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diff --git a/Documentation/security/siphash.rst b/Documentation/security/siphash.rst new file mode 100644 index 000000000..bd9363025 --- /dev/null +++ b/Documentation/security/siphash.rst @@ -0,0 +1,189 @@ +=========================== +SipHash - a short input PRF +=========================== + +:Author: 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`, `sha1_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. + +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. + +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. + +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. + +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. + +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); + +Resources +========= + +Read the SipHash paper if you're interested in learning more: +https://131002.net/siphash/siphash.pdf + +------------------------------------------------------------------------------- + +=============================================== +HalfSipHash - SipHash's insecure younger cousin +=============================================== + +:Author: 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. + +Generating a HalfSipHash 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. + +Using the HalfSipHash 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. + +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. + +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. |