Merkle tree

In cryptography and computer science, a hash tree or Merkle tree is a tree in which every non-leaf node is labelled with the hash of the labels or values (in case of leaves) of its child nodes. Hash trees allow efficient and secure verification of the contents of large data structures. Hash trees are a generalization of hash lists and hash chains.

Demonstrating that a leaf node is a part of the given hash tree requires processing an amount of data proportional to the logarithm of the number of nodes of the tree; this contrasts with hash lists, where the amount is proportional to the number of nodes.

The concept of hash trees is named after Ralph Merkle who patented it in 1979.

Hash trees can be used to verify any kind of data stored, handled and transferred in and between computers. Currently the main use of hash trees is to make sure that data blocks received from other peers in a peer-to-peer network are received undamaged and unaltered, and even to check that the other peers do not lie and send fake blocks. Suggestions have been made to use hash trees in trusted computing systems.

Hash trees are used in the IPFS, Btrfs and ZFS file systems (to counter data degradation), BitTorrent protocol, Apache Wave protocol,Git distributed revision control system, the Tahoe-LAFS backup system, the Bitcoin peer-to-peer network, the Ethereum peer-to-peer network, the Certificate Transparency framework, and a number of NoSQL systems like Apache Cassandra, Riak and Dynamo.

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