add support for PGP and S/MIME encryption and signing

[vanbaal]

This patch is based upon a patch by Stefan Schlott (
http://medien.informatik.uni-ulm.de/~stefan/gpg-mailman.html )

It extends Mailman to:

- A post will be distributed only if the PGP signature on the post is from
  one of the list members.
- For sending encrypted email, a list member encrypts to the public key of
  the list. The post will be decrypted and re-encrypted to the public keys
  of all list members.
(Later, the patch will handle RFC 2633 (S/MIME) messages too, next to RFC 2440
(OpenPGP)).

In order to achieve this, each list has a public and private key, as well
as a key passphrase. Furthermore, new list settings are defined:

 gpg_postings_allowed: Is it allowed to send to this list postings which are
  encrypted with the GPG list key?
 gpg_msg_distribution: Are subscribers allowed (or even forced) to upload
  their GPG public key in order to receive all messages encrypted?
 gpg_post_sign: Should posts be GPG signed with an acknowledged subscriber key
  before being distributed?
 gpg_msg_sign: Should the server sign encrypted messages?

Finally, each subscriber can upload her PGP public key using the webinterface.

Latest version of the patch is available from
http://www.non-gnu.uvt.nl/pub/mailman/ .

[vanbaal (vanbaal)]

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The patch fully supports S/MIME too.

Between 2006-01 and 2006-10, no work has been done on this
patch. It applies to Mailman 2.1.7 only.

[mnaumann (mnaumann-users)]

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This patch has since been updated for 2.1.9:
http://ulm.ccc.de/pipermail/ssls-dev/2008-January/000003.html
http://<email address hidden>/msg10530.html