Miners compete to solve cryptographic puzzles. The first to find a valid solution gets to add the next block and earn rewards.

• Examples: Bitcoin (BTC), Ethereum (pre-Merge), Litecoin.
• Security model: High security through immense computational cost (hashing power).
• Trade-offs: Extremely energy-intensive; slower transaction finality (Bitcoin: ~10 min/block).

Validators are chosen to propose and attest to blocks based on the amount of cryptocurrency they "stake" as collateral.

• Examples: Ethereum (post-Merge), Cardano, Polkadot.
• Security model: Security through economic stake; malicious acts lead to "slashing" of staked funds.
• Trade-offs: More energy-efficient than PoW; potential for centralization among large stakers.

Token holders vote to elect a small set of delegates (e.g., 21 or 101) to validate transactions and produce blocks.

• Examples: EOS, TRON, Steem.
• Security model: Relies on reputation and voter accountability.
• Trade-offs: High throughput and fast finality, but criticized for being less decentralized due to small validator set.

A voting-based consensus where a known set of validators communicate in multiple rounds to agree on a block's validity.

• Examples: Hyperledger Fabric, Stellar (Federated Byzantine Agreement variant), early versions of Ripple.
• Security model: Tolerates up to f faulty nodes in a network of 3f + 1 nodes.
• Trade-offs: Fast finality and high throughput for permissioned networks, but scales poorly with large validator counts.

A cryptographic clock that creates a historical record proving that an event occurred at a specific moment. Used alongside PoS.

• Examples: Solana (uses PoH with a PoS-based consensus called Tower BFT).
• Security model: Enables parallel processing and high throughput by providing verifiable timestamps.
• Trade-offs: Allows for extremely high transaction speeds (Solana: ~400ms block time) but requires high-performance hardware.

Transactions and blocks are validated by approved accounts, known as validators, whose identities are publicly known and verified.

• Examples: Binance Smart Chain (BSC) validator set, xDai/Gnosis Chain, VeChain.
• Security model: Security and trust are derived from the reputation and legal identity of the validators.
• Trade-offs: Highly efficient and fast, but centralized by design. Suitable for private/enterprise blockchains.

A permissioned model where a limited set of pre-approved, reputable validators produce blocks. It prioritizes high throughput and low latency by sacrificing decentralization. This is common in private or consortium blockchains where participants are known and trusted, such as enterprise supply chain networks. Validators are held accountable by their real-world identity.

• Use Case: Hyperledger Besu, VeChainThor
• Throughput: Can process thousands of transactions per second (TPS)
• Trade-off: High performance, but centralized control.

A cryptographic clock that enables nodes to prove the passage of time without waiting for consensus. It's not a standalone mechanism but a verifiable delay function that sequences events before they are processed by a high-speed consensus layer (like Proof-of-Stake). This decoupling allows for parallel transaction processing.

• Use Case: Solana's core sequencing component
• Key Feature: Creates a historical record proving an event occurred at a specific moment
• Benefit: Dramatically reduces messaging overhead, enabling ~65,000 TPS.

A representative democracy model where token holders vote for a small number of delegates (e.g., 21 or 101) to validate blocks on their behalf. This streamlines consensus, enabling faster block times and higher efficiency than standard PoS, but concentrates power among the elected delegates.

• Use Case: EOS, TRON, Steem
• Block Time: Often 0.5–3 seconds
• Governance: Delegates can be voted out, introducing a layer of accountability.

Also called Proof-of-Space, this mechanism uses allocated hard drive space as the resource for mining rights. Validators pre-compute and store solutions to a cryptographic puzzle on their disks. The more storage space you allocate, the higher your chance of being chosen to mine the next block. It is more energy-efficient than Proof-of-Work.

• Use Case: Chia Network, Spacemesh
• Resource: Requires terabytes of available storage, not computational power
• Energy Use: Significantly lower than ASIC or GPU mining.

A family of algorithms designed for distributed systems to reach consensus even when some nodes are malicious (Byzantine faults). Practical BFT (PBFT) is a foundational model used in permissioned settings. Newer variants like Tendermint BFT and HotStuff are optimized for public blockchain performance and are the consensus engines for many Cosmos SDK and Diem-derived chains.

• Use Case: Hyperledger Fabric (PBFT), Cosmos (Tendermint)
• Finality: Provides immediate, deterministic finality
• Node Count: Typically efficient with tens to hundreds of validators.

