At 21:22 24-04-10, Murray S. Kucherawy wrote:
Someone on the opendkim-users list has pointed out that DKIM
signatures are being invalidated when re-mailed through one
particular MLM that rewrites Content-Type: so that its value is all
lowercase. Obviously this is a problem for DKIM since even
"relaxed" requires nothing other than spacing changes in header
field values; however RFC2045 says that the interpretation of
Content-Type: values is case-insensitive. Thus, at least to
consumers of that header field, DKIM is doing something "wrong". In
any case, it was suggested on that list that "relaxed" header
canonicalization be adjusted to accommodate this.
If I am not mistaken, it's a MTA and not a MLM that did that modification.
The response is probably obvious: DKIM operates at a level below the
semantics of the header fields it signs, i.e. closer to SMTP than to
MIME, and so it hasn't, and shouldn't have, knowledge about header
field interpretation. I suspect this isn't something we consider a
problem that needs fixing in the spec.
I would categorize it as operating close to the (signer/verifier)
boundaries to get around in-transit modifications to the mail format.
Regards,
-sm
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