merkle

Merkle is a surname of German origin. In technology, Merkle commonly refers to Ralph C. Merkle, an American computer scientist whose work on cryptography produced several foundational concepts including Merkle trees, Merkle puzzles, and the Merkle–Damgård construction.

Merkle trees: A Merkle tree is a binary hash tree in which each leaf node stores a cryptographic hash of data blocks, and each non-leaf node stores the hash of its two children. The root hash summarizes the entire set and enables efficient proofs of inclusion or exclusion; verifying a single data piece requires only O(log n) hashes.

Merkle–Damgård construction: A hash function construction by Ralph Merkle and Ivan Damgård that turns a variable-length message into a fixed-length output by iteratively hashing blocks; used in many early hash functions such as MD5, SHA-1; known for providing a general framework for building hash functions from compression functions.

Merkle’s puzzles: A 1974 key-exchange scheme in which a sender transmits a large set of encrypted puzzles, each with a different key. A party that solves one puzzle secretly obtains a key to encrypt a message; an eavesdropper must solve many puzzles to discover the same key. It helped motivate ideas around public-key cryptography.

Applications: In modern systems, Merkle trees underpin data integrity in blockchains and distributed file systems. Bitcoin uses a Merkle tree to commit to all transactions in a block; IPFS and Git use Merkle-DAGs to address content by hashes; Ethereum uses Merkle Patricia trees, a variant of Merkle trees.