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Re: [dmarc-ietf] Fwd: New Version Notification for draft-srose-dkim-ecc-00.txt

"John R Levine" <johnl@taugh.com> Sat, 08 April 2017 12:45 UTC

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Date: Sat, 08 Apr 2017 08:45:04 -0400
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From: John R Levine <johnl@taugh.com>
To: Vladimir Dubrovin <dubrovin@corp.mail.ru>
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Subject: Re: [dmarc-ietf] Fwd: New Version Notification for draft-srose-dkim-ecc-00.txt
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> Without marking the published key as obsolete, downgrade attack is 
> possible, because attacker can still use a weaker key to spoof 
> signature.

If you know how to spoof a sha256/rsa1024 signature, I know a lot of 
people who would like to talk to you.

Other than that, please review RFC 6376.  Each signing algorithm has a 
separate key -- if you don't trust an algorithm, don't publish a key for 
it.

R's,
John

>
>>> 1. produce 2 different DKIM-Signatures with 2 different selectors:
>>> slector1  with SHA-1 + RSA and selector2 one with  SHA-512 + ECDSA
>>
>> Of course.
>>
>>> 2. add an additional field to either selector1 DKIM DNS record (need to
>>> consult RFC if it's allowed) or to DKIM-Signature with selector1 (it's
>>> allowed but probably is not enough to protect against downgrade) to
>>> indicate the selector is legacy-only, e.g. o=sha512/eccp256 to indicate
>>> this selector should be ignored if verifier supports sha-512 and eccp256.
>>
>> No.  If the verifier is smart enough to understand new algorithms, it
>> is smart enough to figure out which signature to prefer.  Also keep in
>> mind that the legacy crypto is sha256/rsa1024 which is plenty strong
>> for the forseeable future.

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