Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

backport fix for timing variability in backend/serial/u64/scalar.rs #25

Open
wants to merge 1 commit into
base: main
Choose a base branch
from
Open

backport fix for timing variability in backend/serial/u64/scalar.rs #25

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
12 changes: 11 additions & 1 deletion src/backend/serial/u64/scalar.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -170,6 +170,14 @@ impl Scalar52 {

/// Compute `a - b` (mod l)
pub fn sub(a: &Scalar52, b: &Scalar52) -> Scalar52 {
// Backport of https://github.com/dalek-cryptography/curve25519-dalek/pull/659
// 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 @@ -184,7 +192,9 @@ 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;
}

Expand Down