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The multipart/alternative construct was deemed more useful for backward compatability to non-MIME readers. This is because the "alternative" part is in readable ASCII. The use of multipart/signed infers a requirement that the "signed" text be transported from sender to recipient intact. In practice, this would cause a tendency to encode the text for transport rendering it unreadable by non-MIME readers.
Are you assuming that most messages will be textual or is that simlpy the problem set at hand? If we divide MIME messages into two groups, textual and multi-media, it's easier to identify problems with the above logic. If the message is multi-media, chances are that that a recipient without a MIME-aware user agent will not be able to read it no matter which protocol is used. This is especially true when things are binary (e.g., pictures, sounds, spreadsheets) or not generally meant for direct human consumption (e.g., PostScript), but also applies to messages containing a mix of text and multi-media, as the multi-media will obscure the text. So, in the case of multi-media messages, S/MIME doubles and then some the size of the message and offers no visible advantage. If the message is textual, chances are that it can be included in a MOSS message without any transport encoding. If any part of the text containes characters unsafe for email transport, quoted-printable encoding can be used. Quoted printable messages are generally quite readable without being decoded. While S/MIME would provide a readable message to a recipient without MIME-aware software, it would be double and then some the size of the MOSS message. My reading of the situation is that MOSS messages will be as readable as S/MIME messages for folks without MIME-aware user agents. S/MIME messages will be double the size and that will persist even after backward compatibility is no longer a [major] concern. And given the acceptance of MIME and multi-media in general, I don't see compatibility a long-term problem. Has anyone without a MIME-aware user agent had trouble reading my messages? Mark
binNCa9XrDpQ6.bin
Description: application/moss-signature