Many kinds of attacks can launch on the Proof of Work (PoW) based system. However, one of the popular attacks that people were trying to perform was the Sybil Attack.
Sybil Attack
The attacker tries to fill the network with the clients under his control and suppose the attacker can succeed in his intention. In that case, the attacker can actually control or get the monopoly over the network. Consequently, he can compromise the distributed consensus, which is based on the Proof of Work mechanism.
The plant clients can do many different kinds of actions based on the instruction received from the attacker. They can refuse to relay the valid blocks and only relay the blocks generated by the plant clients and an attacker, and those blocks can lead to double-spending. A more powerful attacker can include multiple nodes in the different networks that can collectively compromise the entire ecosystem.
To solve this problem, the bitcoin network has applied a solution where it diversifies the connections. Bitcoin architecture allows outbound connection to only an IP with subnet mask 16. Example: X.Y.0.0/16
In general, the attacker can not be omnipresent on the entire internet. They may reside in a cluster of the same subnet. But if the outgoing connection is diversified and forwarding the block to multiple nodes in the different subnets rather than nodes in the same network.
The entire idea is something like this. Each node forwards the transactions to one node of a cluster. That way, bitcoin architecture makes the Sybil attack hard to implement on a distributed network. But, this solution makes it hard to launch. It does not make it impossible. It is always possible to launch multiple attackers at multiple subnets, collectively control them, and launch attacks in a distributed way. But again, launching those kinds of attacks is much difficult in the real network.
Denial of Service (DoS) Attack
The attacker sends a lot of data to a particular node. As a consequence, the node will not be able to process the normal bitcoin transactions. This is a typical denial of service attack in the bitcoin network. To solve denial of service attack, the bitcoin architecture proposed a set of rules, and a few of those are as follows:
- No forwarding of orphaned blocks: It means the blocks which have been forked from the main chain, means it does not belong to the main chain (longest blockchain), do not forward those blocks to peer nodes.
- No forwarding of double-spend transactions: It means if a transaction has been already forwarded once, do not forward it anymore.
- No forwarding of same block or transactions
- Disconnect a peer that sends too many messages
- Restrict the block size to 1 MB: It is beneficial as if the block size is more than 1 MB, then a normal node will take more time to process all the transactions and transfer them to the peer nodes. As a result, it will not be able to process all the blocks. However, with the latest version of bitcoin, it supports a maximum of 8 MB of block size.
- Limit the size of each script up to 10,000 bytes: It contains scripts that provide instructions on how to validate or match the input of a transaction to the output of the next transaction or match the output to the input of the next transaction. Having a larger bitcoin script, it is more susceptible to attack.
Breaking Bitcoin Proof of Work (PoW)
Bitcoin PoW is computationally difficult to break but not impossible. The attackers can deploy high-power servers to do more work than the total work of the blockchain.
A known case of successful double-spending was discovered in Nov 2013. The GHash.io mining pool (a mining pool means a set of miners coming together collectively and are trying to mine a new block) appeared to be engaging in repeated payment fraud against BetCoin Dice, a gambling site.
Summary
we have seen the known attacks on the “Proof of Work” based system and the proposed prevention mechanisms.
References
- NPTEL lecture series on Blockchains Architecture, Design and Use Cases by Prof. Sandip Chakraborty, IIT Kharagpur.
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