[tor-bugs] #23415 [Core Tor/Tor]: sample_laplace_distribution() should take multiple random inputs

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#23415: sample_laplace_distribution() should take multiple random inputs
-------------------------+-------------------------------------------------
     Reporter:  teor     |      Owner:  (none)
         Type:  defect   |     Status:  new
     Priority:  Medium   |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core     |    Version:  Tor: 0.2.8.1-alpha
  Tor/Tor                |   Keywords:  tor-relay, security-low, privcount,
     Severity:  Normal   |  031-backport, 030-backport, 029-backport, 028
                         |  -backport-maybe, 026-backport-maybe
Actual Points:           |  Parent ID:  #23061
       Points:  0.5      |   Reviewer:
      Sponsor:           |
-------------------------+-------------------------------------------------
 Currently, sample_laplace_distribution() takes a random double as input,
 and then extracts the following random values:
 * a random sign
 * a random value in (0.0, 1.0]

 This reduces the precision of the result. For details, see:
 https://trac.torproject.org/projects/tor/ticket/23061#comment:32

 Instead, the function could take a random boolean sign and p, and the
 transform becomes:
 {{
 result = mu - b * (sign ? 1.0 : -1.0)
                 * tor_mathlog(1.0 - p);
 }}

 This would increase the precision by one bit, plus whatever precision is
 lost in `2.0 * fabs(p - 0.5)`.

 This may have been introduced in dad5eb7.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23415>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online

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