Hashgraph

Hashgraph is a distributed ledger technology that has been described as an alternative to blockchains. The hashgraph technology is currently patented, is used by the public ledger Hedera, and there is a grant to implement the patent as a result of the Apache 2.0's Grant of Patent License (provision #3) so long as the implementation conforms to the terms of the Apache license.^[1] The native cryptocurrency of the Hedera Hashgraph system is HBAR.

Unlike blockchains, hashgraphs do not bundle data into blocks or use miners to validate transactions. Instead, hashgraphs use a "gossip about gossip" protocol where the individual nodes on the network "gossip" about transactions to create directed acyclic graphs that time-sequence transactions.^[2] Each "gossip" message contains one or more transactions plus a timestamp, a digital signature, and cryptographic hashes of two earlier events. This makes Hashgraph form an asynchronous Byzantine Fault-Tolerant (aBFT) consensus algorithm.^[3]

The process to achieve consensus also relies on a variant of Proof of Stake. Although the content of the transactions themselves is not subjected to consensus because it is handled by the hashgraph itself, their ordering and timestamp still needs to be. A modified version of Proof of Stake is used for this purpose, characterized by not submitting votes as messages in the protocol but rather by "virtual voting", i.e. applying a voting algorithm to calculate what other witnesses of an event would have to vote based on their copies of the hashgraph, and weighting the votes according to the stake of the voter. This is argued to increase efficiency by reducing message-passing.^[4]

Hashgraph was invented in the mid-2010s by the American computer scientist Leemon Baird. Baird is the co-founder and chief technical officer of Swirlds, a company that holds patents covering the hashgraph algorithm.^[5][6]