Abstract out verify logic for fe_cmp_var

Author: sipa

src/field.h

@@ -84,6 +84,7 @@ static const secp256k1_fe secp256k1_const_beta = SECP256K1_FE_CONST(
 #  define secp256k1_fe_clear secp256k1_fe_impl_clear
 #  define secp256k1_fe_is_zero secp256k1_fe_impl_is_zero
 #  define secp256k1_fe_is_odd secp256k1_fe_impl_is_odd
+#  define secp256k1_fe_cmp_var secp256k1_fe_impl_cmp_var
 #endif /* !defined(VERIFY) */
 
 /** Normalize a field element.
@@ -164,7 +165,12 @@ static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b);
  */
 static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b);
 
-/** Compare two field elements. Requires both inputs to be normalized */
+/** Compare the values represented by 2 field elements, without constant-time guarantee.
+ *
+ * On input, a and b must be valid normalized field elements.
+ * Returns 1 if a > b, -1 if a < b, and 0 if a = b (comparisons are done as integers
+ * in range 0..p-1).
+ */
 static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b);
 
 /** Set a field element equal to 32-byte big endian value.

src/field_10x26_impl.h

@@ -285,14 +285,8 @@ SECP256K1_INLINE static void secp256k1_fe_impl_clear(secp256k1_fe *a) {
     }
 }
 
-static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
+static int secp256k1_fe_impl_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
     int i;
-#ifdef VERIFY
-    VERIFY_CHECK(a->normalized);
-    VERIFY_CHECK(b->normalized);
-    secp256k1_fe_verify(a);
-    secp256k1_fe_verify(b);
-#endif
     for (i = 9; i >= 0; i--) {
         if (a->n[i] > b->n[i]) {
             return 1;

src/field_5x52_impl.h

@@ -231,14 +231,8 @@ SECP256K1_INLINE static void secp256k1_fe_impl_clear(secp256k1_fe *a) {
     }
 }
 
-static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
+static int secp256k1_fe_impl_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
     int i;
-#ifdef VERIFY
-    VERIFY_CHECK(a->normalized);
-    VERIFY_CHECK(b->normalized);
-    secp256k1_fe_verify(a);
-    secp256k1_fe_verify(b);
-#endif
     for (i = 4; i >= 0; i--) {
         if (a->n[i] > b->n[i]) {
             return 1;

src/field_impl.h

@@ -228,6 +228,15 @@ SECP256K1_INLINE static int secp256k1_fe_is_odd(const secp256k1_fe *a) {
     VERIFY_CHECK(a->normalized);
     return secp256k1_fe_impl_is_odd(a);
 }
+
+static int secp256k1_fe_impl_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b);
+SECP256K1_INLINE static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
+    secp256k1_fe_verify(a);
+    secp256k1_fe_verify(b);
+    VERIFY_CHECK(a->normalized);
+    VERIFY_CHECK(b->normalized);
+    return secp256k1_fe_impl_cmp_var(a, b);
+}
 #endif /* defined(VERIFY) */
 
 #endif /* SECP256K1_FIELD_IMPL_H */