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diff --git a/Documentation/security/siphash.rst b/Documentation/security/siphash.rst new file mode 100644 index 000000000..023bd95c7 --- /dev/null +++ b/Documentation/security/siphash.rst @@ -0,0 +1,199 @@ +=========================== +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 offsetofend() 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. + +HalfSipHash support is provided through the "hsiphash" family of functions. + +.. warning:: + Do not ever use the hsiphash functions 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. + +On 64-bit kernels, the hsiphash functions actually implement SipHash-1-3, a +reduced-round variant of SipHash, instead of HalfSipHash-1-3. This is because in +64-bit code, SipHash-1-3 is no slower than HalfSipHash-1-3, and can be faster. +Note, this does *not* mean that in 64-bit kernels the hsiphash functions are the +same as the siphash ones, or that they are secure; the hsiphash functions still +use a less secure reduced-round algorithm and truncate their outputs to 32 +bits. + +Generating a hsiphash 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 hsiphash 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 +=========== + +hsiphash() is roughly 3 times slower than jhash(). 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 hsiphash(). |