From bc00f7e58c0c3549cdec8f2922e4038a574f6ef0 Mon Sep 17 00:00:00 2001
From: Brian Smith <brian@briansmith.org>
Date: Wed, 22 Nov 2023 13:05:44 -0800
Subject: [PATCH] ec: NFC: Refactor `scalar_sum` to eliminate `LIMBS_add_mod`
 use.

Use the pattern we typically use where one argument is passed by value.

This lets us use `limbs_add_assign_mod`, eliminating the `unsafe`
direct use of `LIMBS_add_mod`. This will make future refactoring easier.

This also eliminates the need to construct and zeroize a new scalar `r`
for the result.
---
 src/ec/suite_b/ecdsa/signing.rs |  2 +-
 src/ec/suite_b/ops.rs           | 31 ++++++++-----------------------
 2 files changed, 9 insertions(+), 24 deletions(-)

diff --git a/src/ec/suite_b/ecdsa/signing.rs b/src/ec/suite_b/ecdsa/signing.rs
index 587086c89..3320937f8 100644
--- a/src/ec/suite_b/ecdsa/signing.rs
+++ b/src/ec/suite_b/ecdsa/signing.rs
@@ -266,7 +266,7 @@ impl EcdsaKeyPair {
             // Step 6.
             let s = {
                 let dr = scalar_ops.scalar_product(&self.d, &r);
-                let e_plus_dr = scalar_sum(cops, &e, &dr);
+                let e_plus_dr = scalar_sum(cops, &e, dr);
                 scalar_ops.scalar_product(&k_inv, &e_plus_dr)
             };
             if cops.is_zero(&s) {
diff --git a/src/ec/suite_b/ops.rs b/src/ec/suite_b/ops.rs
index 579d05e14..eeb15fe8d 100644
--- a/src/ec/suite_b/ops.rs
+++ b/src/ec/suite_b/ops.rs
@@ -12,7 +12,7 @@
 // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
-use crate::{arithmetic::limbs_from_hex, arithmetic::montgomery::*, c, error, limb::*};
+use crate::{arithmetic::limbs_from_hex, arithmetic::montgomery::*, error, limb::*};
 use core::marker::PhantomData;
 
 pub use self::elem::*;
@@ -326,18 +326,13 @@ pub fn elem_reduced_to_scalar(ops: &CommonOps, elem: &Elem<Unencoded>) -> Scalar
     }
 }
 
-pub fn scalar_sum(ops: &CommonOps, a: &Scalar, b: &Scalar) -> Scalar {
-    let mut r = Scalar::zero();
-    unsafe {
-        LIMBS_add_mod(
-            r.limbs.as_mut_ptr(),
-            a.limbs.as_ptr(),
-            b.limbs.as_ptr(),
-            ops.n.limbs.as_ptr(),
-            ops.num_limbs,
-        )
-    }
-    r
+pub fn scalar_sum(ops: &CommonOps, a: &Scalar, mut b: Scalar) -> Scalar {
+    limbs_add_assign_mod(
+        &mut b.limbs[..ops.num_limbs],
+        &a.limbs[..ops.num_limbs],
+        &ops.n.limbs[..ops.num_limbs],
+    );
+    b
 }
 
 // Returns (`a` squared `squarings` times) * `b`.
@@ -425,16 +420,6 @@ fn parse_big_endian_fixed_consttime<M>(
     Ok(r)
 }
 
-prefixed_extern! {
-    fn LIMBS_add_mod(
-        r: *mut Limb,
-        a: *const Limb,
-        b: *const Limb,
-        m: *const Limb,
-        num_limbs: c::size_t,
-    );
-}
-
 #[cfg(test)]
 mod tests {
     extern crate alloc;