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SECURITY: fix timing variability in backend/serial/u32/scalar.rs #661

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Merged
merged 1 commit into from
Jun 18, 2024
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SECURITY: fix timing variability in backend/serial/u32/scalar.rs #661

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10 changes: 9 additions & 1 deletion curve25519-dalek/src/backend/serial/u32/scalar.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -185,6 +185,14 @@ impl Scalar29 {

/// Compute `a - b` (mod l).
pub fn sub(a: &Scalar29, b: &Scalar29) -> Scalar29 {
// Optimization barrier to prevent compiler from inserting branch instructions
// TODO(tarcieri): find a better home (or abstraction) for this
fn black_box(value: u32) -> u32 {
// SAFETY: `u32` is a simple integer `Copy` type and `value` lives on the stack so
// a pointer to it will be valid.
unsafe { core::ptr::read_volatile(&value) }
}

let mut difference = Scalar29::ZERO;
let mask = (1u32 << 29) - 1;

Expand All @@ -199,7 +207,7 @@ impl Scalar29 {
let underflow_mask = ((borrow >> 31) ^ 1).wrapping_sub(1);
let mut carry: u32 = 0;
for i in 0..9 {
carry = (carry >> 29) + difference[i] + (constants::L[i] & underflow_mask);
carry = (carry >> 29) + difference[i] + (constants::L[i] & black_box(underflow_mask));
difference[i] = carry & mask;
}

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