Add header for mbedtls_mpi_exp_mod_unsafe()

To silence no previous prototype warnings. And this is the proper way to
do it anyway.

Signed-off-by: Janos Follath <janos.follath@arm.com>

library/bignum_internal.h

@@ -0,0 +1,50 @@
+/**
+ * \file bignum_internal.h
+ *
+ * \brief Internal-only bignum public-key cryptosystem API.
+ *
+ * This file declares bignum-related functions that are to be used
+ * only from within the Mbed TLS library itself.
+ *
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+#ifndef MBEDTLS_BIGNUM_INTERNAL_H
+#define MBEDTLS_BIGNUM_INTERNAL_H
+
+/**
+ * \brief          Perform a modular exponentiation: X = A^E mod N
+ *
+ * \warning        This function is not constant time with respect to \p E (the exponent).
+ *
+ * \param X        The destination MPI. This must point to an initialized MPI.
+ *                 This must not alias E or N.
+ * \param A        The base of the exponentiation.
+ *                 This must point to an initialized MPI.
+ * \param E        The exponent MPI. This must point to an initialized MPI.
+ * \param N        The base for the modular reduction. This must point to an
+ *                 initialized MPI.
+ * \param prec_RR  A helper MPI depending solely on \p N which can be used to
+ *                 speed-up multiple modular exponentiations for the same value
+ *                 of \p N. This may be \c NULL. If it is not \c NULL, it must
+ *                 point to an initialized MPI. If it hasn't been used after
+ *                 the call to mbedtls_mpi_init(), this function will compute
+ *                 the helper value and store it in \p prec_RR for reuse on
+ *                 subsequent calls to this function. Otherwise, the function
+ *                 will assume that \p prec_RR holds the helper value set by a
+ *                 previous call to mbedtls_mpi_exp_mod(), and reuse it.
+ *
+ * \return         \c 0 if successful.
+ * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.
+ * \return         #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if \c N is negative or
+ *                 even, or if \c E is negative.
+ * \return         Another negative error code on different kinds of failures.
+ *
+ */
+int mbedtls_mpi_exp_mod_unsafe(mbedtls_mpi *X, const mbedtls_mpi *A,
+                               const mbedtls_mpi *E, const mbedtls_mpi *N,
+                               mbedtls_mpi *prec_RR);
+
+#endif /* bignum_internal.h */

tf-psa-crypto/drivers/builtin/src/bignum.c

@@ -27,6 +27,7 @@
 
 #include "mbedtls/bignum.h"
 #include "bignum_core.h"
+#include "bignum_internal.h"
 #include "bn_mul.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"

tf-psa-crypto/drivers/builtin/src/rsa.c

@@ -29,6 +29,7 @@
 
 #include "mbedtls/rsa.h"
 #include "bignum_core.h"
+#include "bignum_internal.h"
 #include "rsa_alt_helpers.h"
 #include "rsa_internal.h"
 #include "mbedtls/oid.h"
@@ -1226,16 +1227,6 @@ int mbedtls_rsa_check_pub_priv(const mbedtls_rsa_context *pub,
     return 0;
 }
 
-/*
- * This function is identical to mbedtls_mpi_exp_mod() the only difference is that this function is
- * not constant time.
- *
- * WARNING! This function is not constant time.
- */
-int mbedtls_mpi_exp_mod_unsafe(mbedtls_mpi *X, const mbedtls_mpi *A,
-                               const mbedtls_mpi *E, const mbedtls_mpi *N,
-                               mbedtls_mpi *prec_RR);
-
 /*
  * Do an RSA public key operation
  */

tf-psa-crypto/tests/suites/test_suite_bignum.function

@@ -3,6 +3,7 @@
 #include "mbedtls/entropy.h"
 #include "constant_time_internal.h"
 #include "bignum_core.h"
+#include "bignum_internal.h"
 #include "test/constant_flow.h"
 
 #if MBEDTLS_MPI_MAX_BITS > 792