Markus,
At 12:50 PM 12/17/2003, Markus Stumpf wrote:
On Wed, Dec 10, 2003 at 05:05:07PM -0800, Mark Baugher wrote:
> It would not verify if the signature design was secure. That is,
> one should not sign a piece of the message that could be cut and
> pasted in this way. I have not seen anything about the signing
> proposal to suggest that they were doing this or even signing SMTP
> trace record.
>
> So yes, the problem you describe is a problem that's caused by a
> poor signature design.
The problem is that I yet have to see a signature design that will work
and that will not be broken by a sufficiently large number of mailservers/
features out there.
There are many alternative signature designs. One may sign the actual
mail message, or the design could be to "sign" the TLS connection between
mail agents, for example. Furthermore, the signature could attest
to the address or the signature could attest to the fact that a
particular mail operator forwarded the mail. Here are four alternatives
and there may be more.
Which of these are you thinking of or are you thinking of all of them?
> What gets signed is an open question. This does not strike me as
> an insurmountable problem.
:-) IMHO exactly this is the problem that hasn't been solved yet.
Before going further down this path. Is the problem that you address
concerned with adding a signature to the message and having that
message re-written in ways to invalidate the signature?
Cheers, Mark
> I think of each message sent as a separate mail transaction that will
> have a separate signature applied by the mail submission agent.
What is the advantage then?
I know who I am talking to directly. So the sending MTA doesn't need the
hassle to sign the message. The MTAs could simply use TLS or a key
exchange. The advantage of the signing is that I - as the receiver -
can validate the signing of the original sender or - that would be fine -
of all intermediate hosts.
Other than that it's pretty worthless, IMHO.
\Maex
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