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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