[ANNOUNCE] contrib/hashcash.el spam fighter

[Simon Josefsson <jas@extundo.com>]

Pay Hashcash
============

   A novel technique to fight spam is to require senders to do something
costly for each message they send.  This has the obvious drawback that
you cannot rely on that everyone uses this technique, since it is
optional, but it may be useful in smaller communities.

   The "something costly" is to burn CPU time, more specifically to
compute a hash collision up to a certain number of bits.  The resulting
hashcash cookie is inserted in a `X-Hashcash:' header. For more
details, and for the external application `hashcash' you need to
install to use this feature, see
`http://www.cypherspace.org/~adam/hashcash/'.  Even more information
can be found at `http://www.camram.org/'.

   If you wish to call hashcash for each message you send, say something
like:

     (require 'hashcash)
     (add-hook 'message-send-hook 'mail-add-payment)

   The `hashcash.el' library can be found at
`http://users.actrix.gen.nz/mycroft/hashcash.el', or in the Gnus
development contrib directory.

   You will need to set up some additional variables as well:

`hashcash-default-payment'
     This variable indicates the default number of bits the hash
     collision should consist of.  By default this is 0, meaning
     nothing will be done.  Suggested useful values include 17 to 29.

`hashcash-payment-alist'
     Some receivers may require you to spend burn more CPU time than the
     default.  This variable contains a list of `(ADDR AMOUNT)' cells,
     where ADDR is the receiver (email address or newsgroup) and AMOUNT
     is the number of bits in the collision that is needed.  It can also
     contain `(ADDR STRING AMOUNT)' cells, where the STRING is the
     string to use (normally the email address or newsgroup name is
     used).

`hashcash'
     Where the `hashcash' binary is installed.

   Currently there is no built in functionality in Gnus to verify
hashcash cookies, it is expected that this is performed by your hand
customized mail filtering scripts.  Improvements in this area would be
a useful contribution, however.