A proof-of-work system designed by Adam Back in the late 1990s to limit email spam and distributed denial-of-service (DDOS) attacks. For example, if every email system was required to spend an extra 15 seconds before releasing an outgoing message, compromised computers could not quickly spit out thousands of messages. Bitcoin's Satoshi Nakamoto was inspired by the Hashcash proof-of-work concept, which became an integral part of the Bitcoin blockchain process. See
Bitcoin and
botnet.