The question remains: given a message with such a signature, which is
entirely valid in the current DKIM, what will a recipient system do
with it? What will users see? Ask ten people, get ten answers, which
is about as far from interoperable as you can get.
I think the use of "interoperability" here is curious. What qualifies as
interoperable? According to the DKIM spec, if sizeof(message) = 10k and l=100,
then if the first 100 bytes were unchanged since signing, the signature is
valid, and I think everyone would agree that the verifiers would all behave the
same way.
You might get ten answers if you're asking about assessors, but I doubt you'd
get ten answers when you're talking about verifiers.
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