A lottery-based, energy-efficient consensus mechanism developed by Intel for permissioned networks. It relies on a trusted execution environment (TEE), like Intel SGX, to generate a random, verifiable wait time for each validator. The node with the shortest wait time wins the right to produce the next block. It requires specialized hardware for the TEE.

• Use Case: Hyperledger Sawtooth (optional)
• Core Concept: Fair leader election through trusted random timers
• Limitation: Dependent on specific, verifiable hardware security.

A majority attack where a single entity gains control of over 50% of the network's mining hash power (PoW) or staked tokens (PoS). This allows them to:

• Double-spend coins by reorganizing the blockchain.
• Censor transactions by excluding them from blocks.
• Halt block production to freeze the network.

Example: The Ethereum Classic network suffered multiple 51% attacks in 2020, resulting in millions in losses.

A threat to Proof-of-Stake (PoS) chains where an attacker acquires old private keys to rewrite history from an early point in the chain. This is mitigated by:

• Checkpointing: Periodically finalizing blocks to prevent reorganization beyond a certain point.
• Weak Subjectivity: Requiring new nodes to trust a recent, valid block hash.
• Slashing: Penalizing validators for creating conflicting blocks.

Networks like Ethereum use a combination of these defenses.

A theoretical problem in early PoS designs where validators have no cost to vote on multiple blockchain histories during a fork, potentially preventing consensus. Modern PoS protocols solve this with:

• Slashing conditions that destroy a validator's stake for equivocation.
• Economic penalties that make attacking more expensive than honest validation.

Example: Ethereum's Casper FFG slashes a validator's entire stake for signing conflicting messages.

An attack where one entity creates many fake identities (Sybils) to gain disproportionate influence over a peer-to-peer network. Consensus mechanisms defend against it differently:

• Proof-of-Work: Requires expensive computational power per identity.
• Proof-of-Stake: Requires locked capital (stake) per validator identity.
• Proof-of-Authority: Uses a known, permissioned set of validators.

Without these costs, decentralized networks are vulnerable to spam and vote manipulation.

An attack where a malicious validator manipulates the process used to select the next block proposer in PoS systems. By trying different committee seeds or VRF inputs, they can increase their selection odds. Mitigations include:

• Using Verifiable Random Functions (VRFs) with unbiasable entropy.
• Implementing RANDAO or Drand for distributed randomness beacons.

Ethereum's beacon chain uses RANDAO combined with VDFs (Verifiable Delay Functions) for future-proof randomness.

Bitcoin implements Proof of Work (PoW) as defined in the original Nakamoto consensus model. This resource explains how nodes reach agreement on the valid chain using hash-based leader election.

Key areas:

• Difficulty adjustment every 2016 blocks to target a ~10 minute block time
• Chain selection using cumulative work, not block count
• Security tradeoffs tied to hashrate distribution and energy cost
• How PoW prevents double-spend attacks using economic finality

Use this documentation to understand why PoW favors simplicity and adversarial resistance over throughput, and how consensus emerges without explicit voting.

Ethereum transitioned to Proof of Stake (PoS) in 2022 using the Beacon Chain and Casper FFG. This documentation details how validators propose blocks, vote on checkpoints, and achieve economic finality.

Core concepts covered:

• Slot and epoch structure with 12-second slots
• Validator rewards, penalties, and slashing conditions
• Finality guarantees after two justified epochs
• Separation of block production (LMD-GHOST) and finality (Casper)

This is the authoritative source for understanding modern PoS design at scale, including liveness under partial validator failure.

PBFT is a classical consensus protocol designed for small, permissioned networks with known participants. It tolerates up to f faults out of 3f+1 nodes using multi-round message passing.

This paper explains:

• Pre-prepare, prepare, and commit phases
• Deterministic finality without probabilistic forks
• Why communication overhead grows as O(n²)
• How PBFT influenced Tendermint, HotStuff, and enterprise chains

PBFT remains foundational for understanding why many blockchains trade decentralization for fast finality in permissioned or validator-limited systems.

Tendermint implements a Byzantine Fault Tolerant Proof of Stake consensus engine used by Cosmos SDK chains. It combines validator voting with deterministic finality.

Important details:

• Block proposal and prevote/precommit rounds
• Instant finality once 2/3+ stake signs a block
• Safety guarantees under asynchronous network assumptions
• How Tendermint separates consensus from application logic

This specification is useful for developers evaluating BFT-style consensus for application-specific blockchains and sovereign chains.