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

[Eric Allman]

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