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Making field element operations constant-time.
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Original file line number | Diff line number | Diff line change |
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use std::ops; | ||
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use crate::kem::kyber768::{parameters::FIELD_MODULUS, utils::field::FieldElement}; | ||
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#[derive(Debug, Clone, Copy, PartialEq, Eq)] | ||
pub struct KyberFieldElement { | ||
pub value: u16, | ||
} | ||
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impl KyberFieldElement { | ||
pub const MODULUS: u16 = FIELD_MODULUS; | ||
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const BARRETT_SHIFT : u32 = 24; // 2 * ceil(log_2(FIELD_MODULUS)) | ||
const BARRETT_MULTIPLIER : u32 = (1u32 << Self::BARRETT_SHIFT) / (Self::MODULUS as u32); | ||
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pub fn barrett_reduce(value : u32) -> Self { | ||
let product : u64 = u64::from(value) * u64::from(Self::BARRETT_MULTIPLIER); | ||
let quotient : u32 = (product >> Self::BARRETT_SHIFT).try_into().unwrap(); | ||
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let remainder = value - (quotient * u32::from(Self::MODULUS)); | ||
let remainder : u16 = remainder.try_into().unwrap(); | ||
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let remainder_minus_modulus = remainder.wrapping_sub(Self::MODULUS); | ||
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// TODO: Check if LLVM detects this and optimizes it away into a | ||
// conditional. | ||
let selector = 0u16.wrapping_sub((remainder_minus_modulus >> 15) & 1); | ||
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Self { | ||
value: (selector & remainder) | (!selector & remainder_minus_modulus), | ||
} | ||
} | ||
} | ||
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impl FieldElement for KyberFieldElement { | ||
const ZERO: Self = Self { value: 0 }; | ||
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fn new(number: u16) -> Self { | ||
Self::barrett_reduce(u32::from(number)) | ||
} | ||
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fn nth_bit_little_endian(&self, n: usize) -> u8 { | ||
((self.value >> n) & 1) as u8 | ||
} | ||
} | ||
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impl From<u8> for KyberFieldElement { | ||
fn from(number: u8) -> Self { | ||
Self { | ||
value: u16::from(number) | ||
} | ||
} | ||
} | ||
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impl From<KyberFieldElement> for u16 { | ||
fn from(fe: KyberFieldElement) -> Self { | ||
fe.value | ||
} | ||
} | ||
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impl ops::Add for KyberFieldElement { | ||
type Output = Self; | ||
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fn add(self, other: Self) -> Self { | ||
let sum: u16 = self.value + other.value; | ||
let difference: u16 = sum.wrapping_sub(Self::MODULUS); | ||
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let mask = 0u16.wrapping_sub((difference >> 15) & 1); | ||
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Self { | ||
value: (mask & sum) | (!mask & difference), | ||
} | ||
} | ||
} | ||
impl ops::Sub for KyberFieldElement { | ||
type Output = Self; | ||
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fn sub(self, other: Self) -> Self { | ||
let lhs = self.value; | ||
let rhs = Self::MODULUS - other.value; | ||
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let sum: u16 = lhs + rhs; | ||
let difference: u16 = sum.wrapping_sub(Self::MODULUS); | ||
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let mask = 0u16.wrapping_sub((difference >> 15) & 1); | ||
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Self { | ||
value: (mask & sum) | (!mask & difference), | ||
} | ||
} | ||
} | ||
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impl ops::Mul for KyberFieldElement { | ||
type Output = Self; | ||
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fn mul(self, other: Self) -> Self { | ||
let product: u32 = u32::from(self.value) * u32::from(other.value); | ||
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Self::barrett_reduce(product) | ||
} | ||
} | ||
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impl ops::Mul<u16> for KyberFieldElement { | ||
type Output = Self; | ||
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fn mul(self, other: u16) -> Self { | ||
let product: u32 = u32::from(self.value) * u32::from(other); | ||
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Self::barrett_reduce(product) | ||
} | ||
} |
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