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In message <[email protected]
il.com>, Wei Chuang <[email protected]> writes
> Also we can require the From header to be aligned at every SMTP
> transaction boundary which implies From header munging at those
> boundaries.
to a first approximation no forwarding system does this today
this is not going to fly
> FWIW the current I-D calls for that by default, though
> that's pretty onerous, and there is an explicit escape hatch with
> the "unaligned" flag.
so something that no-one will switch over to doing has to be flagged up
specially in such a way that receivers will consider it dubious...
... though please recall the MAIL FROM (mf=) chain of custody already
requires an alignment at every hop and so it documents what has happened
to the mail and what systems did with it. This is of course just an
attestation and not something that anyone later in the chain can check,
but at least there is a clear record of what was said to have happened
and there so culpability can be pinned down if other forensic analysis
determines that systems have been trying to fool you.
Anyway, I recommend rethinking what you are trying to achieve here and
aligning with how forwarding systems actually work.
[then later in another reply....]
> I guess this is saying that
> chain of custody can be verified to i=n (highest) but not to i=1
> because the algebra doesn't work. To me that is WAI. Those
> earlier identities can't be claimed.
if you cannot verify the signing all the way back to i=1 then the
message is NOT validly signed.
The spec says that you only need to validate the highest numbered
DKIM2-Signature header field before accepting the message (and closing
the SMTP conversation and losing the opportunity to 5xx the transfer)
... but before you do anything else you have to do a full check.
Now by local policy (ie laziness) you may skip all the other checking,
but if so you will need to take steps to ensure that the message does
not then leave your organisation.
Now quite WHY your local policy would be to avoid doing all the checks
back to i=1 in order to get all the warm and fuzzy feelings you would
get from knowing that the message was not completely forged by the
system that gave it to you I could not say. But local is local and not
for others to criticise.
- --
richard @ highwayman . com "Nothing seems the same
Still you never see the change from day to day
And no-one notices the customs slip away"
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