Merkle Root
The single 32-byte hash at the top of a block's Merkle tree, summarizing all included transactions.
The Merkle root is the single 32-byte hash sitting in every block header that commits to every transaction in the block.
It's computed by building a Merkle tree over the block's transaction IDs: pair them up, hash each pair, then pair the resulting hashes, then pair those, until one hash remains. That final hash is the Merkle root.
Three things make it load-bearing:
- It binds transactions to the block. Change any transaction's data and the Merkle root changes. Change the Merkle root and the block header changes. Change the header and you have to re-do the proof-of-work. The cost of tampering is the full re-mining cost.
- It's what miners are actually hashing over. The block-header hash includes the Merkle root. When a miner varies their nonce looking for a valid block, the Merkle root is one of the inputs. (When they exhaust nonces, they tweak the coinbase transaction's extranonce, which changes the Merkle root, which gives them a fresh nonce space.)
- It enables SPV. Light clients can verify their transactions are in the chain by downloading just block headers plus a
log2(N)-sized Merkle proof per transaction.
A 32-byte commitment to potentially gigabytes of transaction data, with no information loss as far as inclusion is concerned. One of the quietest pieces of design elegance in Bitcoin.
See Merkle Tree / Merkle Root for the construction in detail.
Key takeaways
- Final hash encapsulating every transaction in the block
- Integral to proof-of-work since miners hash the block header
- One small alteration in transactions would break the chain of hashes