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1 | SipHash - a short input PRF |
2 | ----------------------------------------------- |
3 | Written by Jason A. Donenfeld <jason@zx2c4.com> |
4 | |
5 | SipHash is a cryptographically secure PRF -- a keyed hash function -- that |
6 | performs very well for short inputs, hence the name. It was designed by |
7 | cryptographers Daniel J. Bernstein and Jean-Philippe Aumasson. It is intended |
8 | as a replacement for some uses of: `jhash`, `md5_transform`, `sha_transform`, |
9 | and so forth. |
10 | |
11 | SipHash takes a secret key filled with randomly generated numbers and either |
12 | an input buffer or several input integers. It spits out an integer that is |
13 | indistinguishable from random. You may then use that integer as part of secure |
14 | sequence numbers, secure cookies, or mask it off for use in a hash table. |
15 | |
16 | 1. Generating a key |
17 | |
18 | Keys should always be generated from a cryptographically secure source of |
19 | random numbers, either using get_random_bytes or get_random_once: |
20 | |
21 | siphash_key_t key; |
22 | get_random_bytes(&key, sizeof(key)); |
23 | |
24 | If you're not deriving your key from here, you're doing it wrong. |
25 | |
26 | 2. Using the functions |
27 | |
28 | There are two variants of the function, one that takes a list of integers, and |
29 | one that takes a buffer: |
30 | |
31 | u64 siphash(const void *data, size_t len, const siphash_key_t *key); |
32 | |
33 | And: |
34 | |
35 | u64 siphash_1u64(u64, const siphash_key_t *key); |
36 | u64 siphash_2u64(u64, u64, const siphash_key_t *key); |
37 | u64 siphash_3u64(u64, u64, u64, const siphash_key_t *key); |
38 | u64 siphash_4u64(u64, u64, u64, u64, const siphash_key_t *key); |
39 | u64 siphash_1u32(u32, const siphash_key_t *key); |
40 | u64 siphash_2u32(u32, u32, const siphash_key_t *key); |
41 | u64 siphash_3u32(u32, u32, u32, const siphash_key_t *key); |
42 | u64 siphash_4u32(u32, u32, u32, u32, const siphash_key_t *key); |
43 | |
44 | If you pass the generic siphash function something of a constant length, it |
45 | will constant fold at compile-time and automatically choose one of the |
46 | optimized functions. |
47 | |
48 | 3. Hashtable key function usage: |
49 | |
50 | struct some_hashtable { |
51 | DECLARE_HASHTABLE(hashtable, 8); |
52 | siphash_key_t key; |
53 | }; |
54 | |
55 | void init_hashtable(struct some_hashtable *table) |
56 | { |
57 | get_random_bytes(&table->key, sizeof(table->key)); |
58 | } |
59 | |
60 | static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input) |
61 | { |
62 | return &table->hashtable[siphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)]; |
63 | } |
64 | |
65 | You may then iterate like usual over the returned hash bucket. |
66 | |
67 | 4. Security |
68 | |
69 | SipHash has a very high security margin, with its 128-bit key. So long as the |
70 | key is kept secret, it is impossible for an attacker to guess the outputs of |
71 | the function, even if being able to observe many outputs, since 2^128 outputs |
72 | is significant. |
73 | |
74 | Linux implements the "2-4" variant of SipHash. |
75 | |
76 | 5. Struct-passing Pitfalls |
77 | |
78 | Often times the XuY functions will not be large enough, and instead you'll |
79 | want to pass a pre-filled struct to siphash. When doing this, it's important |
80 | to always ensure the struct has no padding holes. The easiest way to do this |
81 | is to simply arrange the members of the struct in descending order of size, |
82 | and to use offsetendof() instead of sizeof() for getting the size. For |
83 | performance reasons, if possible, it's probably a good thing to align the |
84 | struct to the right boundary. Here's an example: |
85 | |
86 | const struct { |
87 | struct in6_addr saddr; |
88 | u32 counter; |
89 | u16 dport; |
90 | } __aligned(SIPHASH_ALIGNMENT) combined = { |
91 | .saddr = *(struct in6_addr *)saddr, |
92 | .counter = counter, |
93 | .dport = dport |
94 | }; |
95 | u64 h = siphash(&combined, offsetofend(typeof(combined), dport), &secret); |
96 | |
97 | 6. Resources |
98 | |
99 | Read the SipHash paper if you're interested in learning more: |
100 | https://131002.net/siphash/siphash.pdf |
101 | |
102 | |
103 | ~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~ |
104 | |
105 | HalfSipHash - SipHash's insecure younger cousin |
106 | ----------------------------------------------- |
107 | Written by Jason A. Donenfeld <jason@zx2c4.com> |
108 | |
109 | On the off-chance that SipHash is not fast enough for your needs, you might be |
110 | able to justify using HalfSipHash, a terrifying but potentially useful |
111 | possibility. HalfSipHash cuts SipHash's rounds down from "2-4" to "1-3" and, |
112 | even scarier, uses an easily brute-forcable 64-bit key (with a 32-bit output) |
113 | instead of SipHash's 128-bit key. However, this may appeal to some |
114 | high-performance `jhash` users. |
115 | |
116 | Danger! |
117 | |
118 | Do not ever use HalfSipHash except for as a hashtable key function, and only |
119 | then when you can be absolutely certain that the outputs will never be |
120 | transmitted out of the kernel. This is only remotely useful over `jhash` as a |
121 | means of mitigating hashtable flooding denial of service attacks. |
122 | |
123 | 1. Generating a key |
124 | |
125 | Keys should always be generated from a cryptographically secure source of |
126 | random numbers, either using get_random_bytes or get_random_once: |
127 | |
128 | hsiphash_key_t key; |
129 | get_random_bytes(&key, sizeof(key)); |
130 | |
131 | If you're not deriving your key from here, you're doing it wrong. |
132 | |
133 | 2. Using the functions |
134 | |
135 | There are two variants of the function, one that takes a list of integers, and |
136 | one that takes a buffer: |
137 | |
138 | u32 hsiphash(const void *data, size_t len, const hsiphash_key_t *key); |
139 | |
140 | And: |
141 | |
142 | u32 hsiphash_1u32(u32, const hsiphash_key_t *key); |
143 | u32 hsiphash_2u32(u32, u32, const hsiphash_key_t *key); |
144 | u32 hsiphash_3u32(u32, u32, u32, const hsiphash_key_t *key); |
145 | u32 hsiphash_4u32(u32, u32, u32, u32, const hsiphash_key_t *key); |
146 | |
147 | If you pass the generic hsiphash function something of a constant length, it |
148 | will constant fold at compile-time and automatically choose one of the |
149 | optimized functions. |
150 | |
151 | 3. Hashtable key function usage: |
152 | |
153 | struct some_hashtable { |
154 | DECLARE_HASHTABLE(hashtable, 8); |
155 | hsiphash_key_t key; |
156 | }; |
157 | |
158 | void init_hashtable(struct some_hashtable *table) |
159 | { |
160 | get_random_bytes(&table->key, sizeof(table->key)); |
161 | } |
162 | |
163 | static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input) |
164 | { |
165 | return &table->hashtable[hsiphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)]; |
166 | } |
167 | |
168 | You may then iterate like usual over the returned hash bucket. |
169 | |
170 | 4. Performance |
171 | |
172 | HalfSipHash is roughly 3 times slower than JenkinsHash. For many replacements, |
173 | this will not be a problem, as the hashtable lookup isn't the bottleneck. And |
174 | in general, this is probably a good sacrifice to make for the security and DoS |
175 | resistance of HalfSipHash. |
176 |