chain with most work). Afterwards, your node will start ignoring chains with less work. Creating a dishonest chain with the same or more amount of work is equivalent to a 51% hash rate attack.

    Another mitigation is that nodes do not accept a chain below a chain work value hard-coded in the source code. I.e., “the best chain must have at least x amount of work”. This prevents dishonest nodes from sending you low work chains.

    Here’s a link explaining chain work: https://learnmeabitcoin.com/technical/longest-chain

  2. Fake nodes can’t do harm because they can only aggregate information to you, and all of which you can ignore even. Ofc, this works on the assumption that you got connected to some honest node, otherwise you simply won’t receive the “stronger” data.
    A 51% attack refers to the ability to undo an old transaction by rewriting history and having this alternative version becoming stronger (in accumulated PoW) than the original one.

    > In this way, the attacker would be able to put fake transactions on these nodes .. and make the entire network agree on them since he has 51% of all nodes .. in the network.

    Having 51% (or any other number) doesn’t do anything since each honest node convinces itself that the data checks. It doesn’t depend on what other nodes think, much less of what a “majority” thinks.

  3. It’s possible if you can trick the victim into only connecting to your nodes. There might be some situations, the proverbial cafe WiFi for example where it might be possible to hijack all of a victim’s traffic and divert all bitcoin traffic to your own nodes. Large isps would be well placed to do this. You can mitigate it by using tor or a VPN.

What do you think?

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