All too frequently a new user's first experience of S/MIME is
being flamed for wasting bandwidth by signing everything in a public
mailing list or newsgroup.
Well, people shouldn't sign something unless there is value in being able
to authenticate the sender. Nor should the HTML-encode things for which
formatting adds no value.
By contrast the more compact PGP signatures
seem to be tolerated more.
The PGP world tolerates more than the non-PGP world.
S/MIME signatures could be very small, indeed, if the CAs would pare down
what they toss into the signatures. So far, most of the flames have been
aimed at people with bloated VeriSign certs. I've seen much smaller certs
from other CAs.
I would support the specification stating that an S/MIME agent either
SHOULD (or even MUST) give the user the option to not include
certificates in signed messages.
We cannot specify how an MUA interacts with a human user; that's out of
scope for a spec like this. At most, we might put in a sentence saying "A
sending MUA may want to consider the size of certs being added to a message
when it decides (or lets the sender decide) whether or not to include a
cert in the message."
--Paul Hoffman, Director
--Internet Mail Consortium