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.
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.
Just identifying this as another point of interest with respect to MLM BCP
stuff we might wish to produce (which is starting to sound like a more urgent
matter than it did before). Maybe I'll start drafting something.
-MSK
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