Adam Back (born July 1970) is a British cryptographer and crypto-hacker.
He is the inventor of hashcash, the proof-of-work system used by several anti-spam systems. A similar system is used in bitcoin. Hashcash has also been used in a number of other protocols such as combating blog spam, and defending against user namespace pollution.
He is also known for pioneering the use of ultra-compact code with his 3-line RSA in Perl signature file and non-exportable T-shirts to protest the (now relaxed) United States cryptography export regulations.
Today he is CEO of Blockstream, a company that seeks to further the development of bitcoin and blockchain technology.