Subject: [ietf-dkim] (-base document) clarifications on use of l= tag

From: Eric Allman <[email-masked]>
Date: Wed, 15 Feb 2006 14:13:31 -0800

As a result of a discussion at the DKIM Summit on Monday, I have 
added the following explanatory text to the -base document.  Please 
let me know if I haven't been clear enough.

At the end of Section 5.5 (Compute the Message Hash):

        INFORMATIVE NOTE: A possible value to include in the "l=" tag
        would include the entire length of the message being signed,
        thereby allowing intermediate agents to append further
        information to the message without breaking the signature
        (e.g., a mailing list manager might add unsubscribe
        information). A signer wishing to permit such intermediate
        agents to add another MIME body part to a "multipart/mixed"
        message should use a length that covers the entire presented
        message except for the trailing "--CRLF" characters; this is
        known as the "N-4" approach.

At the end of Section 6.3 (Compute the Verification):

        INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the
        body at the indicated body length might pass on a malformed
        MIME message if the signer used the "N-4" trick described in
        the informative note in Section 5.5 (Compute the Message
        Hash). Such verifiers may wish to check for this case and
        include a trailing "--CRLF" to avoid breaking the MIME
        structure. A simple way to achieve this might be to append
        "--CRLF" to any "multipart" message with a body length; if
        the MIME structure is already correctly formed, this will
        appear in the postlude and will not be displayed to the end
        user.

eric
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From: Frank Ellermann <[email-masked]>
Date: Thu, 16 Feb 2006 01:05:29 +0100

Eric Allman wrote:

> DKIM Summit on Monday

In another WG the Chair was quite upset when somebody so much
as proposed an "informal" meeting.  Here it's apparently okay
to have unannounced (?) DKIM summits.

>| (e.g., a mailing list manager might add unsubscribe
>|  information).

Better say "(e.g. some mailing list managers inappropriately
add unsubscribe info to the body, in addition to or lieu of
the header field for this purpose)".

The DKIM l= is merely a kludge for an utter dubious practice.

> use a length that covers the entire presented message except
> for the trailing "--CRLF" characters; this is known as the
> "N-4" approach.

 <http://tools.ietf.org/html/2046#page-22>
| multipart-body := [preamble CRLF]
|                   dash-boundary transport-padding CRLF
|                   body-part *encapsulation
|                   close-delimiter transport-padding
|                   [CRLF epilogue]

That "4" is a minimum, there could be some junk between "--"
and the final CRLF.

The best l= tricks would fail if the original Content-Type is
not any multipart, e.g. a simple text/html or message/rfc822,
and a weird "list manager" transforms it into multipart/mixed.

                           Bye, Frank