Dr Adam Back is a well known British cryptographer and crypto-hacker.

He is the inventor of hashcash , a denial-of-service counter-measure based on the concept of proof-of-work done by the sender or client. Hashcash is currently used in a number of systems, including email (anti-spam), blog-spam defense (Hashcash wordpress ) in some mail2news gateways used by anonymous remailers and in the Self-Certifying Filesystem for resource protection. The initial use for hashcash was in the email setting as an anti-spam technology and for anonymous remailers as a defense against anonymous spam abuse. Hashcash is non-interactive and has no infrastructure requirement.

He has an interest in privacy technology, electronic cash (of the payer and payee anonymous type), and is author of the credlib library that provides implementations of Dr. Stefan Brands and Dr. David Chaum's respective credential technologies. (These technologies can be used as a privacy preserving credential / certificate technology and there are also anonymous electronic cash settings for both systems.)

He was the first to formalize the Non-Interactive Forward-Secrecy security property for email and first to observe that any Identity Based Encryption (IBE) can be used to give Non-Interactive Forward Secrecy .

He is also known for starting the trend of making political statements with ultra compact code -- size-hacked code -- the export-a-crypto-system .signature -- in relation to the then US export regulations preventing export of crypto software from the US. (These regulations have since been relaxed, one could speculate in some very small part because of the export-a-crypt-system effort to ridicule the then regulations). There was also a t-shirt of the signature . The compact code in policital statements trend has been continued more recently with the anti-copyright proponents implementing a compact version of CSS (the DVD scrambling code) and a related t-shirt.