101 lines
3.5 KiB
Plaintext
101 lines
3.5 KiB
Plaintext
SipHash - a short input PRF
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Written by Jason A. Donenfeld <jason@zx2c4.com>
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SipHash is a cryptographically secure PRF -- a keyed hash function -- that
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performs very well for short inputs, hence the name. It was designed by
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cryptographers Daniel J. Bernstein and Jean-Philippe Aumasson. It is intended
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as a replacement for some uses of: `jhash`, `md5_transform`, `sha_transform`,
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and so forth.
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SipHash takes a secret key filled with randomly generated numbers and either
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an input buffer or several input integers. It spits out an integer that is
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indistinguishable from random. You may then use that integer as part of secure
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sequence numbers, secure cookies, or mask it off for use in a hash table.
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1. Generating a key
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Keys should always be generated from a cryptographically secure source of
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random numbers, either using get_random_bytes or get_random_once:
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siphash_key_t key;
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get_random_bytes(&key, sizeof(key));
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If you're not deriving your key from here, you're doing it wrong.
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2. Using the functions
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There are two variants of the function, one that takes a list of integers, and
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one that takes a buffer:
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u64 siphash(const void *data, size_t len, const siphash_key_t *key);
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And:
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u64 siphash_1u64(u64, const siphash_key_t *key);
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u64 siphash_2u64(u64, u64, const siphash_key_t *key);
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u64 siphash_3u64(u64, u64, u64, const siphash_key_t *key);
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u64 siphash_4u64(u64, u64, u64, u64, const siphash_key_t *key);
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u64 siphash_1u32(u32, const siphash_key_t *key);
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u64 siphash_2u32(u32, u32, const siphash_key_t *key);
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u64 siphash_3u32(u32, u32, u32, const siphash_key_t *key);
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u64 siphash_4u32(u32, u32, u32, u32, const siphash_key_t *key);
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If you pass the generic siphash function something of a constant length, it
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will constant fold at compile-time and automatically choose one of the
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optimized functions.
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3. Hashtable key function usage:
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struct some_hashtable {
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DECLARE_HASHTABLE(hashtable, 8);
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siphash_key_t key;
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};
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void init_hashtable(struct some_hashtable *table)
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{
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get_random_bytes(&table->key, sizeof(table->key));
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}
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static inline hlist_head *some_hashtable_bucket(struct some_hashtable *table, struct interesting_input *input)
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{
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return &table->hashtable[siphash(input, sizeof(*input), &table->key) & (HASH_SIZE(table->hashtable) - 1)];
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}
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You may then iterate like usual over the returned hash bucket.
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4. Security
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SipHash has a very high security margin, with its 128-bit key. So long as the
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key is kept secret, it is impossible for an attacker to guess the outputs of
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the function, even if being able to observe many outputs, since 2^128 outputs
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is significant.
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Linux implements the "2-4" variant of SipHash.
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5. Struct-passing Pitfalls
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Often times the XuY functions will not be large enough, and instead you'll
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want to pass a pre-filled struct to siphash. When doing this, it's important
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to always ensure the struct has no padding holes. The easiest way to do this
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is to simply arrange the members of the struct in descending order of size,
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and to use offsetendof() instead of sizeof() for getting the size. For
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performance reasons, if possible, it's probably a good thing to align the
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struct to the right boundary. Here's an example:
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const struct {
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struct in6_addr saddr;
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u32 counter;
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u16 dport;
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} __aligned(SIPHASH_ALIGNMENT) combined = {
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.saddr = *(struct in6_addr *)saddr,
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.counter = counter,
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.dport = dport
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};
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u64 h = siphash(&combined, offsetofend(typeof(combined), dport), &secret);
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6. Resources
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Read the SipHash paper if you're interested in learning more:
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https://131002.net/siphash/siphash.pdf
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