176 строки
6.1 KiB
Plaintext
176 строки
6.1 KiB
Plaintext
SipHash - a short input PRF
|
|
-----------------------------------------------
|
|
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
|
|
|
|
|
|
~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~
|
|
|
|
HalfSipHash - SipHash's insecure younger cousin
|
|
-----------------------------------------------
|
|
Written by Jason A. Donenfeld <jason@zx2c4.com>
|
|
|
|
On the off-chance that SipHash is not fast enough for your needs, you might be
|
|
able to justify using HalfSipHash, a terrifying but potentially useful
|
|
possibility. HalfSipHash cuts SipHash's rounds down from "2-4" to "1-3" and,
|
|
even scarier, uses an easily brute-forcable 64-bit key (with a 32-bit output)
|
|
instead of SipHash's 128-bit key. However, this may appeal to some
|
|
high-performance `jhash` users.
|
|
|
|
Danger!
|
|
|
|
Do not ever use HalfSipHash except for as a hashtable key function, and only
|
|
then when you can be absolutely certain that the outputs will never be
|
|
transmitted out of the kernel. This is only remotely useful over `jhash` as a
|
|
means of mitigating hashtable flooding denial of service attacks.
|
|
|
|
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:
|
|
|
|
hsiphash_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:
|
|
|
|
u32 hsiphash(const void *data, size_t len, const hsiphash_key_t *key);
|
|
|
|
And:
|
|
|
|
u32 hsiphash_1u32(u32, const hsiphash_key_t *key);
|
|
u32 hsiphash_2u32(u32, u32, const hsiphash_key_t *key);
|
|
u32 hsiphash_3u32(u32, u32, u32, const hsiphash_key_t *key);
|
|
u32 hsiphash_4u32(u32, u32, u32, u32, const hsiphash_key_t *key);
|
|
|
|
If you pass the generic hsiphash 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);
|
|
hsiphash_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[hsiphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
|
|
}
|
|
|
|
You may then iterate like usual over the returned hash bucket.
|
|
|
|
4. Performance
|
|
|
|
HalfSipHash is roughly 3 times slower than JenkinsHash. For many replacements,
|
|
this will not be a problem, as the hashtable lookup isn't the bottleneck. And
|
|
in general, this is probably a good sacrifice to make for the security and DoS
|
|
resistance of HalfSipHash.
|