[tor-bugs] #23061 [Core Tor/Tor]: crypto_rand_double() should produce all possible outputs on platforms with 32-bit int

Tor Bug Tracker & Wiki blackhole at torproject.org
Fri Aug 25 05:12:01 UTC 2017


#23061: crypto_rand_double() should produce all possible outputs on platforms with
32-bit int
-------------------------------------------------+-------------------------
 Reporter:  teor                                 |          Owner:  nickm
     Type:  defect                               |         Status:
                                                 |  needs_review
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.2.x-final
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  0.2.2.14-alpha
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-relay, security-low, privcount,  |  Actual Points:  0.5
  031-backport, 030-backport, 029-backport, 028  |
  -backport-maybe, 027-backport-maybe, 026       |
  -backport-maybe, review-group-22               |
Parent ID:                                       |         Points:  0.1
 Reviewer:                                       |        Sponsor:
                                                 |  SponsorQ
-------------------------------------------------+-------------------------

Comment (by teor):

 Replying to [comment:22 catalyst]:
 > Replying to [comment:21 nickm]:
 > > I tried the ldexp() trick too, but that fails the test that
 `uint64_to_dbl_0_1(UINT64_MAX) < 1.0` if have all four ldexp() calls.  If
 I remove the lowest-magnitude one, it's fine, but of course we only get 48
 bits.
 > Oh right, it'll round off the bits past `DBL_MANT_DIG`.  Maybe mask off
 all but the high 5 bits?

 This is still susceptible to the rounding mode in use at the time.
 So yawning's bit twiddling is going to be more reliable and robust.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23061#comment:30>
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