What is Pure Proof of Stake (PPoS)?

Pure Proof of Stake (PPoS) is the consensus mechanism developed by Silvio Micali and used by the Algorand blockchain. PPoS uses a cryptographic sortition process based on verifiable random functions (VRFs) to secretly and randomly select validators for block proposal and committee voting. Each token holder automatically participates in consensus proportional to their stake — there is no delegation or bonding required. A user's VRF output, computed locally using their private key, determines whether they are selected to propose a block or serve on a voting committee for a given round. Because the selection is computed privately, no one knows who the selected participants are until they broadcast their proposal or vote, along with the VRF proof that verifies their selection.

PPoS achieves immediate finality — blocks are final as soon as they appear, with no possibility of forks or chain reorganizations. The protocol completes in two phases: a block proposal phase where selected proposers submit candidate blocks, and a Byzantine Agreement phase (called BA*) where a committee of randomly selected voters reaches agreement on a single block through a series of voting steps. Block times are approximately 3.3 seconds. The secret sortition mechanism provides strong security against targeted attacks, since committee members are unknown until they reveal themselves, and each committee is used only once, making it impossible to corrupt members before they act. PPoS requires honest behavior from only a simple majority of the stake (greater than two-thirds for safety), and its communication cost per round is independent of the total number of users.

PPoS has been running on the Algorand mainnet since June 2019. Algorand was founded by Silvio Micali, a Turing Award-winning MIT professor whose research spans cryptography and complexity theory. The protocol's design reflects rigorous academic foundations, with the sortition mechanism and BA* protocol formally analyzed for security. Algorand has found adoption in various use cases including central bank digital currencies (the Marshall Islands' SOV), decentralized finance, and carbon credit markets. The blockchain emphasizes both performance and decentralization, with all ALGO holders able to participate in consensus without needing to run specialized infrastructure.