Bob
The kind of certificate needed in a system like I'm suggesting
here can be very minimal and even self-generated. You don't, for
instance, have to worry about having any CA that issues the things. You
can just create an arbitrary public key and publish it via the
certificate server of your choice. The reason for this is that you are
asking the server's DNS server itself to tell you where to find the
server's key. As long as you trust that you are speaking to the server's
DNS, you can invest reasonable trust in any statements it makes.
Using signature to authenticate sounds like a good idea ;-). Plus
s/mime has been around for a while and server-to-server s/mime is
available in commercial products for several years now.
But the idea of using self-signed certificates (vs CA) is not
going to work well because they are free. The spammers will simply
do what they do today with the spamvertised URLs' hosts and keep
using new ones because domain cost is low enough (<$10 today).
Using a CA could introduce a new cost in the spammer economics
(plus force some basic validation depending on cert class).
But until the vast majority of the legitimate MTAs support
some verification (with or without digital signature), no one
can start to refuse unauthenticated inbound mails. So the
benefits in that (long) transition period seems to be:
- ability to increase probability of an email being spam if
sender is not verified.
- ability to have reliable white/black sender domain lists on
verified senders _if_ the cost of creating new 'verified
sender domains' is high enough.
jean
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