Agreeing on a history
A blockchain needs a way to resolve competing histories while checking that transactions follow its rules. Consensus mechanisms organize that process. Cryptographic signatures authorize actions; consensus addresses which valid sequence the network accepts.
These are related but separate tasks. A system can correctly order transactions that interact with a vulnerable application.
Work and stake
Proof of work makes block production costly through computation. Proof of stake assigns protocol duties to participants committing value, with incentives and penalties under the network’s rules. Neither label fully describes a network’s implementation.
Compare confirmation or finality rules, participation requirements and responses to dishonest behavior. The distribution of mining resources or stake also affects the practical structure of a network.
Avoid the shortcut comparison
It is too simple to rank every network’s security using only its consensus label. Client software, network conditions, governance and user verification all matter.
For a practice question, ask what would happen if a block producer proposed a transaction creating unauthorized assets. Validating nodes should reject a block that violates their rules. Then ask the different question of how the network resolves two valid competing blocks. Understanding that distinction makes discussions of attacks and forks more precise.
Consensus chooses a history within validation rules
In proof of work, participants compare accumulated work when selecting among valid histories. In proof of stake, a protocol uses validators and stake-based incentives and penalties under its own rules. Neither label means a participant may simply ignore every validation rule and have honest nodes accept an arbitrary balance.
Consensus, execution and custody are separate layers. A network can operate as designed while a user loses funds through a stolen signing key. Likewise, a consensus mechanism does not verify that a company’s off-chain reserves exist.
Compare the actual attack and recovery assumptions
Ask which resource an attacker needs, what actions are possible, how finality is defined and what recovery would require. Costs, penalties and operational concentration differ between implementations. A headline percentage about control should specify the network, time period and measurement method.
Energy consumption is a separate comparison from transaction capacity or application safety. A change in consensus can alter one property without automatically fixing congestion, contract bugs or centralized service risk. Avoid treating a single metric as an overall security score.
Sources & further reading
Educational content. Examples are illustrative. Protocols, products and rules can change; consult the linked documentation for current details.