[ietf-dkim] #1196: Base: Upgrade indication and protection against downgrade attacks

[Douglas Otis]

There are two aspects not covered by this recommendation.  Rather  
than marking preferred keys with a tag, keys that are depreciated  
should be marked instead.  This inversion of the logic allows easier  
upgrading.

- When a verifier detects a signature is using a key marked as  
depreciated, it must verify the existence of an additional signature  
supported by the signing domain not marked as depreciated, and  
confirm the correspondence of the signature algorithm with that of  
the key.

- If the verifier supports the algorithm of the signature using a key  
not marked as depreciated, this signature SHOULD be used instead.

- If there are no additional signatures not marked as depreciated, or  
where the algorithm of the signature is not confirmed to correspond  
with the key, the message signature for that domain SHOULD be  
considered invalid.

-Doug