Hashcash is an anti-spam / denial of service counter-measure tool.
A hashcash stamp constitutes a proof-of-work which takes a parameterizable amount of work to compute for the sender. The recipient (and indeed anyone as it is publicly auditable) can verify received hashcash stamps efficiently.
E-mail senders attach hashcash stamps with the X-Hashcash header. Vendors and authors of anti-spam tools are encouraged to exempt e-mail sent with hashcash from their blacklists and content-based filtering rules.
This library provides basic utilities for hashcash creation and validation.