Subject: [EAI] Proposal for decision, approach for downgrading in UTF8SMTP

From: Harald Alvestrand <[email-masked]>
Date: Tue, 15 Aug 2006 04:41:09 -0700

In draft-ietf-eai-downgrade-01 section 5, the editors have identified a 
design decision that the WG needs to take: Whether to use an 
encapsulation strategy or a conversion strategy when downgrading 
messages from an UTF8SMTP user to an ASCII user.

A few of us (editors + chairs) have had a discussion about this, and 
reached a proposed resolution:

- It is of paramount importance that ASCII users are able to interpret 
downgraded messages without special software. Therefore, a conversion 
approach MUST be used, and this will be documented in the next version.

- In some cases (MUAs that attach both to UTF8SMTP MTAs and ASCII MTAs, 
for instance), accurate reconstruction of the UTF8SMTP headers can 
provide some benefit to the user. One way of accomplishing this is by 
attaching a copy of the headers as they were before downgrading to the 
downgraded message, attached as a special (new) MIME body part. However, 
this also carries security risks, and makes an unpleasant irritation in 
pure ASCII user interfaces.
Edmun Chong has promised to write a separate draft that explores this 
approach.

Main point: Version -02 of the -downgrade- draft will document the 
header conversion approach (currently in section 5.3 of the draft) only.

Of course, no decision is final until consensus has been verified on the 
mailing list. So this is a proposal.
Please comment.

                  Harald, for the chairs

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From: Martin Duerst <[email-masked]>
Date: Wed, 16 Aug 2006 11:51:02 +0900

At 20:41 06/08/15, Harald Alvestrand wrote:
>In draft-ietf-eai-downgrade-01 section 5, the editors have identified a design decision that the WG needs to take: Whether to use an encapsulation strategy or a conversion strategy when downgrading messages from an UTF8SMTP user to an ASCII user.
>
>A few of us (editors + chairs) have had a discussion about this, and reached a proposed resolution:
>
>- It is of paramount importance that ASCII users are able to interpret downgraded messages without special software. Therefore, a conversion approach MUST be used, and this will be documented in the next version.
>
>- In some cases (MUAs that attach both to UTF8SMTP MTAs and ASCII MTAs, for instance), accurate reconstruction of the UTF8SMTP headers can provide some benefit to the user. One way of accomplishing this is by attaching a copy of the headers as they were before downgrading to the downgraded message, attached as a special (new) MIME body part.

This would mean that overall, we would get a multipart even if
the original message wasn't multipart, and straight conversion
wouldn't produce multipart. I'd assume this would be fine,
because we can assume that multipart is widely deployed.
But I just wanted to mention it here.

>However, this also carries security risks,

What in particular? Some scenarios with one set of headers
being tweaked/cheated, but the other not?

>and makes an unpleasant irritation in pure ASCII user interfaces.

We wouldn't downgrade pure US-ASCII email anyway, would we?
Anything non-ASCII will irritate pure ASCII user interfaces
one way or another. So I don't get the point you wanted to
make here, sorry.

Regards,     Martin.

>Edmun Chong has promised to write a separate draft that explores this approach.
>
>Main point: Version -02 of the -downgrade- draft will document the header conversion approach (currently in section 5.3 of the draft) only.
>
>Of course, no decision is final until consensus has been verified on the mailing list. So this is a proposal.
>Please comment.
>
>                  Harald, for the chairs
>
>

#-#-#  Martin J. Du"rst, Assoc. Professor, Aoyama Gakuin University
#-#-#  http://www.sw.it.aoyama.ac.jp       mailto:[email-masked]

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From: Harald Alvestrand <[email-masked]>
Date: Thu, 17 Aug 2006 14:10:34 +0200

> But I just wanted to mention it here.
Yes.
> being tweaked/cheated, but the other not?
This was extensively discussed when we discussed the options for S/MIME 
signatures (some wanted to have the message sent twice, once in 
cleartext and once signed; it was pointed out that an attacker could 
modify the cleartext part and fool anyone who did not bother to decode 
the message).

I remember a similar discussion wrt Multipart/Alternative, but couldn't 
find that in the RFC's security considerations.

> one way or another. So I don't get the point you wanted to
An otherwise pure US-ASCII message sent from an UTF8SMTP address, with 
that address in the From: header, would have such a section added to it. 
I believe that will be a rather common scenario.
> Regards,     Martin.
Thanks for the feedback.

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From: "Charles Lindsey" <[email-masked]>
Date: Tue, 22 Aug 2006 18:10:30 +0100

> approach MUST be used, and this will be documented in the next version.

Yes, I agree.
> pure ASCII user interfaces.

And I don't really like that. It will create multiparts where there were  
none before, and scammers will find some benefit in making the two  
versions different.

Whilst the ability to retain or reconstitute the Utf=8 form of an address  
is useful, I do not regard it as an _essential_ requirement of our project.

-- 
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