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

[Tor Bug Tracker & Wiki]

#23061: crypto_rand_double() should produce all possible outputs on 32-bit
platforms
--------------------------+------------------------------------------------
     Reporter:  teor      |      Owner:
         Type:  defect    |     Status:  new
     Priority:  Medium    |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core      |    Version:  Tor: 0.2.2.14-alpha
  Tor/Tor                 |
     Severity:  Normal    |   Keywords:  tor-relay, security-low, privcount
Actual Points:            |  Parent ID:
       Points:  0.1       |   Reviewer:
      Sponsor:  SponsorQ  |
--------------------------+------------------------------------------------
 On 32-bit platforms, crypto_rand_double() only produces 1 in every 2
 million possible values between 0 and 1.

 This happens because:
 * crypto_rand_double() divides a random unsigned int by UINT_MAX
 * an unsigned int on 32-bit platforms is 32 bits
 * the mantissa on a double is 53 bits

 So crypto_rand_double() doesn't fill the lower 21 bits with random values.

 This makes the rep_hist_format_hs_stats() noise more predictable on 32-bit
 platforms.

 This fix shouldn't affect the unit tests, because they pass on 64-bit.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23061>
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