[tor-bugs] #7801 [Tor]: Our one use of tor_weak_random() is subtly wrong

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#7801: Our one use of tor_weak_random() is subtly wrong
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 Reporter:  nickm      |          Owner:                    
     Type:  defect     |         Status:  new               
 Priority:  minor      |      Milestone:  Tor: 0.2.4.x-final
Component:  Tor        |        Version:                    
 Keywords:  tor-relay  |         Parent:                    
   Points:             |   Actualpoints:                    
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Comment(by cypherpunks):

 Replying to [ticket:7801 nickm]:
 > But that's subtly wrong. Many popular libcs' versions of random() use a
 linear congruential generator with a modulus that's a power of two, for
 which the low-order bits tend to have a much shorter period than the high-
 order bits.

 It's actually total wrong, and not because random() implementations.

 {{{
     int num_streams = 0;
     for (conn = first_conn; conn; conn = conn->next_stream) {
       num_streams++;
       if ((tor_weak_random() % num_streams)==0)
         chosen_stream = conn;
 }}}
 Every first checking of condition happens with num_streams = 1;
 Any (random) number % 1 == 0
 As result chosen_stream = first_conn. always.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/7801#comment:1>
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