I personally favor a message/rfc822 parameter, but I can also see a case for
putting it elsewhere. What do other people think? If there seems to be
consensus that this needs to be on message/rfc822, I'd be happy to write
a short draft defining such a parameter.
It sounds as if this might be a role for content-disposition. In a
non-RFC822 environment, one might wish to apply the "promotion" principle
for content types other than message/rfc822, and to encapsulations other
than signature/encryption.
I considered a content-disposition value, but quickly rejected it. My reasons
include:
(1) This usage is specific to message/rfc822; it doesn't make sense on other
content types. Dispositions are supposed to apply regardless of type, and
definitely shouldn't be highly type-specific.
(2) Applications may want to specify a disposition for a message independent
of security handling. This results in the one field being used for two
things.
(3) This isn't a disposition per se; it is structural handling information.
Ned