“Built on the widely available SHA-1 algorithm, hashcash is a
clever system that requires a parameterizable amount of work on the
part of a requester while staying ‘cheap’ for an evaluator to
check. In other words, the sender has to do real work to put
something into your inbox. You can certainly use hashcash in
preventing spam, but it has other applications as well, including
keeping spam off of Wikis and speeding the work of distributed
parallel applications. In this article, you’ll meet David’s own
Python-based hashcash implementation…”