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Making field element operations constant-time.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,111 @@ | ||
| 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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