1. 03 Jun, 2018 1 commit
  2. 21 Feb, 2017 1 commit
    • antirez's avatar
      SipHash 2-4 -> SipHash 1-2. · 0285c271
      antirez authored
      For performance reasons we use a reduced rounds variant of
      SipHash. This should still provide enough protection and the
      effects in the hash table distribution are non existing.
      If some real world attack on SipHash 1-2 will be found we can
      trivially switch to something more secure. Anyway it is a
      big step forward from Murmurhash, for which it is trivial to
      generate *seed independent* colliding keys... The speed
      penatly introduced by SipHash 2-4, around 4%, was a too big
      price to pay compared to the effectiveness of the HashDoS
      attack against SipHash 1-2, and considering so far in the
      Redis history, no such an incident ever happened even while
      using trivially to collide hash functions.
      0285c271
  3. 20 Feb, 2017 3 commits
    • antirez's avatar
      84fa8230
    • antirez's avatar
      SipHash x86 optimizations. · 05ea8c61
      antirez authored
      05ea8c61
    • antirez's avatar
      Use SipHash hash function to mitigate HashDos attempts. · adeed29a
      antirez authored
      This change attempts to switch to an hash function which mitigates
      the effects of the HashDoS attack (denial of service attack trying
      to force data structures to worst case behavior) while at the same time
      providing Redis with an hash function that does not expect the input
      data to be word aligned, a condition no longer true now that sds.c
      strings have a varialbe length header.
      
      Note that it is possible sometimes that even using an hash function
      for which collisions cannot be generated without knowing the seed,
      special implementation details or the exposure of the seed in an
      indirect way (for example the ability to add elements to a Set and
      check the return in which Redis returns them with SMEMBERS) may
      make the attacker's life simpler in the process of trying to guess
      the correct seed, however the next step would be to switch to a
      log(N) data structure when too many items in a single bucket are
      detected: this seems like an overkill in the case of Redis.
      
      SPEED REGRESION TESTS:
      
      In order to verify that switching from MurmurHash to SipHash had
      no impact on speed, a set of benchmarks involving fast insertion
      of 5 million of keys were performed.
      
      The result shows Redis with SipHash in high pipelining conditions
      to be about 4% slower compared to using the previous hash function.
      However this could partially be related to the fact that the current
      implementation does not attempt to hash whole words at a time but
      reads single bytes, in order to have an output which is endian-netural
      and at the same time working on systems where unaligned memory accesses
      are a problem.
      
      Further X86 specific optimizations should be tested, the function
      may easily get at the same level of MurMurHash2 if a few optimizations
      are performed.
      adeed29a