william(at)elan.net wrote:
- I find it odd that the header field that will contain the
signature (DKIM-Signature) must also be included in the
signing/verification process.
The reasons for including data from signature header itself is to make
it less useful for replay attacks, for example an attacker could take a
signature and replace some of its key parts (like change expiration,
change signer name and domain, etc) and then introduce it as his own.
By including key data tags, the range of replay attacks is reduced.
It's not really replay, per se, but to protect the fields
so they can't be modified in transit. By protecting everything
now, you get forward compatibility of new tags being introduced.
What I'm slightly concerned however is that DKIM says that all tags
are to be included (except 'b') but there maybe reasons to introduce
extensions as new tags with data that is not to be included so I think
opt-out option for unknown tags should be made available.
Receivers MUST ignore unknown tags is in the current draft,
so you are future proofed.
Mike