A Merkle proof in blockchain is a cryptographic proof that allows someone to verify that a specific piece of data (such as a transaction) is included in a larger dataset (such as a block in the blockchain) without having to reveal or download the entire dataset. It leverages the properties of Merkle trees (also known as hash trees), which are data structures with a digital fingerprint of the entire set of transactions in a blockchain, to organise and verify large amounts of data efficiently and securely.
A Merkle tree is a binary tree where leaves of the tree represent individual pieces of data (like transactions). Each non-leaf node in the tree is a hash of its two child nodes, and the root of the tree, called the Merkle root, is a single hash that represents all the data in the tree.
To prove that a specific piece of data (e.g., a transaction) is part of the Merkle tree, only a small subset of the tree is needed — specifically, the path from the transaction (leaf node) to the Merkle root. This path includes the hash of the target transaction and the hashes of the sibling nodes along the way up to the Merkle root.
The verifier, who knows the Merkle root, can use the Merkle proof to hash the transaction and the intermediate hashes and compare the final computed hash to the known Merkle root. If the computed hash matches the Merkle root, the verifier can be confident that the transaction is indeed part of the dataset (block) without needing the entire block’s data.
Verifiers can confirm the inclusion of a transaction without downloading the entire block or dataset, saving bandwidth and storage. Additionally, the cryptographic nature of Merkle proofs ensures that altering any data (like a transaction) in the tree would produce a completely different Merkle root, making it easy to detect tampering and malicious activity.
