Validator Set

Validators are chosen based on the amount of cryptocurrency they stake as collateral. Selection is typically probabilistic, weighted by stake size, but often includes mechanisms to prevent centralization. Key methods include:

• Leader Election: A pseudo-random algorithm selects the next block proposer.
• Committee Selection: A subset of validators is chosen to attest to each block's validity.
• Example: Ethereum uses the RANDAO + VDF for proposer selection and randomly samples committees for attestation.

Token holders vote to elect a fixed number of delegates or block producers. This set is smaller and often fixed (e.g., 21 on EOS, 100 on TRON). Rotation occurs on a scheduled basis (e.g., every block, every day) based on the latest vote tally. This creates a known, high-throughput validator set but with a more centralized governance model reliant on voter participation.

The validator set is permissioned or based on identity/reputation.

• PoA: A fixed, known set of authorized nodes run by trusted entities (e.g., validators on a consortium chain).
• Nominated PoS (NPoS): As used by Polkadot, token holders (nominators) back candidates they trust. The protocol runs a complex election algorithm to select the final validator set that maximizes stake distribution, ensuring robust security and decentralization.

Regular validator rotation is critical to mitigate long-range attacks and ensure network liveness. Mechanisms include:

• Epoch Boundaries: Sets are finalized and rotated at the end of each epoch (e.g., every ~6.4 minutes on Ethereum).
• Jailing/Slashing: Faulty validators are forcibly slashed (penalized) and removed (jailed) from the active set.
• Churning: A subset of validators is periodically swapped out to prevent the formation of stable, potentially colluding groups.

Some protocols use multi-dimensional selection criteria beyond simple stake weight.

• Proof-of-History (PoH): Solana's leader schedule is deterministic and known in advance, based on a verifiable delay function.
• Proof-of-Elapsed-Time (PoET): Uses a fair lottery system (trusted execution environment) to select the next leader.
• Hybrid PoS/PoW: Networks like Decred use both stake and work for block validation and governance, creating a dual-layer selection process.

The selection mechanism is designed around economic security. Key concepts:

• Staking Minimums: Protocols often require a minimum stake (e.g., 32 ETH) to become a validator, balancing accessibility with network overhead.
• Stake Decay/Unbonding: To exit the set, validators enter an unbonding period (e.g., 7-28 days), preventing a rapid, destabilizing exodus.
• Sybil Resistance: The cost to create multiple validator identities (stake requirements) protects against Sybil attacks, making it economically irrational to attack the network.

A Sybil attack occurs when a single entity creates many fake identities (Sybils) to gain disproportionate influence. Proof-of-Stake (PoS) mitigates this by requiring validators to stake economic value (cryptocurrency). The cost of acquiring enough stake to attack the network is typically prohibitive, as it requires controlling a significant portion of the total supply. This creates a strong crypto-economic barrier against Sybil attacks, unlike permissionless systems with no cost of entry.

A long-range attack involves an attacker creating an alternative blockchain history from a point far in the past. This is a risk if an attacker can acquire a majority of private keys from past validators whose stake has since been withdrawn or slashed. Defenses include:

• Checkpointing: Periodically finalizing blocks so the chain cannot be rewritten before them.
• Weak Subjectivity: Requiring new nodes to trust a recent, verified block hash when syncing.
• Stake Lock-up Periods: Preventing validators from withdrawing and selling their stake immediately, reducing the availability of old keys.

If validator ownership becomes too concentrated among a few entities (e.g., large exchanges or staking pools), the network risks censorship and collusion. A cartel controlling over one-third of the stake can cause liveness failures, and over two-thirds can finalize invalid blocks. Risks include:

• Geopolitical risk: Validators concentrated in one jurisdiction.
• Infrastructure centralization: Over-reliance on a few cloud providers.
• Protocol changes may be blocked by cartels protecting their interests.

Stake grinding is an attack where a validator manipulates their local input (like a nonce or timestamp) to influence the pseudo-random selection of future block proposers or committee members. If successful, it can bias the validator set in their favor. Modern PoS protocols use Verifiable Random Functions (VRFs) or RANDAO—which incorporate each block's unpredictable proposer signature—to create unpredictable and unbiasable randomness for leader election, making grinding computationally infeasible.

In early PoS designs, validators had an incentive to vote on multiple blockchain forks during a reorg because it cost them nothing and they might gain rewards on whichever fork won. This could prevent consensus. The solution is slashing: penalizing validators a portion of their stake for provably malicious actions like double-signing (signing two conflicting blocks). Slashing creates a clear cost for supporting multiple chains, aligning validator incentives with a single canonical chain.

Validators are vulnerable to targeted DoS attacks that aim to knock them offline, causing them to miss their duties and be slashed for inactivity. Attack vectors include:

• Network-level DDoS: Flooding a validator's public IP address.
• Transaction spam: Filling blocks with computationally heavy transactions to slow down block processing.
• Resource exhaustion: Targeting the validator's mempool or consensus message queue. Mitigations involve using DDoS-protected infrastructure, sentry node architectures (hiding validator IPs), and gas mechanisms to limit spam.