Re: [tor-bugs] #3122 [Tor Client]: Write and use constant-time comparison functions

["Tor Bug Tracker & Wiki" <torproject-admin@xxxxxxxxxxxxxx>]

#3122: Write and use constant-time comparison functions
-------------------------+--------------------------------------------------
 Reporter:  rransom      |          Owner:  ioerror           
     Type:  enhancement  |         Status:  needs_review      
 Priority:  major        |      Milestone:  Tor: 0.2.1.x-final
Component:  Tor Client   |        Version:                    
 Keywords:               |         Parent:                    
   Points:               |   Actualpoints:                    
-------------------------+--------------------------------------------------

Comment(by rransom):

 {{{
 @@ -640,31 +640,36 @@ find_whitespace_eos(const char *s, const char *eos)
  int
  tor_mem_is_zero(const char *mem, size_t len)
  {
    static const char ZERO[] = {
      0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0,
 0,0,0,0,
    };
    while (len >= sizeof(ZERO)) {
 -    if (tor_memcmp(mem, ZERO, sizeof(ZERO)))
 +    /* It's safe to use fast_memcmp here, since the very worst thing an
 +     * attacker could learn is how many initial bytes of a secret were
 zero */
 +    if (fast_memcmp(mem, ZERO, sizeof(ZERO)))
        return 0;
 }}}
 It's also safe to use fast_memneq here.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/3122#comment:21>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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