Re: Keywords for "SMTP Service Extension for Content Negotiation"

On Sun, 14 Jul 2002 06:50:34 +0900, Dave Crocker said:

> Ever since multi-recipient addressing was introduced, we have had 
> downgrading, by virtue of users having to choose the least common 
> denominator approach to sending attachments.
> 
> So the fact that this new mechanism allows that downgrading to happen later 
> in the transmission sequence should not confuse anyone into thinking that 
> downgrading is a new or unusual requirement.

My MUA is authorized to make decisions regarding downgrading and choice of
C-T-Es for attachments.  It listens to my choices regarding base64 versus QP,
For the most part, said conversions are lossless (and a few, such as
PGP-signing are even information-added), but I have authorized it to do lossy
conversions under a a few sets of circumstances, and I *know* about my MUA's
actions *before* I hit 'send'.  This is not news, and hasn't been for at least
a decade.

On the other hand, once my MUA hands it to my MTA, *I DO NOT EXPECT THE MTA TO
PERFORM LOSSY CONVERSIONS*. Period. End Of Discussion.

To quote the 'Security Considerations' of the original draft:

     A man-in-the-middle attack might change the capabilities reported 
     for a given recipient. For example: Suppose the sender knows the 
     recipient has the ability to view color documents so they mark 
     some things in red in what is otherwise a black and white 
     document. But someone interferes with the returned capabilities, 
     indicating that the recipient only supports black and white. The 
     document is duly downgraded, with the result that the recipient 
     doesn't see what the sender marked. 

Hey hey... What happens if this happens by *ACCIDENT*?  Currently, if I
think the recipient has color, and I do this, the recipient hopefully
gets at least a *warning* "trying to open a color image, converting to
gray scale", and can notify me that there was a problem.  However, the spec
as written doesn't even require that the MTA add a 'X-Downgraded-by:' header,
and the recipient has *no clue* that what he received wasn't what I sent.

And that, Dave, *IS* a "new and unusual" state of affairs.
-- 
				Valdis Kletnieks
				Computer Systems Senior Engineer
				Virginia Tech

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