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.k1, pad: rand_state.k0, 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 { folded_multiply(self.buffer, 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 { folded_multiply(self.buffer, self.pad) //self.buffer } #[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::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); } }