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Fix MontFp issue in fields with 64 * k bits #550

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Merged
weikengchen merged 5 commits into from
Dec 21, 2022
Merged

Fix MontFp issue in fields with 64 * k bits #550

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46315e5
fix
onewayfunc Dec 18, 2022
fa16d1f
use the spare bit
weikengchen Dec 21, 2022
bde0c7c
mont macro
weikengchen Dec 21, 2022
86e9d2e
remove unused
weikengchen Dec 21, 2022
068f89f
Merge branch 'master' into fix-montfp
Pratyush Dec 21, 2022
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20 changes: 11 additions & 9 deletions ff/src/fields/models/fp/montgomery_backend.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -199,12 +199,14 @@ pub trait MontConfig<const N: usize>: 'static + Sync + Send + Sized {
}
(a.0).0 = r;
}
a.subtract_modulus();
} else {
// Alternative implementation
// Implements CIOS.
*a = a.mul_without_cond_subtract(b);
let (carry, res) = a.mul_without_cond_subtract(b);
*a = res;
a.subtract_modulus_with_carry(carry);
}
a.subtract_modulus();
}

#[inline(always)]
Expand Down Expand Up @@ -734,7 +736,7 @@ impl<T: MontConfig<N>, const N: usize> Fp<MontBackend<T, N>, N> {
}
}

const fn mul_without_cond_subtract(mut self, other: &Self) -> Self {
const fn mul_without_cond_subtract(mut self, other: &Self) -> (bool, Self) {
let (mut lo, mut hi) = ([0u64; N], [0u64; N]);
crate::const_for!((i in 0..N) {
let mut carry = 0;
Expand Down Expand Up @@ -768,12 +770,12 @@ impl<T: MontConfig<N>, const N: usize> Fp<MontBackend<T, N>, N> {
crate::const_for!((i in 0..N) {
(self.0).0[i] = hi[i];
});
self
(carry2 != 0, self)
}

const fn mul(mut self, other: &Self) -> Self {
self = self.mul_without_cond_subtract(other);
self.const_subtract_modulus()
const fn mul(self, other: &Self) -> Self {
let (carry, res) = self.mul_without_cond_subtract(other);
res.const_subtract_modulus_with_carry(carry)
}

const fn const_is_valid(&self) -> bool {
Expand All @@ -788,8 +790,8 @@ impl<T: MontConfig<N>, const N: usize> Fp<MontBackend<T, N>, N> {
}

#[inline]
const fn const_subtract_modulus(mut self) -> Self {
if !self.const_is_valid() {
const fn const_subtract_modulus_with_carry(mut self, carry: bool) -> Self {
if carry || !self.const_is_valid() {
self.0 = Self::sub_with_borrow(&self.0, &T::MODULUS);
}
self
Expand Down