Add inter-module private header for EC functions

Internal submodules of libcrypto may require non-public functions from
the EC submodule.

In preparation to use `ec_group_do_inverse_ord()` (from #6116) inside
the SM2 submodule to apply a SCA mitigation on the modular inversion,
this commit moves the `ec_group_do_inverse_ord()` prototype declaration
from the EC-local `crypto/ec/ec_lcl.h` header to the
`crypto/include/internal/ec_int.h` inter-module private header.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6521)

crypto/ec/ec_lcl.h

@@ -14,6 +14,7 @@
 #include <openssl/ec.h>
 #include <openssl/bn.h>
 #include "internal/refcount.h"
+#include "internal/ec_int.h"
 #include "curve448/curve448_lcl.h"
 
 #if defined(__SUNPRO_C)
@@ -636,7 +637,4 @@ int X25519(uint8_t out_shared_key[32], const uint8_t private_key[32],
 void X25519_public_from_private(uint8_t out_public_value[32],
                                 const uint8_t private_key[32]);
 
-int ec_group_do_inverse_ord(const EC_GROUP *group, BIGNUM *res,
-                            const BIGNUM *x, BN_CTX *ctx);
-
 int ec_point_blind_coordinates(const EC_GROUP *group, EC_POINT *p, BN_CTX *ctx);

crypto/include/internal/ec_int.h

+/*
+ * Copyright 2018 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the OpenSSL license (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+/* Internal EC functions for other submodules: not for application use */
+
+#ifndef HEADER_OSSL_EC_INTERNAL_H
+# define HEADER_OSSL_EC_INTERNAL_H
+# include <openssl/opensslconf.h>
+
+# ifndef OPENSSL_NO_EC
+
+#  include <openssl/ec.h>
+
+/*-
+ * Computes the multiplicative inverse of x in the range
+ * [1,EC_GROUP::order), where EC_GROUP::order is the cardinality of the
+ * subgroup generated by the generator G:
+ *
+ *         res := x^(-1) (mod EC_GROUP::order).
+ *
+ * This function expects the following two conditions to hold:
+ *  - the EC_GROUP order is prime, and
+ *  - x is included in the range [1, EC_GROUP::order).
+ *
+ * This function returns 1 on success, 0 on error.
+ *
+ * If the EC_GROUP order is even, this function explicitly returns 0 as
+ * an error.
+ * In case any of the two conditions stated above is not satisfied,
+ * the correctness of its output is not guaranteed, even if the return
+ * value could still be 1 (as primality testing and a conditional modular
+ * reduction round on the input can be omitted by the underlying
+ * implementations for better SCA properties on regular input values).
+ */
+__owur int ec_group_do_inverse_ord(const EC_GROUP *group, BIGNUM *res,
+                                   const BIGNUM *x, BN_CTX *ctx);
+
+# endif /* OPENSSL_NO_EC */
+#endif