Candidate Block Definition in Crypto

A candidate block is a temporary block of transactions that a miner or validator proposes to add to a blockchain. It has not been accepted yet, so the network can either include it in the chain or reject it.

How a candidate block is assembled

Nodes pick unconfirmed transactions from the mempool and package them into a block-sized container. Because block space is limited, only a subset of pending transactions fits inside. Until the network validates it, that container remains a candidate block.

What goes into the block header

After selecting transactions, miners compute a Merkle tree over them to produce a single Merkle root. They place that root, the previous block’s hash, and a changing value called a nonce into the block header. Hashing this header yields the block’s hash, which uniquely identifies the candidate.

Role in proof of work

In proof of work, miners repeatedly hash the candidate block’s header while tweaking the nonce. The goal is to find a hash below a target value, often described as starting with a certain number of zeros. The first miner to find a valid hash broadcasts the candidate to the network for verification. If peers accept it, the block is recorded on chain and the miner earns the block reward.

Role in proof of stake

In proof of stake systems, the protocol selects a validator to propose a candidate block. Other validators check the proposal. With enough approvals, the candidate becomes a confirmed block.

From candidate to confirmed

Once a candidate passes the network’s checks, nodes add it to their local copy of the ledger and the proposer receives the reward. If the network rejects the candidate, its transactions go back to the mempool to be considered for future blocks.

A confirmed block is a block that has already been accepted into the chain. Any block before confirmation is simply a candidate or proposed block.

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