[exim] batv expiry time

Hi,

exim4 -be '${prvs{test@email}{secret}}'

Gives me: "prvs=test/073117d635@email"

Then:

exim4 -be '${prvscheck{prvs=test/073117d635@email}{secret}}'

Correctly gives me back: "test@email"

As I understand it the "073117d635" in this case is basically a
timestamp, encrypted with "secret". This is so that after a period of x
${prvscheck} "fails" to decode it due to the expiry time having been
reached.

Is my understanding correct? If so, what is the amount of time until
expiry? I can't see anywhere to configure this so I'm guessing I've
either misunderstood how it works, or the value is hardcoded and
unchangable?

Also, in the examples I've seen in the documentation and on the wiki
there are different, "secrets", for each sender address. Is that
necessary? I don't see the logic behind it...

Thanks,

Mike