[Cryptography] How should we encrypt external mail attachments

[Derek Fawcus]

On Sun, Aug 01, 2021 at 02:59:39PM -0400, John Levine wrote:
> It turns out that back in the 1990s MIME defined message/external-body in
> RFCs 2017, 2046 and 2912, which lets you include the pointer and metadata
> like the size and an expiration date, and treat the external file as though
> it was part of the message.  We're trying to resuscitate it
> and make the semantics as close as possible to an attachment in the message itself.

Just FYI, the mutt MUA makes use of that encoding if one deletes an attachment/part:

 --XXX
  Content-Type: message/external-body; access-type=x-mutt-deleted;
          expiration="Mon, 2 Aug 2021 17:42:46 +0100"; length=6244

  Content-Type: text/html; charset="UTF-8"
  Content-Transfer-Encoding: quoted-printable

DF