Amir Herzberg wrote:
Jim Fenton wrote:
I have just submitted a new threat analysis of DKIM as an Internet
Draft. Hopefully this will form the basis for a meaningful
discussion on the utility and effectiveness of DKIM that were voiced
at the last BoF.
I think it is very good.
I have only one real reservation. In section 6.3, discussing the
message replay attack, esp. in 2nd paragraph... It is presented as if
DKIM cannot be applied against replay since replay is
indistinguishable from acceptable acts e.g. forwarding. This is not
necessarily true. A legitimate application of DKIM may require senders
to indicate specific recipient; this would allow replay prevention, of
course in the price of requiring additional support to deal with
legitimate forwarding. I'm not suggesting DKIM should be modified to
support that, indeed this is not required at DKIM level at all, but I
think the text now seems to exclude this usage, and this should be
fixed imho.
What matters here is really the envelope-to address. There are some
fairly large obstacles to signing the envelope-to address, including the
fact that it's not available to MUAs and the fact that it gets changed
when a message is legitimately forwarded, so I'm not sure how this would
work. Can you provide more details of what you have in mind?
Here are few additional, minor comments:
1. You use the term `zombie` without definition in p. 2, then
`compromised computers` later (in 5.1)... pick one; my suggestion: use
`zombie` and in the first use, add `(compromised computers)`.
Good idea.
2. First paragraph of 4.1: I think should be clarified.
I'll try, but if you have some text to suggest that would be helpful.
3. First sentence of 4.3: s/with/within/
Thanks.
4. Last paragraph of 4.3: this is only MTA-MTA or MTA-MDA
authentication, so I think we should explicitly recommend
cryptographic authentication of submitting MTA, e.g. using SSL or
IP-Sec. Or do you really think SMTP AUTH is better here?
SMTP AUTH was just meant as one example. What technique is actually
used is at the discretion of the recipient domain, and whether it needs
to be cryptographic or not depends on the security of that domain.
5. In 5.2.1: last sentence is imho misleading. Such malware
usually/often does not use the email address of the owner of the
infected machine, but selects other email addresses as sender, to
avoid detection. In this case, DKIM may help. I also think the term
`malware` is better than `worm` here.
I prefer 'malware' too.
Thanks!
-Jim
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