Arvel Hathcock:
The purpose of a valid DKIM signature is to identify the party that
signed the message.
Here, you are completely correct.
Whether this is a first-party or third-party signature is largely
irrelevant.
Here, you are correct only if you restrict your vision to DKIM-BASE.
Once we start talking about DKIM-SSP, first-party vs third-party
becomes extremely relevant.
The importance of first/third party is easily overstated.
Here is an example why first-party signatures can be dangerous.
If I get mail with a perfectly valid first-party DKIM signature,
it could very well be a cleverly disguised domain clone attack
(say, bigbank versus big-bank etc.). Naively believing a valid
first-party DKIM signature can be a costly mistake.
And here is an example why third-party signatures can be safe.
If I receive mail from my bank and I know their signing domain,
then it does not matter what the from domain says. I already know
that the mail comes from the bank, regardless of whether this mail
has a first-party or third-party signature. The signing domain is
the basis for trust.
To summarize: naively believing a valid first-party DKIM signature
can be a costly mistake. The signing domain is a better basis for
trust.
Wietse
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