I'm not as allergic to S/MIME as Ned and Dave, simply because I haven't
perceived S/MIME as a "competitor" for MOSS. It doesn't secure MIME
message elements, it simply specifies a single MIME encapsulation for
PKCS7 messages. The two offer different services at different levels.
If I want to be able to sign/verify/encrypt MIME messages and message
elements, I'll use MOSS. If I want to allow two PKCS7 messaging systems
to interoperate via SMTP, I'll use S/MIME. I actually see no reason
to integrate the two--S/MIME is simply a standard labeling for a particular
file format, and such is more akin to "application/pdf" or
"video/quicktime" than "multipart/signed" or "multipart/encrypted".
Actually, S/MIME is not for encapsulating general files. It is for
MIME encapsulation. From the spec:
4.2 Format of the signed or enveloped data
PKCS #7 places no requirements on the format of the data
which is signed or enveloped. However, for use in
application/x-pkcs7-mime, the signed or enveloped data must
itself be a MIME body part. Therefore, when a MIME agent
receives an application/x-pkcs7-mime, the result of removing
the signature or envelope can be passed directly to the
normal MIME-processing software.
That's why it's application/x-pkcs7-mime and not just
application/x-pkcs7.