Proof-of-activity (PoA)

Proof of activity is a consensus mechanism used in decentralized blockchain networks to validate transactions. It is a hybrid approach that combines aspects of proof of work and proof of stake consensus mechanisms.

Proof of work and proof of stake are both designed to prevent 51% attacks - a malicious action in which one individual or party controls a majority of the mining power in the cryptocurrency network and can then include invalid transactions in the blockchain. Proof of activity combines these mechanisms into a single two step-process:

1. Proof of work for miners to compete to find the block and earn the block reward. The greater percentage of the network's total computing power a miner has, the greater the probability that they will find a block.
2. Proof of stake to validate/sign the new block. The greater the percentage of the total cryptocurrency supply a staker has, the greater the probability that they will be chosen to validate the block.

Proof of activity adds a second line of defense against 51% attacks because an attacker would theoretically need to have both 51% or more of the network's total mining power and 51% or more of the coins staked in the network in order to successfully pull off the attack.

Decred is a cryptocurrency that uses the proof of activity consensus mechanism. Espers is another.