[ietf-dkim] Base issue: multiple linked signatures

[Charles Lindsey]

On Wed, 03 Jan 2007 21:38:41 -0000, william(at)elan.net <william at elan.net>  
wrote:

> On Tue, 2 Jan 2007, John Levine wrote:

> No. What you'd want to do is to rename modified header field into some
> sort of trace data and copy the original data in its place. Displaying
> modification would then be up to the MUA which could display small
> icon allowing user to see what the modification was as an option.

An example of that sort of thing is the downrading process in EAI.

You start with some header

Foo-Header: <utf-8 stuff that wont pass through current MTAs>

and when you find yourself compelled to downgrade it because the next MTA  
doesn't advertise the UTF8SMTP capability, you replace it with:

Foo-Header: <inadequate/partial reformulation of the original in ASCII>
Downgraded: Foo-Header: <RFC 2047 encoded representation of the original  
utf-8>

So if the original Foo-Header was included in the signature, the signature  
is now broken.

One possiblity for subsequently verifying the signature, assuming the  
verifier can be made aware of what has happened (that is where the 'z='  
tag might be needed) is for the verifier to reconstruct the original  
Foo-Header from the Downgraded header, and test the signature on that.

I am not saying that this is the only way or even the best way of handling  
DKIM in an EAI envireonment, because it is not yet clear whether the  
mechanisms that have been proposed for downgrading are sufficiently  
robust, but it is certainly one possibility that has been suggested. And  
whether people will bother to write verifiers that do all that is another  
issue again, but at least the possibility exists.

-- 
Charles H. Lindsey ---------At Home, doing my own thing------------------------
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   Web: http://www.cs.man.ac.uk/~chl
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