Re: [ietf-smtp] Quoted-Printable-8bit and downgrade

Viktor Dukhovni <ietf-dane@dukhovni.org> Wed, 31 March 2021 20:53 UTC

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From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Subject: Re: [ietf-smtp] Quoted-Printable-8bit and downgrade
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> On Mar 31, 2021, at 3:40 PM, John Levine <johnl@taugh.com> wrote:
> 
> Interesting question.  I presume there are still MTAs that can do downgrades
> but how common is it in practice?

Postfix will do 7bit downgrade by default if the content is designated
or detected as 8bit and the remote MTA does not advertise 8BITIME support:

  http://www.postfix.org/postconf.5.html#disable_mime_output_conversion

> It breaks DKIM signatures unless the MTA can find the signing keys and resign
> on the fly which seems a bit much.

For outbound mail, where the MTA/MSA is also the one doing the signing, the
safest thing is to just downgrade all messages before signing, so that
there's never any post-signature dynamic downgrade during delivery.  This
can be done by passing through an internal SMTP content filter that does
not offer 8BITMIME.  I expect most administrators don't do this.  In which
case any submitted 8BITMIME message may get downgraded after signing.

Since most MUAs do quoted-printable defensively, actual downgrading at
the MSA is rare.

One can also of course configure "disable_mime_output_conversion = yes",
but I expect most administrators don't.

I don't do DKIM, so I have personal experience to report.

-- 
	Viktor.