[ietf-dkim] DKIM vs. MIME

[SM]

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