51% Attacks
Where blockchains have consensus rules based on a simple majority, there is a risk that malign actors will act together to influence the outcomes of the system. In the case of a cryptocurrency, this would mean a group of miners controlling more than 50% of the mining computing power can influence what transactions are validated and added (or omitted) from the chain. On a blockchain that uses the Proof of Work (PoW) consensus protocol system, a 51% attack can also take the form of a “rival” chain – including fraudulent transactions – being created by malicious parties.
Through their superior mining capacity, these fraudsters can build an alternative chain which ends up being longer than the “true” chain and therefore – because part of the Bitcoin Nakamoto consensus protocol is “the longest chain wins” – all participants must follow the fraudulent chain going forward.
In a large blockchain like Bitcoin this is increasingly difficult, but where a blockchain has ‘split’ and the pool of miners is smaller, as in the case of Bitcoin Gold, a 51% attack is possible.
A 51% double spend attack was successfully executed on the Bitcoin Gold and Ethereum Classic blockchains in 2018, where fraudsters misappropriated millions of dollars of value.
Proof of Work vs Proof of Stake
A 51% attack on a new blockchain called Ethereum Classic in January 2019 prompted a change in strategic direction from Proof-of-Work (PoW) mining to Proof-of-Stake (PoS) voting for the Ethereum blockchain.
However, Proof of Stake is more vulnerable to schisms or splits known as “forks”, where large stakeholders make different decisions about the transactions that should comprise blocks and end up creating yet another new currency. Ethereum briefly tried this validation method but, due to forking issues, reverted back to Proof of Work. It is expected to introduce a revised Proof of Stake validation system in 2020.
Double Spending
There is a risk that a participant with, for example, one bitcoin can spend it twice and fraudulently receive goods to the value of two bitcoins before one of the providers of goods or services realises that the money has already been spent. But this is, in fact, an issue with any system of electronic money, and is one of the principal reasons behind clearing and settlement systems in traditional currency systems.
