Fix VERIFY calculations in _fe_cmov methods

src/field_10x26_impl.h

@@ -1083,8 +1083,10 @@ static SECP256K1_INLINE void secp256k1_fe_cmov(secp256k1_fe_t *r, const secp256k
     r->n[8] = (r->n[8] & mask0) | (a->n[8] & mask1);
     r->n[9] = (r->n[9] & mask0) | (a->n[9] & mask1);
 #ifdef VERIFY
-    r->magnitude = (r->magnitude & mask0) | (a->magnitude & mask1);
+    if (a->magnitude > r->magnitude) {
-    r->normalized = (r->normalized & mask0) | (a->normalized & mask1);
+        r->magnitude = a->magnitude;
+    }
+    r->normalized &= a->normalized;
 #endif
 }
 

src/field_5x52_impl.h

@@ -414,8 +414,10 @@ static SECP256K1_INLINE void secp256k1_fe_cmov(secp256k1_fe_t *r, const secp256k
     r->n[3] = (r->n[3] & mask0) | (a->n[3] & mask1);
     r->n[4] = (r->n[4] & mask0) | (a->n[4] & mask1);
 #ifdef VERIFY
-    r->magnitude = (r->magnitude & mask0) | (a->magnitude & mask1);
+    if (a->magnitude > r->magnitude) {
-    r->normalized = (r->normalized & mask0) | (a->normalized & mask1);
+        r->magnitude = a->magnitude;
+    }
+    r->normalized &= a->normalized;
 #endif
 }
 

src/tests.c

@@ -737,6 +737,15 @@ void run_field_convert(void) {
     CHECK(memcmp(&fes2, &fes, sizeof(fes)) == 0);
 }
 
+int fe_memcmp(const secp256k1_fe_t *a, const secp256k1_fe_t *b) {
+    secp256k1_fe_t t = *b;
+#ifdef VERIFY
+    t.magnitude = a->magnitude;
+    t.normalized = a->normalized;
+#endif
+    return memcmp(a, &t, sizeof(secp256k1_fe_t));
+}
+
 void run_field_misc(void) {
     secp256k1_fe_t x;
     secp256k1_fe_t y;
@@ -757,12 +766,13 @@ void run_field_misc(void) {
         q = x;
         secp256k1_fe_cmov(&x, &z, 0);
         secp256k1_fe_cmov(&x, &x, 1);
-        CHECK(memcmp(&x, &z, sizeof(x)) != 0);
+        CHECK(fe_memcmp(&x, &z) != 0);
-        CHECK(memcmp(&x, &q, sizeof(x)) == 0);
+        CHECK(fe_memcmp(&x, &q) == 0);
         secp256k1_fe_cmov(&q, &z, 1);
-        CHECK(memcmp(&q, &z, sizeof(q)) == 0);
+        CHECK(fe_memcmp(&q, &z) == 0);
         /* Test storage conversion and conditional moves. */
-        secp256k1_fe_normalize(&z);
+        secp256k1_fe_normalize_var(&x);
+        secp256k1_fe_normalize_var(&z);
         CHECK(!secp256k1_fe_equal_var(&x, &z));
         secp256k1_fe_to_storage(&xs, &x);
         secp256k1_fe_to_storage(&ys, &y);