Hashcash is a denial-of-service counter measure tool. Its main current use
is to help hashcash users avoid losing email due to content based and
blacklist based anti-spam systems.

The hashcash tool allows you to create hashcash stamp to attach to emails
you send, and to verify hashcash stamp attached to emails you receive.
Email senders attach hashcash stamps with the X-Hashcash: header. Vendors
and authors of anti-spam tools are encouraged to exempt mail sent with
hashcash from their blacklists and content based filtering rules.

A hashcash stamp constitutes a proof-of-work which takes a parameterizable
amount of work to compute for the sender. The recipient can verify received
stamps efficiently. This package also includes a sha1 implementation which
behaves somewhat like md5sum, but with SHA1.