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Diffstat (limited to 'src/fallback_hash.rs')
| -rw-r--r-- | src/fallback_hash.rs | 368 |
1 files changed, 368 insertions, 0 deletions
diff --git a/src/fallback_hash.rs b/src/fallback_hash.rs new file mode 100644 index 0000000..f78074d --- /dev/null +++ b/src/fallback_hash.rs @@ -0,0 +1,368 @@ +use crate::convert::*; +use crate::operations::folded_multiply; +use crate::operations::read_small; +use crate::operations::MULTIPLE; +use crate::random_state::PI; +use crate::RandomState; +use core::hash::Hasher; + +const ROT: u32 = 23; //17 + +/// A `Hasher` for hashing an arbitrary stream of bytes. +/// +/// Instances of [`AHasher`] represent state that is updated while hashing data. +/// +/// Each method updates the internal state based on the new data provided. Once +/// all of the data has been provided, the resulting hash can be obtained by calling +/// `finish()` +/// +/// [Clone] is also provided in case you wish to calculate hashes for two different items that +/// start with the same data. +/// +#[derive(Debug, Clone)] +pub struct AHasher { + buffer: u64, + pad: u64, + extra_keys: [u64; 2], +} + +impl AHasher { + /// Creates a new hasher keyed to the provided key. + #[inline] + #[allow(dead_code)] // Is not called if non-fallback hash is used. + pub(crate) fn new_with_keys(key1: u128, key2: u128) -> AHasher { + let pi: [u128; 2] = PI.convert(); + let key1: [u64; 2] = (key1 ^ pi[0]).convert(); + let key2: [u64; 2] = (key2 ^ pi[1]).convert(); + AHasher { + buffer: key1[0], + pad: key1[1], + extra_keys: key2, + } + } + + #[allow(unused)] // False positive + pub(crate) fn test_with_keys(key1: u128, key2: u128) -> Self { + let key1: [u64; 2] = key1.convert(); + let key2: [u64; 2] = key2.convert(); + Self { + buffer: key1[0], + pad: key1[1], + extra_keys: key2, + } + } + + #[inline] + #[allow(dead_code)] // Is not called if non-fallback hash is used. + pub(crate) fn from_random_state(rand_state: &RandomState) -> AHasher { + AHasher { + buffer: rand_state.k0, + pad: rand_state.k1, + extra_keys: [rand_state.k2, rand_state.k3], + } + } + + /// This update function has the goal of updating the buffer with a single multiply + /// FxHash does this but is vulnerable to attack. To avoid this input needs to be masked to with an + /// unpredictable value. Other hashes such as murmurhash have taken this approach but were found vulnerable + /// to attack. The attack was based on the idea of reversing the pre-mixing (Which is necessarily + /// reversible otherwise bits would be lost) then placing a difference in the highest bit before the + /// multiply used to mix the data. Because a multiply can never affect the bits to the right of it, a + /// subsequent update that also differed in this bit could result in a predictable collision. + /// + /// This version avoids this vulnerability while still only using a single multiply. It takes advantage + /// of the fact that when a 64 bit multiply is performed the upper 64 bits are usually computed and thrown + /// away. Instead it creates two 128 bit values where the upper 64 bits are zeros and multiplies them. + /// (The compiler is smart enough to turn this into a 64 bit multiplication in the assembly) + /// Then the upper bits are xored with the lower bits to produce a single 64 bit result. + /// + /// To understand why this is a good scrambling function it helps to understand multiply-with-carry PRNGs: + /// https://en.wikipedia.org/wiki/Multiply-with-carry_pseudorandom_number_generator + /// If the multiple is chosen well, this creates a long period, decent quality PRNG. + /// Notice that this function is equivalent to this except the `buffer`/`state` is being xored with each + /// new block of data. In the event that data is all zeros, it is exactly equivalent to a MWC PRNG. + /// + /// This is