Ouroboros Praos

Ouroboros Praos is a proof-of-stake consensus protocol that decides who may add the next block to a blockchain in proportion to the amount of cryptocurrency they have staked. It is the version of the Ouroboros protocol family that underpins Cardano. Praos improved on the original Ouroboros design by letting each stakeholder learn privately whether they have won the right to produce a block, which makes it harder for attackers to target upcoming block producers.

The protocol was developed by researchers associated with IOHK, the engineering company behind Cardano, and academic partners, and was presented in a peer-reviewed paper published in 2018. "Praos" comes from a Greek word meaning gentle or mild. Later members of the family, such as Ouroboros Genesis and Leios, build on Praos to improve how new nodes join the network and how much data it can process.

Imagine a lottery for the right to write the next page in a shared ledger, held every second. Everyone who has put money into the system gets tickets in proportion to their stake. Crucially, each participant scratches their own ticket in private. Only the winner knows they have won, and they prove it to everyone else at the moment they publish their page.

Because nobody can see in advance who will write next, a troublemaker cannot knock the next writer offline or bribe them ahead of time. Most seconds have no winner, and occasionally two people win at once, but the rules ensure everyone ends up agreeing on one version of the book.

• Slots and epochs. Time is divided into short slots, one second each on Cardano, grouped into epochs of five days. Stake distribution is fixed at the start of each epoch for the purposes of selection.
• Private leader selection. In each slot, every stake pool runs a verifiable random function, a cryptographic method that produces a random output anyone can later check. If the result falls below a threshold set by the pool's share of stake, the pool is a slot leader and may produce a block.
• Sparse slots. The protocol is tuned so that only a small fraction of slots, about one in twenty on Cardano, produce a block, which gives blocks time to spread and keeps forks short.
• Chain selection. If two leaders produce competing blocks, nodes follow the longest valid chain, within limits that stop very deep rollbacks.
• Randomness. Values gathered from blocks during an epoch seed the randomness used in a later epoch, so outcomes cannot easily be predicted or steered.

On Cardano, block production is carried out by stake pools run by operators who keep servers online. ADA holders who do not want to run infrastructure can delegate their stake to a pool without giving up custody or locking their coins, and rewards are shared after the pool's fees. Rewards are paid in lovelace-denominated amounts of ADA at the end of each epoch. This is a form of delegated staking, but it differs from delegated proof of stake systems that elect a small, fixed set of producers: Cardano supports a large number of independent pools.

Praos is designed to remain secure as long as honest participants control a majority of the stake, under realistic network conditions in which messages may be delayed by a bounded amount. Its main advance over earlier versions is resistance to adaptive attackers, who try to corrupt or attack specific participants once they learn who will produce blocks. Because leadership is revealed only when a block is published, there is no schedule to exploit. Finality is probabilistic: a block becomes harder to reverse as more blocks are added after it, similar in principle to proof of work but without the energy cost.

Cardano moved from its earlier federated phase to stake-pool block production during the Shelley era in 2020, initially using a transitional variant, and adopted Praos itself with a later hard fork. Supporters point to its academic foundations and formal analysis as strengths. Critics note that Cardano's research-first approach made feature delivery slower than on rival chains, that stake remains concentrated among some large holders and pool groups, and that sparse slots limit throughput, which is one reason later protocols such as Leios are being developed. As with other proof-of-stake networks, the security of Praos also depends on the value and distribution of the staked token, discussed under staking and consensus mechanism.