Re: A permission to re-sign header

On Fri 18/Apr/2014 14:37:21 +0200 John Levine wrote:
>> Even with the local part (marissam) an M-R is not really hard to
>> forge, otherwise DKIM-Signature wouldn't have had to include all the
>> other tags.  If we worry about replay attacks, we can enhance M-R so
>> that it includes them too.  For example, we could make M-R exactly
>> like a regular DKIM-Signature, except that it would be a very very
>> weak one, something that the MLM won't break.
> 
> BTDT.  If we could invent a weak signature that the MLM won't break,
> we wouldn't have this problem.  The M-R token is signed so it should
> be impossible to forge, and we don't expect anyone to change it in
> transit.

I reckon M-R is equivalent to a DKIM-Signature having h=From:To; l=0;
assuming "From: [email protected]" and "To: r=ietf.org".  Correct?

The equivalent signature is even weaker than it needs to.  IME, the
only problem is that some sites (e.g. netease) have a local policy
whereby they discard weak signatures.  (Their DMARC reports say
"DKIM-Signature could not parse or has bad tags/values".)

Ale

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