Hello Neal,
You are getting into a topic that has my interest. To me, a mailing
list is just an example of encryption things to a group.
I read your proposal of back-then, but it is not wholly clear to me:
* You want to protect the list of subscribers you say; are these the
email addresses or key identities that you wish to protect?
* You say that you don't like re-encryption; is this for reasons of
efficiency, or for reasons of passing the plaintext through the control
of the list owner (who is likely to subscribe and therefore receive the
text anyway)?
Since mailing lists are sort-of a hack in the mail system, you might
consider doing it entirely differently. For instance, downloading list
mail over IMAP, which gives subscribers the initiative so they don't
need to present an email address. Sending might be done over SMTP or
even over IMAP. Being searchable, this also makes for a great document
repository :)
As for re-encryption efficiency, you could decide to re-package the
session key to (only authorized) public keys; one way you could find
those is from STARTTLS with an OpenPGP credential, but that would impose
restrictions on the mail client, or require it go through a SOCKS proxy
or such. Towards the latter, we are working on a TLS Pool that could
make it straightforward to build such a proxy, http://tlspool.arpa2.net
and which implements OpenPGP over TLS.
-Rick
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