[tor-bugs] #24815 [Core Tor/Tor]: Validate shared random state dates before each voting period

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#24815: Validate shared random state dates before each voting period
-------------------------------------------------+-------------------------
 Reporter:  teor                                 |          Owner:  dgoulet
     Type:  defect                               |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.3.x-final
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  0.2.9.1-alpha
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-sr, tor-ddos, 031-backport,      |  Actual Points:
  032-backport                                   |
Parent ID:                                       |         Points:  1
 Reviewer:                                       |        Sponsor:
-------------------------------------------------+-------------------------

Comment (by asn):

 Replying to [comment:8 teor]:
 > The bug is in get_state_valid_until_time():
 > {{{
 >   voting_interval = get_voting_interval();
 >   /* Find the time the current round started. */
 >   beginning_of_current_round = get_start_time_of_current_round();
 >
 >   /* Find how many rounds are left till the end of the protocol run */
 >   current_round = (now / voting_interval) % total_rounds;
 >   rounds_left = total_rounds - current_round;
 >
 >   /* To find the valid-until time now, take the start time of the
 current
 >    * round and add to it the time it takes for the leftover rounds to
 >    * complete. */
 >   valid_until = beginning_of_current_round + (rounds_left *
 voting_interval);
 > }}}
 >
 > If rounds_left is zero, then valid_until is beginning_of_current_round.

 Hmm that was a pretty good hypothesis. However, can `rounds_left` actually
 be 0? Due to the modulo in the `current_round` calculation, it's
 guaranteed that `current_round` will be between 0 and `total_rounds-1`, so
 `rounds_left` can never be 0.

 Pretty close, but I think the current bug argument won't work exactly as
 is.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24815#comment:10>
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