Despite the security features of Proof-of-Work blockchains, they are still susceptible to 51% attacks. Cryptocurrency projects like Bitcoin Gold, Zencash, Verge, Litecoin Cash and more recently, Ethereum Classic have all been victims of the 51% attack. Although pulling off a 51% attack requires a significant amount of resources (about $1.4 billion in the case of Bitcoin), smaller blockchains are more prone to this form of attack.

A 51% attack occurs when a single individual (or group of individuals operating as a single entity) controls more than half of a network’s computational power. This allows for double spending.

PAC will be adopting a mitigation patch as a secondary layer of security against 51% attacks. The patch which was introduced by the Horizen team proposes a penalty system for delayed block submission.

One of the basic assumptions of the patch is that a malicious actor would need to privately mine a parallel forked chain after initiating a transaction on the public chain. He capitalizes on the time required to validate the transaction to mine a longer chain, faster than the public network. Once the transaction is confirmed and withdrawal is made, the malicious actor publishes his longer private chain, which will be adopted by the network.

To mitigate the possibility of a 51% attack and exponentially increase the associated cost, the patch uses a fork acceptance delay, which sets the delay function to zero in order to accept a longer blockchain as the main chain. Additionally, submitted blocks that are greater than the current normal block heights are flagged.

PAC team went further to state that:

While this solution cannot prevent natural forks from happening or if an attacker mines his chain publicly, it still helps in preventing malicious 51 % attacks by adding a layer of complexity to the issue, this is why this patch was dubbed ‘51% attack MITIGATION’ by the team. This solution also gives time for an exchange to react and freeze potentially malicious deposits until the fork has been resolved.