[ietf-dkim] Proposal: get rid of x=

[Paul Hoffman]

Greetings again.

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Summary:
Remove the x= tag from the base spec because it conflicts with some 
of the purpose of DKIM and adds unneeded semantics.

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Rationale:
DKIM permits a signing domain to assert responsibility for a message. 
Saying "I only take responsibility for sending the message until this 
time, and then I don't take responsibility even though I previously 
took responsibility" makes little sense in the real world. Note that 
the informative note only says what x= is not. Leaving x= in can also 
lead to silly states.

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Changes:

In section 3.5, remove the x= section. Also in section 3.5, remove 
"x=1118006938;" from the example for z=.