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Merged
tarcieri merged 1 commit into from
Mar 9, 2026
Merged

Improve performance of Monty pow/pow_vartime #1222

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25 changes: 13 additions & 12 deletions src/modular/pow.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@ use core::{array, mem};
use alloc::vec::Vec;

const WINDOW: u32 = 4;
const WINDOW_COUNT: usize = 1 << WINDOW;
const WINDOW_MASK: Word = (1 << WINDOW) - 1;
#[allow(clippy::integer_division_remainder_used, reason = "constant")]
const BITS_PER_WINDOW: u32 = Limb::BITS / WINDOW;
Expand Down Expand Up @@ -59,10 +60,10 @@ where
let mut power = x.clone();

// powers[i] contains x^i
let powers: [U; 1 << WINDOW] = array::from_fn(|n| {
let powers: [U; WINDOW_COUNT] = array::from_fn(|n| {
if n == 0 {
one.clone()
} else if n == (1 << WINDOW) - 1 {
} else if n == WINDOW_COUNT - 1 {
power.clone()
} else {
let mut new_power = power.clone();
Expand Down Expand Up @@ -102,7 +103,7 @@ where

// Constant-time lookup in the array of powers
power.as_mut_limbs().copy_from_slice(powers[0].as_limbs());
for i in 1..(1 << WINDOW) {
for i in 1..WINDOW_COUNT {
power.ct_assign(&powers[i], (i as Word).ct_eq(&idx));
}

Expand All @@ -129,7 +130,7 @@ pub const fn multi_exponentiate_montgomery_form_array<
}

let mut powers_and_exponents =
[([Uint::<LIMBS>::ZERO; 1 << WINDOW], Uint::<RHS_LIMBS>::ZERO); N];
[([Uint::<LIMBS>::ZERO; WINDOW_COUNT], Uint::<RHS_LIMBS>::ZERO); N];

let mut i = 0;
while i < N {
Expand Down Expand Up @@ -165,7 +166,7 @@ pub fn multi_exponentiate_montgomery_form_slice<
return *params.one(); // 1 in Montgomery form
}

let powers_and_exponents: Vec<([Uint<LIMBS>; 1 << WINDOW], Uint<RHS_LIMBS>)> =
let powers_and_exponents: Vec<([Uint<LIMBS>; WINDOW_COUNT], Uint<RHS_LIMBS>)> =
bases_and_exponents
.iter()
.map(|(base, exponent)| (compute_powers(base, params), *exponent))
Expand All @@ -181,9 +182,9 @@ pub fn multi_exponentiate_montgomery_form_slice<
const fn compute_powers<const LIMBS: usize>(
x: &Uint<LIMBS>,
params: &FixedMontyParams<LIMBS>,
) -> [Uint<LIMBS>; 1 << WINDOW] {
) -> [Uint<LIMBS>; WINDOW_COUNT] {
// powers[i] contains x^i
let mut powers = [*params.one(); 1 << WINDOW];
let mut powers = [*params.one(); WINDOW_COUNT];
powers[1] = *x;

let mut i = 2;
Expand All @@ -201,7 +202,7 @@ const fn multi_exponentiate_montgomery_form_internal<
const RHS_LIMBS: usize,
const VARTIME: bool,
>(
powers_and_exponents: &[([Uint<LIMBS>; 1 << WINDOW], Uint<RHS_LIMBS>)],
powers_and_exponents: &[([Uint<LIMBS>; WINDOW_COUNT], Uint<RHS_LIMBS>)],
exponent_bits: u32,
params: &FixedMontyParams<LIMBS>,
) -> Uint<LIMBS> {
Expand Down Expand Up @@ -232,7 +233,7 @@ const fn multi_exponentiate_montgomery_form_internal<

let mut i = 0;
while i < powers_and_exponents.len() {
let (powers, exponent) = powers_and_exponents[i];
let (powers, exponent) = &powers_and_exponents[i];
let w = exponent.as_limbs()[limb_num].0;
let mut idx = (w >> (window_num * WINDOW)) & WINDOW_MASK;

Expand All @@ -253,9 +254,9 @@ const fn multi_exponentiate_montgomery_form_internal<
// Constant-time lookup in the array of powers
let mut power = powers[0];
let mut j = 1;
while j < 1 << WINDOW {
let choice = word::choice_from_eq(j, idx);
power = Uint::<LIMBS>::select(&power, &powers[j as usize], choice);
while j < WINDOW_COUNT {
let choice = word::choice_from_eq(j as Word, idx);
power = Uint::<LIMBS>::select(&power, &powers[j], choice);
j += 1;
}
z = mul_montgomery_form(&z, &power, params.modulus(), params.mod_neg_inv());
Expand Down