I have a lot more trouble understanding why t= needs to be kept than why
x= needs to be kept.
Without t= we have no idea when a message was signed, since there's no
particular reason that the Date: header has to contain the current date,
or even that there be one.
As a signer, I would much rather specify an expiration time for the
signature than to specify the time at which it was signed than to
have the verifier add a fudge factor to the signing time and use
that as the expiration. On this list, I have already heard numbers
between 1 and 2 weeks for the fudge factor, so the signer would
really have no idea how long the signatures are valid.
But the signer is likely to have little idea of what the transit time
to the recipient will be. The basic justification for x= is that the
sender knows the transit time and the recipient doesn't. I've never
seen any justification for that, and it's easy to think of scenarios
where it's just wrong.
Regards,
John Levine, johnl(_at_)iecc(_dot_)com, Primary Perpetrator of "The Internet
for Dummies",
Information Superhighwayman wanna-be, http://www.johnlevine.com, Mayor
"A book is a sneeze." - E.B. White, on the writing of Charlotte's Web
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