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SECURITY: fix timing variability in backend/serial/u64/scalar.rs #659

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Jun 18, 2024
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SECURITY: fix timing variability in backend/serial/u64/scalar.rs #659

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12 changes: 11 additions & 1 deletion curve25519-dalek/src/backend/serial/u64/scalar.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -174,6 +174,14 @@ impl Scalar52 {

/// Compute `a - b` (mod l)
pub fn sub(a: &Scalar52, b: &Scalar52) -> Scalar52 {
// Optimization barrier to prevent compiler from inserting branch instructions
// TODO(tarcieri): find a better home (or abstraction) for this
fn black_box(value: u64) -> u64 {
// SAFETY: `u64` 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 = Scalar52::ZERO;
let mask = (1u64 << 52) - 1;

Expand All @@ -188,7 +196,9 @@ impl Scalar52 {
let underflow_mask = ((borrow >> 63) ^ 1).wrapping_sub(1);
let mut carry: u64 = 0;
for i in 0..5 {
carry = (carry >> 52) + difference[i] + (constants::L[i] & underflow_mask);
// SECURITY: `black_box` prevents LLVM from inserting a `jns` conditional on x86(_64)
// which can be used to bypass this section when `underflow_mask` is zero.
carry = (carry >> 52) + difference[i] + (constants::L[i] & black_box(underflow_mask));
difference[i] = carry & mask;
}

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