[tor-bugs] #21420 [Core Tor/Tor]: Link certificate start date in the future

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#21420: Link certificate start date in the future
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 Reporter:  mmcloughlin   |          Owner:  nickm
     Type:  defect        |         Status:  needs_review
 Priority:  Medium        |      Milestone:  Tor: 0.3.0.x-final
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:  029-backport  |  Actual Points:
Parent ID:                |         Points:
 Reviewer:                |        Sponsor:
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Comment (by arma):

 I think you're right that commit 0196647 is the problem here.

 I suspect you're right that fixing the underlying math is the right
 answer.

 However, I'm unable to follow the math here:
 {{{
 -  start_time = crypto_rand_time_range(now - cert_lifetime, now) +
 2*24*3600;
 +  const time_t min_real_lifetime = 2*24*3600;
 +  time_t earliest_start_time = now - cert_lifetime + min_real_lifetime;
 +  if (earliest_start_time < now)
 +    earliest_start_time = now;
 +  start_time = crypto_rand_time_range(earliest_start_time, now);
 }}}

 Maybe some more comments to explain what we're computing, and *why*, would
 help?

 Looking at the origin commit, it seems maybe I wanted to say "- 2 days",
 not "+ 2 days". Would that resolve everything here?

 Maybe I am deeply confused, but won't
 {{{
 +  if (earliest_start_time < now)
 +    earliest_start_time = now;
 +  start_time = crypto_rand_time_range(earliest_start_time, now);
 }}}
 trigger the assert in crypto_rand_time_range() that min < max, since we'll
 be passing it "now, now"?

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21420#comment:5>
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