Re: lists and DMARC and ARC, was Incoming SMTP in the year 2017 and absence of DKIM

[Michael Thomas <mike () mtcc com>]

On 11/29/2017 02:40 PM, Grant Taylor via NANOG wrote:
> On 11/29/2017 03:24 PM, Michael Thomas wrote:
> > Message footers and subject lines can be dealt with. That's already 
> > been proven within the current DKIM spec.
>
> Please humor my ignorance and explain how a subject line (which is 
> (over)signed) can be dealt with in the current DKIM spec?
>
> I get how footers can be dealt with, read appended.  At least as long 
> as DKIM only signs a given amount of the (original) body. (Though HTML 
> (read: MIME structures) can complicate this.)  -  Or are you referring 
> to something else?
You know what the original header was via the signature. You can take 
the delta of the current subject line and
remove any additions and validate the signature. Whether you're happy 
with the additions is a different concern,

If I were constructing a spam filter out of it, I'd give a lot of 
prejudice to anything added, but that's outside of
what you can do within the bounds of the spec.

Mike