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[WIP] BoxedResidue: fix constant-time TODO in montgomery_mul #415

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tarcieri wants to merge 1 commit into from
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[WIP] BoxedResidue: fix constant-time TODO in montgomery_mul #415

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13 changes: 9 additions & 4 deletions src/modular/boxed_residue/mul.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ use core::{
borrow::Borrow,
ops::{Mul, MulAssign},
};
use subtle::{ConditionallySelectable, ConstantTimeLess};
use subtle::ConditionallySelectable;

#[cfg(feature = "zeroize")]
use zeroize::Zeroize;
Expand Down Expand Up @@ -211,7 +211,7 @@ fn montgomery_mul(z: &mut [Limb], x: &[Limb], y: &[Limb], m: &[Limb], k: Limb) {
let cx = c.wrapping_add(c2);
let cy = cx.wrapping_add(c3);
z[n + i] = cy;
c = Limb((cx.ct_lt(&c2) | cy.ct_lt(&c3)).unwrap_u8() as Word);
c = limb_ct_lt(cx, c2, cy, c3);
}

let (lower, upper) = z.split_at_mut(n);
Expand Down Expand Up @@ -255,8 +255,7 @@ fn sub_vv(z: &mut [Limb], x: &[Limb], y: &[Limb]) -> Limb {
fn add_ww(x: Limb, y: Limb, c: Limb) -> (Limb, Limb) {
let yc = y.wrapping_add(c);
let z0 = x.wrapping_add(yc);
// TODO(tarcieri): eliminate data-dependent branches
let z1 = Limb((z0.0 < x.0 || yc.0 < y.0) as Word);
let z1 = limb_ct_lt(z0, x, yc, y);
(z1, z0)
}

Expand All @@ -266,3 +265,9 @@ fn mul_add_www(x: Limb, y: Limb, c: Limb) -> (Limb, Limb) {
let z = x.0 as WideWord * y.0 as WideWord + c.0 as WideWord;
(Limb((z >> Word::BITS) as Word), Limb(z as Word))
}

/// Compare limbs in constant time, returning `Limb::ONE` if the left size is less than the right.
#[inline(always)]
fn limb_ct_lt(a1: Limb, b1: Limb, a2: Limb, b2: Limb) -> Limb {
(a1.sbb(b1, Limb::ZERO).1 | a2.sbb(b2, Limb::ZERO).1) & Limb::ONE
}
Comment on lines +269 to +273
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@tarcieri tarcieri Dec 10, 2023

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This was faster than using the ct_lt approach previously used on L214, but still slower than using branch instructions