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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