Improve comments/check for fe_equal{,_var}

Author: sipa

src/field.h

@@ -150,10 +150,18 @@ static int secp256k1_fe_is_zero(const secp256k1_fe *a);
  */
 static int secp256k1_fe_is_odd(const secp256k1_fe *a);
 
-/** Compare two field elements. Requires magnitude-1 inputs. */
+/** Determine whether two field elements are equal.
+ *
+ * On input, a and b must be valid field elements with magnitudes not exceeding
+ * 1 and 31, respectively.
+ * Returns a = b (mod p).
+ */
 static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b);
 
-/** Same as secp256k1_fe_equal, but may be variable time. */
+/** Determine whether two field elements are equal, without constant-time guarantee.
+ *
+ * Identical in behavior to secp256k1_fe_equal, but not constant time in either a or 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 */

src/field_impl.h

@@ -20,13 +20,25 @@
 
 SECP256K1_INLINE static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b) {
     secp256k1_fe na;
+#ifdef VERIFY
+    secp256k1_fe_verify(a);
+    secp256k1_fe_verify(b);
+    VERIFY_CHECK(a->magnitude <= 1);
+    VERIFY_CHECK(b->magnitude <= 31);
+#endif
     secp256k1_fe_negate(&na, a, 1);
     secp256k1_fe_add(&na, b);
     return secp256k1_fe_normalizes_to_zero(&na);
 }
 
 SECP256K1_INLINE static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b) {
     secp256k1_fe na;
+#ifdef VERIFY
+    secp256k1_fe_verify(a);
+    secp256k1_fe_verify(b);
+    VERIFY_CHECK(a->magnitude <= 1);
+    VERIFY_CHECK(b->magnitude <= 31);
+#endif
     secp256k1_fe_negate(&na, a, 1);
     secp256k1_fe_add(&na, b);
     return secp256k1_fe_normalizes_to_zero_var(&na);