Use of multipart/encrypted

Bill Wohler (wohler@uluru.worldtalk.com)
Thu, 22 Feb 1996 14:57:00 -0800 

  I created an example to explain to a colleague our discussion at the
  end of the day regarding multipart/encrypted and thought I'd share
  it with the rest of you.  By requiring S/MIME to use
  multipart/encrypted, it would make it easy for mailers to use
  plug-in encrypters/decrypters.  The "protocol" parameter would
  specify which plug-in to use.

  Since parameters can't (yet) be used in mailcap files to specify a
  body part viewer, a shell-script front-end might be necessary to
  read the protocol parameter and execute the specified decrypter.

  For example, assume the following message based on RFC 1874:

	To: wohler@uluru.worldtalk.com
	From:    Bill Wohler <wohler@uluru.worldtalk.com>
	Subject: test
	Mime-Version: 1.0
	Content-Type: multipart/mixed; boundary="----- =_aaaaaaaaaa0"

	------- =_aaaaaaaaaa0
	Content-Type: multipart/encrypted; boundary="----- =_aaaaaaaaaa1";
		protocol="foo/bar"

	------- =_aaaaaaaaaa1
	Content-Type: foo/bar

	Control information...

	------- =_aaaaaaaaaa1
	Content-Type: application/octet-stream
	Content-Transfer-Encoding: base64

	TGludXggMS4xLjU5LiAoUE9TSVgpLgo=

	------- =_aaaaaaaaaa1--

	------- =_aaaaaaaaaa0--


  and a mailcap (mhn_defaults in MH, grrr!) entry like:

	mhn-show-multipart/encrypted: /usr/local/bin/decrypt '%f' %a

  Decrypt would accept 3 parameters (in this case, mhn would pass two
  filenames for the single %f escape).  The first two parameters would
  be filenames containing the control and encrypted information
  respectively.  The third argument would be `protocol="foo/bar"',
  which would be used to execute a suitable decrypter which would act
  upon the two files.

  Questions: Is this a reasonable example of "plug-in" usage?  Is the
  %a escape common in mailcap files?  (Is there a mailcap standard?)
  Is there a security problem with using intermediary files?  Is this
  why we were shooting towards or did we have a loftier goal of moving
  more control information into the control block so that the
  encrypted block might actually be used by different decryption
  schemes (PGP, S/MIME), but using a convergent technology?
  
Bill Wohler <wohler@newt.com>   ph: +1-415-854-1857  fax: +1-415-854-3195
Say it with MIME.  Maintainer of comp.mail.mh and news.software.nn FAQs.
If you're passed on the right, you're in the wrong lane.