John,
As long as you mentioned a PEM/MIME content type,
let me suggest one of my favorites: a canonical form
for digitally signed paper archives.
I know this may sound silly -- isn't PEM and digital
signatures supposed to replace all this old-fashioned
technology and save a few trees?
Well, I hope so. But experience teaches that
the newer the technology the shrter its life.
Cunieform tablets have been found that are
thousands of years old, and still readable.
Unfortunately, they are a little heavy for
bedtime reading.
Paper has been around for at least 1000 years, and
maybe close to 2000 if you count some of the Dead
Sea scrolls. Printed books have been around for 500
years, and most are still quite readable. Wax cylinder
records, 78s, and 33s are still playable, if a little
scratchy.
However, my stereo reel-to-reel tapes are now
in some cases unplayable, because of magnetic
print through and stretch distortion of the tape against
the flange. Cassette tape backups that I made for
my Radio Shack Pocket Computer just three years
ago can no longer be read.
Incompatible technology takes its toll, also.
I no longer have the ability to read some of the
quad-densitity disks I used in the mid-80s,
including some of my income tax records,
and my laptop can't read my 5-1/4" disks.
So I would like to be able to print out a hard copy
of my document and print the digital signature as well.
Then I would like to be able to OCR scan the
document back in again, and validate the signature.
However, this requires that the text be printed in
a canonical form that eliminates all multiple spaces,
especially at the ends of lines, eliminates blank lines
at the ends of pages, and eliminates such tricks as
backspaces and forms controls, especially things
like space-backspace-space.
In addition to the archival possibilities, this format
ought to be particularly useful when you want to FAX
a digitally signed document to someone, for
example to your hotel when you are on a business
trip, but you would like to have the ability to
authenticate the signature if necessary. It would also
add significant strength to contracts, notarized
documents, etc., where traditional paper may be
required for some time to come, instead of having to
initial each page.
Bob