Order of header field names in h= tag

From: Maarten Oelering <maarten_at_postmastery.net>
Date: Wed, 23 Mar 2016 21:58:49 +0100

I noticed that opendkim and other DKIM implementations list header field names from top to bottom in the “h=“ tag.
However only fields that occur once can be signed in that order. Multiple instances of the same field must be signed bottom first. See RFC 6376 section 5.4.2.

For example this header:

Resent-From: … (a)
Date: ...
From: …
Subject: ...
Resent-From: …. (b)

Results in "h=Resent-From:Date:From:Subject:Resent-From”. But the first field signed/verified is (b), and the last one is (a).

Does anyone know a reason for wanting to order field names top to bottom in “h=“?
If they were ordered bottom to top than signing implementations would be much simpler.

Thanks,

Maarten
Received on Wed Mar 23 2016 - 20:59:05 PST