Proof of stake

The low amount of computing power involved allows a class of attacks that replace a non-negligible portion of the main blockchain with a hijacked version. These attacks are called in literature by different names, Long-Range, Alternative History, Alternate History, History Revision, and are unfeasible in the PoW schemes due to the sheer volume of calculations required.[9] The early stages of a blockchain are much more malleable for rewriting, as they likely have much smaller group of stakeholders involved, simplifying the collusion. If the per-block and per-transaction rewards are offered, the malicious group can, for example, redo the entire history and collect these rewards.[10]

The classic "Short-Range" attack (bribery attack) that rewrites just a small tail portion of the chain is also possible.[9]

Since validators do not need to spend a considerable amount of computing power (and thus money) on the process, they are prone to the Nothing-at-Stake attack: the participation in a successful validation increases the validator's earnings, so there is a built-in incentive for the validators to accept all chain forks submitted to them, thus increasing the chances of earning the validation fee. The PoS schemes enable low-cost creation of blockchain alternatives starting at any point in history (costless simulation), submitting these forks to eager validators endangers the stability of the system.[8] If this situation persists, it can allow double-spending, where a digital token can be spent more than once.[10] This can be mitigated through penalizing validators who validate conflicting chains[10] ("economic finality"[11]) or by structuring the rewards so that there is no economic incentive to create conflicts.[3] Byzantine Fault Tolerance based PoS are generally considered robust against this threat.[12]

Bribery attack, where the attackers financially induce some validators to approve their fork of blockchain, is enhanced in PoS, as rewriting a large portion of history might enable the collusion of once-rich stakeholders that no longer hold significant amounts at stake to claim a necessary majority at some point back in time, and grow the alternative blockchain from there, an operation made possible by the low computing cost of adding blocks in the PoS scheme.[10]

This is essentially a modification of the PoW scheme, where the competition is based not on applying brute force to solving the identical puzzle in the smallest amount of time, but instead on varying the difficulty of the puzzle depending on the stake of the participant; the puzzle is solved if on a tick of the clock (|| is concatenation):

${\displaystyle Hash(ProposedNewBlock||ClockTime)<target*StakeValue}$

The smaller amount of calculations required for solving the puzzle for high-value stakeholders helps to avoid excessive hardware.[13]

Also known as "committee-based", this scheme involves an election of a committee of validators using a verifiable random function with probabilities of being elected higher with higher stake. Validators then randomly take turns producing blocks. NPoS is utilized by Ouroboros Praos and BABE.[14]

The outline of the BFT PoS "epoch" (adding a block to the chain) is as follows:[15]

1. A "proposer" with a "proposed block" is randomly selected by adding it to the temporary pool used to select just one consensual block;
2. The other participants, validators, obtain the pool, validate, and vote for one;
3. The BFT consensus is used to finalize the most-voted block.

The scheme works as long as no more than a third of validators are dishonest. BFT schemes are used in Tendermint and Casper FFG.[15]

Proof of stake delegated systems use a two-stage process: first,[16] the stakeholders elect a validation committee,[17] a.k.a. witnesses, by voting proportionally to their stakes, then the witnesses take tuns in a round-robin fashion to propose new blocks that are then voted upon by the witnesses, usually in the BFT-like fashion. Since there are fewer validators in the DPoS than in many other PoS schemes, the consensus can be established faster. The scheme is used in many chains, including EOS, Lisk, Tron.[16]

In the liquid PoS anyone with a stake can declare themselves a validator, but for the small holders is makes sense to delegate their voting rights instead to larger players in exchange for some benefits (like periodic payouts). A market is established where the validators compete on the fees, reputation, and other factors. Token holders are free to switch their support to another validator at any time. LPoS is used in Tezos.[18]

The exact definition of "stake" varies from implementation to implementation. For instance, some cryptocurrencies use the concept of "coin age", the product of the number of tokens with the amount of time that a single user has held them, rather than merely the number of tokens, to define a validator's stake.[4][13]

Critics have argued that the proof of stake model is less secure compared to the proof of work model.[26]

Critics have argued that the proof of stake will likely lead cryptocurrency blockchains being more centralized in comparison to proof of work as the system favors users who have a large amount of cryptocurrency, which in turn could lead to users who have a large amount of cryptocurrency having major influence on the management and direction for a crypto blockchain.[27][28]