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SipHash 2-4 -> SipHash 1-2.

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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.
This commit is contained in:
antirez 2017-02-21 17:07:28 +01:00
parent a8cbc3ec87
commit 5a4133034e
1 changed files with 15 additions and 14 deletions
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29
src/siphash.c
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@ -19,18 +19,23 @@
This version was modified by Salvatore Sanfilippo <antirez@gmail.com>
in the following ways:
1. Hard-code 2-4 rounds in the hope the compiler can optimize it more
1. We use SipHash 1-2. This is not believed to be as strong as the
suggested 2-4 variant, but AFAIK there are not trivial attacks
against this reduced-rounds version, and it runs at the same speed
as Murmurhash2 that we used previously, why the 2-4 variant slowed
down Redis by a 4% figure more or less.
2. Hard-code rounds in the hope the compiler can optimize it more
in this raw from. Anyway we always want the standard 2-4 variant.
2. Modify the prototype and implementation so that the function directly
3. Modify the prototype and implementation so that the function directly
returns an uint64_t value, the hash itself, instead of receiving an
output buffer. This also means that the output size is set to 8 bytes
and the 16 bytes output code handling was removed.
3. Provide a case insensitive variant to be used when hashing strings that
4. Provide a case insensitive variant to be used when hashing strings that
must be considered identical by the hash table regardless of the case.
If we don't have directly a case insensitive hash function, we need to
perform a text transformation in some temporary buffer, which is costly.
4. Remove debugging code.
5. Modified the original test.c file to be a stand-alone function testing
5. Remove debugging code.
6. Modified the original test.c file to be a stand-alone function testing
the function in the new form (returing an uint64_t) using just the
relevant test vector.
*/
@ -131,7 +136,6 @@ uint64_t siphash(const uint8_t *in, const size_t inlen, const uint8_t *k) {
m = U8TO64_LE(in);
v3 ^= m;
SIPROUND;
SIPROUND;
v0 ^= m;
@ -150,14 +154,11 @@ uint64_t siphash(const uint8_t *in, const size_t inlen, const uint8_t *k) {
v3 ^= b;
SIPROUND;
SIPROUND;
v0 ^= b;
v2 ^= 0xff;
SIPROUND;
SIPROUND;
SIPROUND;
SIPROUND;
@ -195,7 +196,6 @@ uint64_t siphash_nocase(const uint8_t *in, const size_t inlen, const uint8_t *k)
m = U8TO64_LE_NOCASE(in);
v3 ^= m;
SIPROUND;
SIPROUND;
v0 ^= m;
@ -214,14 +214,11 @@ uint64_t siphash_nocase(const uint8_t *in, const size_t inlen, const uint8_t *k)
v3 ^= b;
SIPROUND;
SIPROUND;
v0 ^= b;
v2 ^= 0xff;
SIPROUND;
SIPROUND;
SIPROUND;
SIPROUND;
@ -308,7 +305,11 @@ const uint8_t vectors_sip64[64][8] = {
/* Test siphash using a test vector. Returns 0 if the function passed
* all the tests, otherwise 1 is returned. */
* all the tests, otherwise 1 is returned.
*
* IMPORTANT: The test vector is for SipHash 2-4. Before running
* the test revert back the siphash() function to 2-4 rounds since
* now it uses 1-2 rounds. */
int siphash_test(void) {
uint8_t in[64], k[16];
int i;