I hear plenty of buzz that blockchains are supersecure and attack proof and so are used global scale shared public ledgers. Before I try to understand what and how blockchains are made attack proof, I was wanted to know what attacks are possible on a blockchain. This is what I found.
The two main methods that we can attack a blockchain are-
1. Sybil Attack
2. 51% Attack
A Sybil attack is wherein a single user creates multiple identities with the idea of increasing or decreasing the reputation of a network. In Twitter terms, it can be like creating a horde of Twitter bots that like and retweet your content in order to increase your credibility.
The main vulnerability to Sybil attacks, therefore, comes from the ease of creating and maintaining fake identities a.k.a. sockpuppets.
In terms of blockchain, it would be the ability of a single miner/validator to pose as multiple validators in order to mine as many blocks as blocks possible and get higher rewards.
There have been no Sybil attacks on any blockchains due to the use of PoW, PoS, and other Proof Mechanisms. Fun fact, the makers of StackOverflow in order to prevent Sybil attacks require a certain reputation before you can start using all the functionalities.
A 51% Attack is wherein a malicious agent captures a majority of the population and controls them to its benefit. In real-world terms, it is like booth capturing/ballot box stuffing during elections, where a candidate of choice can be elected by simply adding more and more votes in favor of them by a small group of people.
In terms of blockchain, it would happen when miners/validators with 51% of compute power(hash rate) of cryptocurrency and allow for double-spending.
A plethora of altcoins with wildly differing market capitalizations has launched. This has made 51% of attacks against altcoins realistic because only a small proportion of miners from larger coins need to switch to a smaller coin in order to control 51% of the smaller coin’s network hash rate. Here is a list maintained by James Lovejoy on some interesting 51% attacks on blockchain.
In order to do a 51% attack, Sybil attack is not necessary as different real agents can come together to do an attack.