[ietf-dkim] z= question with X headers

[Eric Allman]

>> The z= tag is only supposed to be used for "diagnostic purposes",
>> not for  computing the hash.  Changing that would have major
>> implications that we  would have to examine very carefully.
>
> So if mail list changed Subject header field (and for purposes of
> this question did not add other fields or changed content data) and
> there was a signature in message before that contained original
> Subject in the 'z' tag AND now message got to verifying agent -
> that agent is supposed to say the signature is invalid rather then
> use data from 'z' tag to attempt to verify the signature?

Yes.  The mailing list should have re-signed the message.

eric