1===========================
2SipHash - a short input PRF
3===========================
4
5:Author: Written by Jason A. Donenfeld <jason@zx2c4.com>
6
7SipHash is a cryptographically secure PRF -- a keyed hash function -- that
8performs very well for short inputs, hence the name. It was designed by
9cryptographers Daniel J. Bernstein and Jean-Philippe Aumasson. It is intended
10as a replacement for some uses of: `jhash`, `md5_transform`, `sha_transform`,
11and so forth.
12
13SipHash takes a secret key filled with randomly generated numbers and either
14an input buffer or several input integers. It spits out an integer that is
15indistinguishable from random. You may then use that integer as part of secure
16sequence numbers, secure cookies, or mask it off for use in a hash table.
17
18Generating a key
19================
20
21Keys should always be generated from a cryptographically secure source of
22random numbers, either using get_random_bytes or get_random_once::
23
24	siphash_key_t key;
25	get_random_bytes(&key, sizeof(key));
26
27If you're not deriving your key from here, you're doing it wrong.
28
29Using the functions
30===================
31
32There are two variants of the function, one that takes a list of integers, and
33one that takes a buffer::
34
35	u64 siphash(const void *data, size_t len, const siphash_key_t *key);
36
37And::
38
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);
47
48If you pass the generic siphash function something of a constant length, it
49will constant fold at compile-time and automatically choose one of the
50optimized functions.
51
52Hashtable key function usage::
53
54	struct some_hashtable {
55		DECLARE_HASHTABLE(hashtable, 8);
56		siphash_key_t key;
57	};
58
59	void init_hashtable(struct some_hashtable *table)
60	{
61		get_random_bytes(&table->key, sizeof(table->key));
62	}
63
64	static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
65	{
66		return &table->hashtable[siphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
67	}
68
69You may then iterate like usual over the returned hash bucket.
70
71Security
72========
73
74SipHash has a very high security margin, with its 128-bit key. So long as the
75key is kept secret, it is impossible for an attacker to guess the outputs of
76the function, even if being able to observe many outputs, since 2^128 outputs
77is significant.
78
79Linux implements the "2-4" variant of SipHash.
80
81Struct-passing Pitfalls
82=======================
83
84Often times the XuY functions will not be large enough, and instead you'll
85want to pass a pre-filled struct to siphash. When doing this, it's important
86to always ensure the struct has no padding holes. The easiest way to do this
87is to simply arrange the members of the struct in descending order of size,
88and to use offsetendof() instead of sizeof() for getting the size. For
89performance reasons, if possible, it's probably a good thing to align the
90struct to the right boundary. Here's an example::
91
92	const struct {
93		struct in6_addr saddr;
94		u32 counter;
95		u16 dport;
96	} __aligned(SIPHASH_ALIGNMENT) combined = {
97		.saddr = *(struct in6_addr *)saddr,
98		.counter = counter,
99		.dport = dport
100	};
101	u64 h = siphash(&combined, offsetofend(typeof(combined), dport), &secret);
102
103Resources
104=========
105
106Read the SipHash paper if you're interested in learning more:
107https://131002.net/siphash/siphash.pdf
108
109-------------------------------------------------------------------------------
110
111===============================================
112HalfSipHash - SipHash's insecure younger cousin
113===============================================
114
115:Author: Written by Jason A. Donenfeld <jason@zx2c4.com>
116
117On the off-chance that SipHash is not fast enough for your needs, you might be
118able to justify using HalfSipHash, a terrifying but potentially useful
119possibility. HalfSipHash cuts SipHash's rounds down from "2-4" to "1-3" and,
120even scarier, uses an easily brute-forcable 64-bit key (with a 32-bit output)
121instead of SipHash's 128-bit key. However, this may appeal to some
122high-performance `jhash` users.
123
124Danger!
125
126Do not ever use HalfSipHash except for as a hashtable key function, and only
127then when you can be absolutely certain that the outputs will never be
128transmitted out of the kernel. This is only remotely useful over `jhash` as a
129means of mitigating hashtable flooding denial of service attacks.
130
131Generating a HalfSipHash key
132============================
133
134Keys should always be generated from a cryptographically secure source of
135random numbers, either using get_random_bytes or get_random_once:
136
137hsiphash_key_t key;
138get_random_bytes(&key, sizeof(key));
139
140If you're not deriving your key from here, you're doing it wrong.
141
142Using the HalfSipHash functions
143===============================
144
145There are two variants of the function, one that takes a list of integers, and
146one that takes a buffer::
147
148	u32 hsiphash(const void *data, size_t len, const hsiphash_key_t *key);
149
150And::
151
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);
156
157If you pass the generic hsiphash function something of a constant length, it
158will constant fold at compile-time and automatically choose one of the
159optimized functions.
160
161Hashtable key function usage
162============================
163
164::
165
166	struct some_hashtable {
167		DECLARE_HASHTABLE(hashtable, 8);
168		hsiphash_key_t key;
169	};
170
171	void init_hashtable(struct some_hashtable *table)
172	{
173		get_random_bytes(&table->key, sizeof(table->key));
174	}
175
176	static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
177	{
178		return &table->hashtable[hsiphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
179	}
180
181You may then iterate like usual over the returned hash bucket.
182
183Performance
184===========
185
186HalfSipHash is roughly 3 times slower than JenkinsHash. For many replacements,
187this will not be a problem, as the hashtable lookup isn't the bottleneck. And
188in general, this is probably a good sacrifice to make for the security and DoS
189resistance of HalfSipHash.
190