rsa: introduce rsa_internal_rsassa_pss_sign_no_mode_check()

And use it in the non-PSA version of mbedtls_pk_sign_ext()
to bypass checks that didn't succeed when used by TLS 1.3.

That is because in the failing scenarios the padding of
the RSA context is not set to PKCS_V21.

See the discussion on PR #7930 for more details.

Signed-off-by: Tomi Fontanilles <129057597+tomi-font@users.noreply.github.com>

library/rsa_internal.h

@@ -0,0 +1,42 @@
+/**
+ * \file rsa_internal.h
+ *
+ * \brief Internal-only RSA public-key cryptosystem API.
+ *
+ * This file declares RSA-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_RSA_INTERNAL_H
+#define MBEDTLS_RSA_INTERNAL_H
+
+#include "mbedtls/rsa.h"
+
+#if defined(MBEDTLS_PKCS1_V21)
+/**
+ * \brief This function is analogue to \c mbedtls_rsa_rsassa_pss_sign().
+ *        The only difference between them is that this function is more flexible
+ *        on the parameters of \p ctx that are set with \c mbedtls_rsa_set_padding().
+ *
+ * \note  Compared to its counterpart, this function:
+ *        - does not check the padding setting of \p ctx.
+ *        - allows the hash_id of \p ctx to be MBEDTLS_MD_NONE,
+ *          in which case it uses \p md_alg as the hash_id.
+ *
+ * \note  Refer to \c mbedtls_rsa_rsassa_pss_sign() for a description
+ *        of the functioning and parameters of this function.
+ */
+int mbedtls_rsa_rsassa_pss_sign_no_mode_check(mbedtls_rsa_context *ctx,
+                                              int (*f_rng)(void *, unsigned char *, size_t),
+                                              void *p_rng,
+                                              mbedtls_md_type_t md_alg,
+                                              unsigned int hashlen,
+                                              const unsigned char *hash,
+                                              unsigned char *sig);
+#endif /* MBEDTLS_PKCS1_V21 */
+
+#endif /* rsa_internal.h */