Content-Type: multipart/signed;
--Signed Message
Content-Type: text/plain
Note: please send refunded monies to Jeff Thompson
We have returned item #4425 for full refund.
--Signed Message
Content-Type: application/signature
<pemsig>
--Signed Message--
The recipient may look at the original cleartext version of the
message and see the note to divert the refunded money. The PEM
application will indicate that the signature is valid since the
signed text has not been altered. Is this a security problem?
Some possibilities:
A. It is not a problem since the MIME agent will process the
formatted message before the user sees it and only display the
signed text. (But then why did the MIME standard go to the
trouble of making unprocessed messages readable?)
According to the latest draft of the "Security Multiparts for MIME"
document, the headers:
Content-Type: text/plain
Note: please send refunded monies to Jeff Thompson
are included in the signature calculation. If the message had been
encrypted, they would have been included in the encrypted text (and thus
unrecognizable to all but a valid recipient).
When we wrote the security multiparts specification for MIME we
explicitly included the headers in the signature calculation for just
this reason.
Therefore, none of the remaining options:
B. The MIME agent should "beep real loud" when it comes across
unrecognized header fields. (This makes it hard to use experimental
or new fields.)
C. It is serious enough that unrecognized fields should be disallowed.
D. Designing the standard to allow casual inspection of secured
messages is a Bad Idea. (Perhaps the message text should always be
base64 encoded.)
are necessary.
A still relevant question is what should a MIME agent display to a user.
In principle, the MIME agent should only display the content to the
user, not the headers. Thus, the header:
Note: please send refunded monies to Jeff Thompson
would normally not be displayed to the user. However, this question
more precisely belongs on the ietf-822(_at_)dimacs(_dot_)rutgers(_dot_)edu
mailing list.
Jim
binpeH7pyPlcf.bin
Description: application/pem-signature