1 ===========================
2 SipHash - a short input PRF
3 ===========================
5 :Author: Written by Jason A. Donenfeld <jason@zx2c4.com>
7 SipHash is a cryptographically secure PRF -- a keyed hash function -- that
8 performs very well for short inputs, hence the name. It was designed by
9 cryptographers Daniel J. Bernstein and Jean-Philippe Aumasson. It is intended
10 as a replacement for some uses of: `jhash`, `md5_transform`, `sha_transform`,
13 SipHash takes a secret key filled with randomly generated numbers and either
14 an input buffer or several input integers. It spits out an integer that is
15 indistinguishable from random. You may then use that integer as part of secure
16 sequence numbers, secure cookies, or mask it off for use in a hash table.
21 Keys should always be generated from a cryptographically secure source of
22 random numbers, either using get_random_bytes or get_random_once::
25 get_random_bytes(&key, sizeof(key));
27 If you're not deriving your key from here, you're doing it wrong.
32 There are two variants of the function, one that takes a list of integers, and
33 one that takes a buffer::
35 u64 siphash(const void *data, size_t len, const siphash_key_t *key);
39 u64 siphash_1u64(u64, const siphash_key_t *key);
40 u64 siphash_2u64(u64, u64, const siphash_key_t *key);
41 u64 siphash_3u64(u64, u64, u64, const siphash_key_t *key);
42 u64 siphash_4u64(u64, u64, u64, u64, const siphash_key_t *key);
43 u64 siphash_1u32(u32, const siphash_key_t *key);
44 u64 siphash_2u32(u32, u32, const siphash_key_t *key);
45 u64 siphash_3u32(u32, u32, u32, const siphash_key_t *key);
46 u64 siphash_4u32(u32, u32, u32, u32, const siphash_key_t *key);
48 If you pass the generic siphash function something of a constant length, it
49 will constant fold at compile-time and automatically choose one of the
52 Hashtable key function usage::
54 struct some_hashtable {
55 DECLARE_HASHTABLE(hashtable, 8);
59 void init_hashtable(struct some_hashtable *table)
61 get_random_bytes(&table->key, sizeof(table->key));
64 static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
66 return &table->hashtable[siphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
69 You may then iterate like usual over the returned hash bucket.
74 SipHash has a very high security margin, with its 128-bit key. So long as the
75 key is kept secret, it is impossible for an attacker to guess the outputs of
76 the function, even if being able to observe many outputs, since 2^128 outputs
79 Linux implements the "2-4" variant of SipHash.
81 Struct-passing Pitfalls
82 =======================
84 Often times the XuY functions will not be large enough, and instead you'll
85 want to pass a pre-filled struct to siphash. When doing this, it's important
86 to always ensure the struct has no padding holes. The easiest way to do this
87 is to simply arrange the members of the struct in descending order of size,
88 and to use offsetendof() instead of sizeof() for getting the size. For
89 performance reasons, if possible, it's probably a good thing to align the
90 struct to the right boundary. Here's an example::
93 struct in6_addr saddr;
96 } __aligned(SIPHASH_ALIGNMENT) combined = {
97 .saddr = *(struct in6_addr *)saddr,
101 u64 h = siphash(&combined, offsetofend(typeof(combined), dport), &secret);
106 Read the SipHash paper if you're interested in learning more:
107 https://131002.net/siphash/siphash.pdf
109 -------------------------------------------------------------------------------
111 ===============================================
112 HalfSipHash - SipHash's insecure younger cousin
113 ===============================================
115 :Author: Written by Jason A. Donenfeld <jason@zx2c4.com>
117 On the off-chance that SipHash is not fast enough for your needs, you might be
118 able to justify using HalfSipHash, a terrifying but potentially useful
119 possibility. HalfSipHash cuts SipHash's rounds down from "2-4" to "1-3" and,
120 even scarier, uses an easily brute-forcable 64-bit key (with a 32-bit output)
121 instead of SipHash's 128-bit key. However, this may appeal to some
122 high-performance `jhash` users.
126 Do not ever use HalfSipHash except for as a hashtable key function, and only
127 then when you can be absolutely certain that the outputs will never be
128 transmitted out of the kernel. This is only remotely useful over `jhash` as a
129 means of mitigating hashtable flooding denial of service attacks.
134 Keys should always be generated from a cryptographically secure source of
135 random numbers, either using get_random_bytes or get_random_once:
138 get_random_bytes(&key, sizeof(key));
140 If you're not deriving your key from here, you're doing it wrong.
145 There are two variants of the function, one that takes a list of integers, and
146 one that takes a buffer::
148 u32 hsiphash(const void *data, size_t len, const hsiphash_key_t *key);
152 u32 hsiphash_1u32(u32, const hsiphash_key_t *key);
153 u32 hsiphash_2u32(u32, u32, const hsiphash_key_t *key);
154 u32 hsiphash_3u32(u32, u32, u32, const hsiphash_key_t *key);
155 u32 hsiphash_4u32(u32, u32, u32, u32, const hsiphash_key_t *key);
157 If you pass the generic hsiphash function something of a constant length, it
158 will constant fold at compile-time and automatically choose one of the
161 Hashtable key function usage
162 ============================
166 struct some_hashtable {
167 DECLARE_HASHTABLE(hashtable, 8);
171 void init_hashtable(struct some_hashtable *table)
173 get_random_bytes(&table->key, sizeof(table->key));
176 static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
178 return &table->hashtable[hsiphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
181 You may then iterate like usual over the returned hash bucket.
186 HalfSipHash is roughly 3 times slower than JenkinsHash. For many replacements,
187 this will not be a problem, as the hashtable lookup isn't the bottleneck. And
188 in general, this is probably a good sacrifice to make for the security and DoS
189 resistance of HalfSipHash.