Add _fe_verify_magnitude (no-op unless VERIFY is enabled)

Co-authored-by: Tim Ruffing <crypto@timruffing.de>
BlockstreamResearch · Jul 21, 2023 · 4e9661f · 4e9661f
1 parent 690b0fc
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diff --git a/src/field.h b/src/field.h
@@ -352,4 +352,7 @@ static int secp256k1_fe_is_square_var(const secp256k1_fe *a);
 /** Check invariants on a field element (no-op unless VERIFY is enabled). */
 static void secp256k1_fe_verify(const secp256k1_fe *a);
 
+/** Check that magnitude of a is at most m (no-op unless VERIFY is enabled). */
+static void secp256k1_fe_verify_magnitude(const secp256k1_fe *a, int m);
+
 #endif /* SECP256K1_FIELD_H */
diff --git a/src/field_impl.h b/src/field_impl.h
@@ -159,6 +159,7 @@ static int secp256k1_fe_sqrt(secp256k1_fe * SECP256K1_RESTRICT r, const secp256k
 
 #ifndef VERIFY
 static void secp256k1_fe_verify(const secp256k1_fe *a) { (void)a; }
+static void secp256k1_fe_verify_magnitude(const secp256k1_fe *a, int m) { (void)a; (void)m; }
 #else
 static void secp256k1_fe_impl_verify(const secp256k1_fe *a);
 static void secp256k1_fe_verify(const secp256k1_fe *a) {
@@ -172,6 +173,12 @@ static void secp256k1_fe_verify(const secp256k1_fe *a) {
     secp256k1_fe_impl_verify(a);
 }
 
+static void secp256k1_fe_verify_magnitude(const secp256k1_fe *a, int m) {
+    VERIFY_CHECK(m >= 0);
+    VERIFY_CHECK(m <= 32);
+    VERIFY_CHECK(a->magnitude <= m);
+}
+
 static void secp256k1_fe_impl_normalize(secp256k1_fe *r);
 SECP256K1_INLINE static void secp256k1_fe_normalize(secp256k1_fe *r) {
     secp256k1_fe_verify(r);