Uses
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. Sun Microsystems has used Hash Trees in the ZFS filesystem. Hash Trees are used in Google Wave protocol, Git distributed revision control system, the Tahoe-LAFS backup system, the Bitcoin peer-to-peer network, and a number of NoSQL systems like Apache Cassandra & Riak.
Hash trees were invented in 1979 by Ralph Merkle. The original purpose was to make it possible to efficiently handle many Lamport one-time signatures. Lamport signatures are believed to still be secure in the event that quantum computers become reality. Unfortunately each Lamport key can only be used to sign a single message. But combined with hash trees they can be used for many messages and then become a fairly efficient digital signature scheme.
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