Add constant-flow testing for PKCS7 padding

Signed-off-by: David Horstmann <david.horstmann@arm.com>

library/cipher_invasive.h

@@ -0,0 +1,27 @@
+/**
+ * \file cipher_invasive.h
+ *
+ * \brief Cipher module: interfaces for invasive testing only.
+ *
+ * The interfaces in this file are intended for testing purposes only.
+ * They SHOULD NOT be made available in library integrations except when
+ * building the library for testing.
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+#ifndef MBEDTLS_CIPHER_INVASIVE_H
+#define MBEDTLS_CIPHER_INVASIVE_H
+
+#include "common.h"
+
+#if defined(MBEDTLS_TEST_HOOKS) && defined(MBEDTLS_CIPHER_C)
+
+MBEDTLS_STATIC_TESTABLE int get_pkcs_padding(unsigned char *input,
+                                             size_t input_len,
+                                             size_t *data_len);
+
+#endif
+
+#endif /* MBEDTLS_CIPHER_INVASIVE_H */