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Definition: Merkle tree


A hierarchy of hashes, which are mathematical summaries of the given data. Each leaf in the tree is a hash of data, and the root of the tree (the Merkle root) is a hashed summary of the entire tree. Merkle trees (hash trees) are used to verify that the data have not been tampered with.

The Root Is the Hash of Everything
In a blockchain such as Bitcoin, the Merkle root is the hash of all the crypto transactions in the block, and the Merkle root is stored in the header of each block.

Obtaining the Root
Transaction data are recursively hashed twice using the SHA-256 cryptographic algorithm until there is only one hash value remaining: the "Merkle root." The root is very small in size and can be quickly processed to determine if a transaction resides within the block. For example, 16 transactions in a block produce a 128-byte Merkle path, but 65,535 transactions take up only 512 bytes. See blockchain, hash function and cryptographic hash function.




Blockchain Headers
The Merkle root is stored in the headers of the blocks in the blockchain.