- 03 Jun, 2018 1 commit
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Yossi Gottlieb authored
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- 21 Feb, 2017 1 commit
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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.
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- 20 Feb, 2017 3 commits
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antirez authored
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antirez authored
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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.
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