impervious to attack because every bit buffer at the end is dependent on every bit in + /// `new_data ^ buffer`. For example suppose two inputs differed in only the 5th bit. Then when the + /// multiplication is performed the `result` will differ in bits 5-69. More specifically it will differ by + /// 2^5 * MULTIPLE. However in the next step bits 65-128 are turned into a separate 64 bit value. So the + /// differing bits will be in the lower 6 bits of this value. The two intermediate values that differ in + /// bits 5-63 and in bits 0-5 respectively get added together. Producing an output that differs in every + /// bit. The addition carries in the multiplication and at the end additionally mean that the even if an + /// attacker somehow knew part of (but not all) the contents of the buffer before hand, + /// they would not be able to predict any of the bits in the buffer at the end. + #[inline(always)] + fn update(&mut self, new_data: u64) { + self.buffer = folded_multiply(new_data ^ self.buffer, MULTIPLE); + } + + /// Similar to the above this function performs an update using a "folded multiply". + /// However it takes in 128 bits of data instead of 64. Both halves must be masked. + /// + /// This makes it impossible for an attacker to place a single bit difference between + /// two blocks so as to cancel each other. + /// + /// However this is not sufficient. to prevent (a,b) from hashing the same as (b,a) the buffer itself must + /// be updated between calls in a way that does not commute. To achieve this XOR and Rotate are used. + /// Add followed by xor is not the same as xor followed by add, and rotate ensures that the same out bits + /// can't be changed by the same set of input bits. To cancel this sequence with subsequent input would require + /// knowing the keys. + #[inline(always)] + fn large_update(&mut self, new_data: u128) { + let block: [u64; 2] = new_data.convert(); + let combined = folded_multiply(block[0] ^ self.extra_keys[0], block[1] ^ self.extra_keys[1]); + self.buffer = (self.buffer.wrapping_add(self.pad) ^ combined).rotate_left(ROT); + } + + #[inline] + #[cfg(feature = "specialize")] + fn short_finish(&self) -> u64 { + self.buffer.wrapping_add(self.pad) + } +} + +/// Provides [Hasher] methods to hash all of the primitive types. +/// +/// [Hasher]: core::hash::Hasher +impl Hasher for AHasher { + #[inline] + fn write_u8(&mut self, i: u8) { + self.update(i as u64); + } + + #[inline] + fn write_u16(&mut self, i: u16) { + self.update(i as u64); + } + + #[inline] + fn write_u32(&mut self, i: u32) { + self.update(i as u64); + } + + #[inline] + fn write_u64(&mut self, i: u64) { + self.update(i as u64); + } + + #[inline] + fn write_u128(&mut self, i: u128) { + self.large_update(i); + } + + #[inline] + #[cfg(any( + target_pointer_width = "64", + target_pointer_width = "32", + target_pointer_width = "16" + ))] + fn write_usize(&mut self, i: usize) { + self.write_u64(i as u64); + } + + #[inline] + #[cfg(target_pointer_width = "128")] + fn write_usize(&mut self, i: usize) { + self.write_u128(i as u128); + } + + #[inline] + #[allow(clippy::collapsible_if)] + fn write(&mut self, input: &[u8]) { + let mut data = input; + let length = data.len() as u64; + //Needs to be an add rather than an xor because otherwise it could be canceled with carefully formed input. + self.buffer = self.buffer.wrapping_add(length).wrapping_mul(MULTIPLE); + //A 'binary search' on sizes reduces the number of comparisons. + if data.len() > 8 { + if data.len() > 16 { + let tail = data.read_last_u128(); + self.large_update(tail); + while data.len() > 16 { + let (block, rest) = data.read_u128(); + self.large_update(block); + data = rest; + } + } else { + self.large_update([data.read_u64().0, data.read_last_u64()].convert()); + } + } else { + let value = read_small(data); + self.large_update(value.convert()); + } + } + + #[inline] + fn finish(&self) -> u64 { + let rot = (self.buffer & 63) as u32; + folded_multiply(self.buffer, self.pad).rotate_left(rot) + } +} + +#[cfg(feature = "specialize")] +pub(crate) struct AHasherU64 { + pub(crate) buffer: u64, + pub(crate) pad: u64, +} + +/// A specialized hasher for only primitives under 64 bits. +#[cfg(feature = "specialize")] +impl Hasher for AHasherU64 { + #[inline] + fn finish(&self) -> u64 { + let rot = (self.pad & 63) as u32; + self.buffer.rotate_left(rot) + } + + #[inline] + fn write(&mut self, _bytes: &[u8]) { + unreachable!("Specialized hasher was called with a different type of object") + } + + #[inline] + fn write_u8(&mut self, i: u8) { + self.write_u64(i as u64); + } + + #[inline] + fn write_u16(&mut self, i: u16) { + self.write_u64(i as u64); + } + + #[inline] + fn write_u32(&mut self, i: u32) { + self.write_u64(i as u64); + } + + #[inline] + fn write_u64(&mut self, i: u64) { + self.buffer = folded_multiply(i ^ self.buffer, MULTIPLE); + } + + #[inline] + fn write_u128(&mut self, _i: u128) { + unreachable!("Specialized hasher was called with a different type of object") + } + + #[inline] + fn write_usize(&mut self, _i: usize) { + unreachable!("Specialized hasher was called with a different type of object") + } +} + +#[cfg(feature = "specialize")] +pub(crate) struct AHasherFixed(pub AHasher); + +/// A specialized hasher for fixed size primitives larger than 64 bits. +#[cfg(feature = "specialize")] +impl Hasher for AHasherFixed { + #[inline] + fn finish(&self) -> u64 { + self.0.short_finish() + } + + #[inline] + fn write(&mut self, bytes: &[u8]) { + self.0.write(bytes) + } + + #[inline] + fn write_u8(&mut self, i: u8) { + self.write_u64(i as u64); + } + + #[inline] + fn write_u16(&mut self, i: u16) { + self.write_u64(i as u64); + } + + #[inline] + fn write_u32(&mut self, i: u32) { + self.write_u64(i as u64); + } + + #[inline] + fn write_u64(&mut self, i: u64) { + self.0.write_u64(i); + } + + #[inline] + fn write_u128(&mut self, i: u128) { + self.0.write_u128(i); + } + + #[inline] + fn write_usize(&mut self, i: usize) { + self.0.write_usize(i); + } +} + +#[cfg(feature = "specialize")] +pub(crate) struct AHasherStr(pub AHasher); + +/// A specialized hasher for a single string +/// Note that the other types don't panic because the hash impl for String tacks on an unneeded call. (As does vec) +#[cfg(feature = "specialize")] +impl Hasher for AHasherStr { + #[inline] + fn finish(&self) -> u64 { + self.0.finish() + } + + #[inline] + fn write(&mut self, bytes: &[u8]) { + if bytes.len() > 8 { + self.0.write(bytes) + } else { + let value = read_small(bytes); + self.0.buffer = folded_multiply(value[0] ^ self.0.buffer, value[1] ^ self.0.extra_keys[1]); + self.0.pad = self.0.pad.wrapping_add(bytes.len() as u64); + } + } + + #[inline] + fn write_u8(&mut self, _i: u8) {} + + #[inline] + fn write_u16(&mut self, _i: u16) {} + + #[inline] + fn write_u32(&mut self, _i: u32) {} + + #[inline] + fn write_u64(&mut self, _i: u64) {} + + #[inline] + fn write_u128(&mut self, _i: u128) {} + + #[inline] + fn write_usize(&mut self, _i: usize) {} +} + +#[cfg(test)] +mod tests { + use crate::convert::Convert; + use crate::fallback_hash::*; + + #[test] + fn test_hash() { + let mut hasher = AHasher::new_with_keys(0, 0); + let value: u64 = 1 << 32; + hasher.update(value); + let result = hasher.buffer; + let mut hasher = AHasher::new_with_keys(0, 0); + let value2: u64 = 1; + hasher.update(value2); + let result2 = hasher.buffer; + let result: [u8; 8] = result.convert(); + let result2: [u8; 8] = result2.convert(); + assert_ne!(hex::encode(result), hex::encode(result2)); + } + + #[test] + fn test_conversion() { + let input: &[u8] = "dddddddd".as_bytes(); + let bytes: u64 = as_array!(input, 8).convert(); + assert_eq!(bytes, 0x6464646464646464); + } +} |