Re: [dmarc-ietf] Reversing modifications from mailing lists

[John Levine <[e-mail]>]

It appears that  <[e-mail]> said:
>> It appears that Wei Chuang  <[e-mail]> said:
>> > If the RFC2045 canonical representation at the final destination can be the
>> > same as the canonical representation at the original sender, ...
>
>> When we were working on DKIM canonicalization we had lengthy discussions about
>> what to do about MIME and we decided not to even try.
>
>A mistake IMO.

This was part of the discussion about what sort of body modifications
to allow. We ended up with optionally ignoring white space changes,
and l= to ignore added text. My impression is that neither is useful.
Very few messages pass with relaxed canonicalization that don't also
pass strict. The goal of l= was to allow mailing lists to add footers,
but as we've seen in this discussion, if a list adds a footer it's
likely to make other changes too.  I think the main use case for
relaxed mode was an old bug in sendmail that added an extra \r\n
on the way through, but it's long gone.

For MIME, the question wasn't just whether two versions of messages
were equivalent, but the impossible question of what other changes
keep the message "the same" and which are too different. As you note
there are lots of ways that a message could be recoded into equivalent
MIME parts, but again it is my impression that those sorts of changes
are rare without also adding or removing body parts which gets us into
the swamp of how different is too different. So we didn't try.

R's,
John