SipHash - a short input PRF
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Written by Jason A. Donenfeld <jason@zx2c4.com>

SipHash is a cryptographically secure PRF -- a keyed hash function -- that
performs very well for short inputs, hence the name. It was designed by
cryptographers Daniel J. Bernstein and Jean-Philippe Aumasson. It is intended
as a replacement for some uses of: `jhash`, `md5_transform`, `sha_transform`,
and so forth.

SipHash takes a secret key filled with randomly generated numbers and either
an input buffer or several input integers. It spits out an integer that is
indistinguishable from random. You may then use that integer as part of secure
sequence numbers, secure cookies, or mask it off for use in a hash table.

1. Generating a key

Keys should always be generated from a cryptographically secure source of
random numbers, either using get_random_bytes or get_random_once:

siphash_key_t key;
get_random_bytes(&key, sizeof(key));

If you're not deriving your key from here, you're doing it wrong.

2. Using the functions

There are two variants of the function, one that takes a list of integers, and
one that takes a buffer:

u64 siphash(const void *data, size_t len, const siphash_key_t *key);

And:

u64 siphash_1u64(u64, const siphash_key_t *key);
u64 siphash_2u64(u64, u64, const siphash_key_t *key);
u64 siphash_3u64(u64, u64, u64, const siphash_key_t *key);
u64 siphash_4u64(u64, u64, u64, u64, const siphash_key_t *key);
u64 siphash_1u32(u32, const siphash_key_t *key);
u64 siphash_2u32(u32, u32, const siphash_key_t *key);
u64 siphash_3u32(u32, u32, u32, const siphash_key_t *key);
u64 siphash_4u32(u32, u32, u32, u32, const siphash_key_t *key);

If you pass the generic siphash function something of a constant length, it
will constant fold at compile-time and automatically choose one of the
optimized functions.

3. Hashtable key function usage:

struct some_hashtable {
	DECLARE_HASHTABLE(hashtable, 8);
	siphash_key_t key;
};

void init_hashtable(struct some_hashtable *table)
{
	get_random_bytes(&table->key, sizeof(table->key));
}

static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
{
	return &table->hashtable[siphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
}

You may then iterate like usual over the returned hash bucket.

4. Security

SipHash has a very high security margin, with its 128-bit key. So long as the
key is kept secret, it is impossible for an attacker to guess the outputs of
the function, even if being able to observe many outputs, since 2^128 outputs
is significant.

Linux implements the "2-4" variant of SipHash.

5. Struct-passing Pitfalls

Often times the XuY functions will not be large enough, and instead you'll
want to pass a pre-filled struct to siphash. When doing this, it's important
to always ensure the struct has no padding holes. The easiest way to do this
is to simply arrange the members of the struct in descending order of size,
and to use offsetendof() instead of sizeof() for getting the size. For
performance reasons, if possible, it's probably a good thing to align the
struct to the right boundary. Here's an example:

const struct {
	struct in6_addr saddr;
	u32 counter;
	u16 dport;
} __aligned(SIPHASH_ALIGNMENT) combined = {
	.saddr = *(struct in6_addr *)saddr,
	.counter = counter,
	.dport = dport
};
u64 h = siphash(&combined, offsetofend(typeof(combined), dport), &secret);

6. Resources

Read the SipHash paper if you're interested in learning more:
https://131002.net/siphash/siphash.pdf